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Reinterpreting the tectono-metamorphic evolution of the Tonga Formation, North Cascades: A new perspective from multiple episodes of folding and metamorphism

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Reinterpreting the tectono-metamorphic evolution of the Tonga Formation, North Cascades: A new perspective from multiple episodes of folding and metamorphism

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Content REINTERPRETING THE TECTONO-METAMORPHIC EVOLUTION OF THE
TONGA FORMATION, NORTH CASCADES: A NEW PERSPECTIVE FROM
MULTIPLE EPISODES OF FOLDING AND METAMORPHISM
Copyright 2004
by
Luke Anthony Jensen
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
August 2004
Luke Anthony Jensen
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UMI Number: 1422392
Copyright 2004 by
Jensen, Luke Anthony
All rights reserved.
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DEDICATION
This thesis is dedicated to Jenny, without whom this work would have been
impossible, personally or professionally.
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ACKNOWLEDGEMENTS
Into the Teton landscape enter many elements which are ceaselessly changing, producing
combinations that are new and beautiful. Even the contour o f the range undergoes change, as
we have observed, but in the large view this is imperceptible, so that seeking permanence in a
universe o f change we turn to the mountains such as these for a symbol o f everlastingness.
-Fritiof Fryxell, from The Tetons: Interpretations o f a Mountain Landscape
It is due to many journeys to the high ranges and valleys of Wyoming as a
child that inspired my interest in geology. Foremost, I have my parents to thank for
that. Many walks through the Teton landscape had me wondering how and why the
craggy peaks, moraines, and stunning rocks had formed. The study of geology, as a
contemplation of the changing features of our planet and particularly its structures,
had me hooked. I encourage anyone who has not had the opportunity to visit the
Tetons to take some time to do so, whether it be to marvel at the geology or simply
for quiet reflection.
I would like to acknowledge the support of the many people who have helped
me a great deal during the research process related to this project. First I want to
thank my advisor, Scott Paterson, for always pushing his students to become experts
in their fields, think critically, and of course to write more. Hermann Lebit
(University of Louisiana), whom I had the chance to spend time with in the field
during my last field season, was critical to helping me recognize large-scale folding
in the Tonga Formation and learn the intricacies of fold interference patterns
throughout the Cascades. Ron Vernon (Macquarie University) has been instrumental
in supporting my Cascades research focusing on porphyroblasts, and partial funding
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for the work was provided through NSF grant EAR-0208394. Additional support for
field research and fold studies was provided by a GSA Student Research Grant,
which I am also grateful for. I wish to thank Bob Miller (San Jose State University)
for providing a base camp in the Cascades for the work, and also for his generous
hospitality and discussion. I wish to acknowledge Ned Brown (Western Washington
University) and Kathleen Harper (now at the Wyoming NASA Space Grant
Consortium) for the access to thin sections, maps, and Rb-Sr powders, and the
University of Maryland for the cost of the geochemical analyses. Tim Johnson
(University of Maryland) constructed the pseudosection for the Tonga Formation and
kindly allowed me to use it in this work. I also thank Mike Brown (University of
Maryland) for pseudosection discussion. Jenny Matzel (MIT) provided the high-
resolution U-Pb zircon dating of the Excelsior Mountain orthogneiss. Last, but not
least, are the other USC structure and tectonics graduate students, whose
companionship and counsel made this process richer, more fulfilling, and more
enjoyable.
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TABLE OF CONTENTS
DEDICATION ii
ACKNOWLEDGEMENTS iii
LIST OF FIGURES vi
ABSTRACT ix
CHAPTER I: INTRODUCTION 1
Purpose 1
Tectonic Setting 4
Tonga Formation 5
CHAPTER II: TECTONO-METAMORPHIC HISTORY OF THE TONGA
FORMATION 8
Tonga Formation Protolith 8
Structural Features 9
Intrusion and Metamorphism 19
Discussion 27
Conclusions 28
CHAPTER III: INTERPRETATION OF MICROSTRUCTURES AND
PORPHYROBLAST-MATRIX RELATIONSHIPS 31
Introduction 31
Inclusion Trail Patterns: Timing of Deformation and Porphyroblast Growth 35
Porphyroblast-matrix Micro-mapping 39
Conclusions 49
CHAPTER IV: COMPARISON OF FOLDING UNDER DIFFERENT
CRUSTAL CONDITIONS 51
Introduction 51
Temperature as a Primary Control of Fold Formation 51
Comparison of Naturally Deformed Folds 56
Conclusions 60
REFERENCES CITED 61
v
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LIST OF FIGURES
CHAPTER I: INTRODUCTION
Figure 1-1: North Cascades System Map 6
CHAPTER II: TECTONO-METAMORPHIC HISTORY OF THE TONGA
FORMATION
Figure 2-1: Photomicrograph: Isoclinally Folded Quartz Veins 11
Figure 2-2: Photomicrograph: Crenulation Cleavage Adjacent to Rim of
Andalusite Porphyroblast Preserving Si 11
Figure 2-3: Photomicrograph: Recrystallized Biotite Fish Preserving Si 12
Figure 2-4: Structural Map and S2 Cleavage Stereonet Plot 14
Figure 2-5: Overturned Limb Bedding-Cleavage Relationship 16
Figure 2-6: Normal Limb Bedding-Cleavage Relationship 16
Figure 2-7: Structural Map and So, U Stereonet Plots 17
Figure 2-8: E-W Cross-section of the Central Tonga Formation 18
Figure 2-9: Steeply Plunging Microcrenulation Lineations (L2) 18
Figure 2-10: Concordia Diagram of U-Pb (Zircon) Dates for the Excelsior
Ridge Orthogneiss 20
Figure 2-11: Photomicrograph: Symplectites of Staurolite and Quartz after
Andalusite in the Beckler Peak Pluton Aureole 22
Figure 2-12: Map of Compiled Spatial Porphyroblast Distribution in the
Tonga Formation 24
Figure 2-13: Subsolidus MnNCKFMASH P-T Pseudosection Showing
Stable Phase Assemblages for an Average Tonga Formation Metapelite
Composition 26
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CHAPTER III: INTERPRETATION OF MICROSTRUCTURES AND
PORPHYROBLAST-MATRIX RELATIONSHIPS
Figure 3-1: Schematic Orientations of Maximum Contractional Phases of
Deformation in the Tonga Formation in Sample TF-97 (YZ)
Figure 3-2: Photomicrograph: Fibrous Sillimanite in Fractures Within
Andalusite Porphyroblast
Figure 3-3: Photomicrograph: Cordierite Porphyroblast Preserving a
Crenulated Si
Figure 3-4a: High-resolution Photomicrograph and Micro-map of Sample
TF-14XY
Figure 3-4b: High-resolution Photomicrograph and Micro-map of Sample
TF-14XZ
Figure 3-4c: High-resolution Photomicrograph and Micro-map of Sample
TF-14 YZ
Figure 3-5a: High-resolution Photomicrograph and Micro-map of Sample
CS-117XY
Figure 3-5b: High-resolution Photomicrograph and Micro-map of Sample
CS-117XZ
Figure 3-5c: High-resolution Photomicrograph and Micro-map of Sample
CS-117 YZ
CHAPTER IV: COMPARISON OF FOLDING UNDER DIFFERENT
CRUSTAL CONDITIONS
Figure 4-1: Temperature-induced Competence Contrast Modifications
Southwest of the Adamello Pluton, Italian Alps
Figure 4-2: Four Theoretical End-member Fold Deformation Mechanisms
Figure 4-3: Modeled Temperature Effects on Elasto-viscous Folding
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37
38
42
43
44
45
46
47
52
53
55
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Figure 4-4: Schematics Examples of Field Grid Maps in the Tonga
Formation and Chiwaukum Schist
Figure 4-5: Photomicrographs: Micro-scale Folds in the Tonga Formation
and Chiwaukum Schist
Figure 4-6: Strain Contour Map Showing Results from Folds of the
Tonga Formation and Chiwaukum Schist
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ABSTRACT
Detailed structural and metamorphic mapping of the Tonga Formation, on the
western boundary of the Cascades crystalline core, reveals a more complex, multi
phase history of deformation than previously recognized similar to the first
transposition cycles in the Chiwaukum Schist. The presence of staurolite-quartz
symplectites suggests that the unit records an overprint of regional over contact
metamorphism, including the northern Beckler Peak pluton. Porphyroblast inclusion
trails and matrix relationships throughout the Cascades were examined in the context
of regional metamorphism and found to record primarily local, heterogeneous strain
over several periods of deformation. Mechanical modeling and strain analysis of
meso- and micro-scale folds from the Tonga Formation and neighboring Chiwaukum
Schist over a regional-scale thermal gradient indicate a strong effect of temperature
on fold nucleation, growth, geometry, and ultimately crustal rheology.
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CHAPTER I: INTRODUCTION
Purpose
The crystalline core of the North Cascades provides a unique crustal section
where the dynamics of and relationship between deformation and metamorphism can
be studied under different crustal conditions. This project began as an attempt to
specifically compare the effects of temperature on deformation over a well-
constrained gradient throughout the crystalline core, and grew into an effort to map
in detail the relatively low-temperature Tonga Formation on the western margin of
the core, interpret and constrain the implications of porphyroblast inclusion trail
patterns and related microstructures, and better understand the role of temperature in
fold nucleation and growth.
Previous studies of the Tonga Formation focused on general mapping over a
broad area (Yeats, 1958) and metamorphic petrology (Duggan, 1992, Duggan and
Brown, 1994), but lacked a detailed consideration of structural features of the unit.
Mapping and thin section analysis of the Tonga Formation were undertaken to
exclusively address timing of multi-phase deformation relative to plutonism and
metamorphism, and to compare these findings with those of the neighboring
Chiwaukum Schist to the east.
Microstructural relationships between porphyroblasts and foliations in the
surrounding matrix of metamorphic rocks are potentially useful for inferring several
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aspects of metamorphic and deformational histories. Their use has dramatically
increased in the last twenty years but remains highly controversial. (1) The extent to
which these relationships can be used to support tectonic interpretations and (2) the
extent to which they can be used to link deformation and metamorphism are critical
factors. The first issue concerns recent proposals suggesting that inclusion trail
patterns can be used to determine the regional evolution of orogenic belts (Bell and
Johnson, 1989; Johnson, 1990; Bell et al., 1995; Bell et al., 1998; Bell and Hickey,
1999; Johnson, 1999) and/or past plate motions (Aerden, 1994; Bell et al., 1995).
The second issue concerns the degree to which porphyroblast-matrix relationships
can be used to address questions about the duration and heat source for
metamorphism, the duration and periodicity of deformation, the importance of
heterogeneous deformation and metamorphism in orogenic belts, and the timing of
mineral growth relative to development of cleavages (Zwart, 1962; Bell et al., 1986;
Vernon, 1989; Johnson, 1992; Williams, 1994; Johnson, 1999).
In trying to evaluate these matters, another philosophical issue often arises -
whether greater weight should be given to a single data set (porphyroblast-matrix
relationships) or multiple data sets (regional structural, metamorphic, and
geochronologic data), when the two approaches result in incompatible conclusions
(e.g., Paterson and Vernon, 2001). In the context of and with support from a National
Science Foundation Tectonics Program grant (EAR-0208394), Cascades
porphyroblasts, their inclusion trails, and whether they record primarily information
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about orogen-scale processes or local heterogeneities are being addressed through
“micro-mapping” of thin-section-scale, porphyroblast-matrix relationships. A suite
of sections were chosen from the Tonga Formation and Chiwaukum Schist to
address porphyroblast dynamics related to regional metamorphism, contact
metamorphism, and within folds. A selection of such “micro-maps” and their
implications is presented herein.
Current fold theory and models do not account for many features, such as
asymmetries, multiple wavelengths, and layer heterogeneities, which are observed in
folded geologic materials. Existing disagreements on the range and relevance of
competence contrasts between folding media and the role of initial perturbations
highlight current ambiguities. A new approach is presented that highlights
temperature as a dominant control over many aspects of folding. Several variables
prevalent during the deformation process, such as temperature, mechanical
anisotropy produced by primary or secondary layering and on the grain-scale, and
microstructural deformation mechanisms have largely been neglected with respect to
folding. Studying these parameters therefore provides an innovative way to address
the disconnect between fold models and folds observed in the field. Not only do
these factors influence material behavior and fold nucleation and development under
static conditions, they also exert a profound effect when they vary dynamically
during deformation, which is likely the case in a naturally deforming system on
geologic timescales.
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The role of temperature in folding is addressed initially by forward numerical
modeling and the study of folds throughout the Cascades core. The goal is to bridge
the current gap between modelers and field geologists by carefully incorporating
observations from the field to generate more realistic fold models and a more
complete understanding of multi-scale folding mechanisms. Folds that formed under
different thermal conditions in the Tonga Formation and Chiwaukum Schist were
systematically measured in the field and in thin-section to evaluate the effects of
temperature on the folding process. These folds were analyzed qualitatively and
quantitatively using the method outlined by Schmalholz and Podladchikov (2001) to
estimate percent shortening due to folding, competence contrast between layers, and
dominant folding deformation mechanisms.
Tectonic Setting
The North Cascades crystalline core is comprised of several, largely fault-
bounded tectono-stratigraphic terranes, which experienced composite episodes of
intrusion, deformation, and metamorphism during Cretaceous to Paleogene time
(Misch, 1966; Rubin et al., 1990). The Wenatchee block, which is bounded to the
west by the Fraser-Straight Creek fault and separated from the Chelan block to the
east by the high-angle Entiat fault (Miller et al., 2003), consists of 96-91 Ma plutons,
the largest of which is the Mt. Stuart batholith, and multiply deformed and
metamorphosed schists and gneisses, predominantly the Chiwaukum Schist host
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rock, which reaches upper-amphibolite facies. The Tonga Formation lies in the
southwestern portion of the block along the Fraser-Straight Creek fault, and was first
mapped in detail and named by Yeats in the 1950s.
Tonga Formation
The Tonga Formation is a narrow (~4 x 20 km), multiply deformed, intruded,
and metamorphosed, predominantly pelite-psammite sequence that lies on the
westernmost margin of the Nason terrane in the Cascades crystalline core (figure 1-
1). It is bounded to the west by the Paleogene Fraser-Straight Creek fault, which
displays dextral displacement of up to 192 km (Misch, 1977; Vance and Miller,
1981; Monger, 1985), and offsets the unit with what is interpreted to be its
equivalent, the Cayoosh Formation (Mahoney and Journeay, 1993; Duggan and
Brown, 1994) to the north in British Columbia. Off to the east, oblique-slip
displacement along the steeply dipping Evergreen fault increases from north to south
in a “scissors” fashion, juxtaposing phyllites of the southernmost Tonga Formation,
near Skykomish, Washington, with the Mt. Stuart batholith and amphibolite-grade,
melt-present Chiwaukum Schist to the east. The Tonga Formation is intruded by the
Beckler Peak granodiorite near its southern margin, and by the granodioritic
Excelsior Ridge orthogneiss and the pyroxene-bearing Goblin Creek granodiorite to
the north, which obscure its contact with sillimanite- and kyanite-bearing
Chiwaukum Schist. All three plutons are elongate in a north-south direction, parallel
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to the long dimension of the Tonga Formation itself. Local dike intrusions of
intermediate to felsic compositions are also present, but not abundant.
Major Geologic Units
Quatcrnars and leriiarv
volcanics and sedim ents
I'ertiars plutons
C retaceous plutons
C hiw aukum schist
and banded gneiss
baston metamorphic:
KILOM ETERS
Figure 1-1. North Cascades system map, showing major
geologic units of the Wenatchee block (after Duggan, 1992).
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Duggan (1992) and Duggan and Brown (1994) have established by field
mapping, petrologic studies, and Rb-Sr whole rock isotopic analyses that the Tonga
Formation is correlative with the Chiwaukum Schist and not the blueschist-grade
Easton Metamorphic Suite (Zen, 1988; Monger, 1986, 1991) or more specifically the
Darrington Phyllite, as previously suggested by Yeats (1958). This paper
corroborates the relationship between the Tonga Formation and Chiwaukum Schist
with further structural and metamorphic evidence that also links the histories of the
units. Deformation of the Tonga Formation is not as simple as previously suggested
(Duggan, 1992; Duggan and Brown, 1994), as multiple episodes of deformation
recorded in complex structures are preserved that are often syn-metamorphic.
Detailed field mapping and microstructural analysis with respect to development of
structures and their timing in the context of distinct Buchan and subsequent
Barrovian metamorphic events are presented. Mineral occurrences previously used to
construct metamorphic isograds and imply a steep metamorphic gradient (Tabor et
al., 1993; Duggan and Brown; 1994) are not tightly constrained, and may instead
reflect compositional variation and heterogeneity of the protolith. Symplectites of
staurolite plus quartz replacing andalusite in the aureole of the Beckler Peak pluton
imply that the southern Tonga Formation evidence higher pressures in the southern
part of the unit than previously assumed. Finally a pseudosection illustrating P-T
conditions for unit’s bulk composition is presented and the unit’s history summarized
in terms of P-T evolution.
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CHAPTER II: TECTONO-METAMORPHIC HISTORY OF THE TONGA
FORMATION
(For submission to the Canadian Journal of Earth Science, including sections of the
introductory chapter)
Tonga Formation Protolith
The Tonga Formation consists of a metasedimentary package containing
interbedded fine-grained highly graphitic phyllite and semi-schist in the south to
coarser-grained, more quartz-rich phyllite, schist, and metasandstone in the north. On
a regional scale, the unit coarsens upward (toward the north), suggesting deposition
in a prograde fan. Significant protolith compositional heterogeneity exists throughout
the sequence, as noted by Duggan (1992). Local metabasite pods with textures
varying from plutonic to volcanic, now metamorphosed to greenschists, are also
present within the sequence of metasediments, and in some areas cut compositional
layering. Local dikes, mostly of intermediate to felsic compositions, are found
throughout the unit, although are not pervasive. Field relationships and thin-section
analysis indicate that the Tonga Formation metaclastics were deposited as an off
shore, arc-derived turbidite sequence similar to that of the neighboring Chiwaukum
Schist (Anderson and Paterson, 1991; Magloughlin, 1993). The presence of several
interbedded slices of oceanic- and arc-related rocks suggests accretion of the unit in a
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wedge before the initiation of large-scale arc magmatism (Paterson et al., 1994). Rb-
Sr whole rock isotopic data indicate that the depositional age of the Tonga Formation
is 204 ± 28 Ma, similar to the 210 ± 44 Ma age of the Chiwaukum Schist (Duggan
and Brown, 1994). However, no sandstones containing detrital zircons appropriate
for high-precision U-Pb dating have been found, leaving the exact depositional age
constrained only to between the mid-Triassic and mid-Jurassic.
Structural Features
Within the Wenatchee block, the Tonga Formation is distinct in that it
preserves primary sedimentary structures remarkably well, despite multiple episodes
of deformation and recrystallization at up to amphibolite-facies conditions. Bedding
or compositional layering (So) is clearly visible at both outcrop and thin-section
scales. Locally, graded bedding, laminae, cross beds, rip-up clasts and lode casts are
preserved. Field mapping and microstructural analysis indicate some revisions to the
deformational history of the Tonga Formation. Several phases of deformation are
evident in thin section, indicating that structures are not as simple as the single,
through-going foliation described by Duggan & Brown (1994). Instead, up to three
cycles of transposition (in the sense of Paterson and Tobisch, 1988) are preserved in
the Tonga Formation.
Unlike the majority of the Cascades, the Tonga Formation preserves bedding
relatively well and therefore does not necessitate the use of transposition cycle
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nomenclature as in the Chiwaukum Schist (Paterson and Tobisch, 1994) because it
appears that all episodes of deformation, and therefore evidence for all primary and
secondary structures, are preserved. The nature of deformation in the Tonga
Formation is still complex, but detailed mapping, petrography, and study of inclusion
trails in porphyroblasts indicate a finite number of deformation episodes and also
highlight the similarity between the Tonga Formation and early Chiwaukum Schist
transposition cycles. The Si cleavage, which in some instances was sub-parallel to
bedding, was accompanied by a metamorphic event (Mi) of at least garnet grade
(Evans and Davidson, 1999). Si is largely eradicated, and is now preserved only as
isoclinal folds (figure 2-1), in the pressure shadows of andalusite porphyroblasts
(figure 2-2), and as inclusion trails in early biotite porphyroblasts (figure 2-3). The
cycle 1 cleavage is best displayed as rootless folds of quartz segregations most easily
visible in thin section (figure 2-1).
The pervasive cleavage observed throughout the Tonga Formation
corresponds to a second cycle of transposition. This cleavage is relatively uniform
throughout the unit, strikes to the NNW, and dips moderately toward the NE (figure
2-4). Formation of S2 was accompanied by substantial metamorphic differentiation
during which quartz segregations, in addition to pre-existing veins, developed
parallel to the dominant cleavage. Small-scale folding and boudinage of these
segregations and veins followed. Field and petrographic observations make it clear
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Figure 2-1. Plane-polarized photomicrograph of Si quartz veins, now visible
as isoclinal, rootless folds transposed by cycle 2 deformation. Base of photo
is 2.5 cm.
Figure 2-2. Plane-polarized photomicrograph showing matrix crenulation
cleavage (top) adjacent to pseudomorphed andalusite porphyroblast
(bottom, high relief) with distinct graphitic arcs at boundary. Crenulation
axial planes run from top to bottom of photo. Base of photo is 2 mm.
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Figure 2-3. Recrystallized biotite “fish” preserving the Si cleavage. Base of
photo is 1.0 cm.
that, unlike in the neighboring Chiwaukum Schist, none of these felsic segregations
are melt-related, probably due to the relatively small nature of the plutonic bodies in
the Tonga Formation.
Though bedding is difficult to recognize in all parts of the Tonga Formation,
detailed field mapping of bedding-cleavage relationships and relatively consistent
intersection lineations (figures 2-5, 2-6) reveals that the dominant cleavage is related
to formation of a regional, close, WSW-vergent, cylindrical, gently north-plunging
anticline (figures 2-7, 2-8). The geometry of the fold profile is remarkably consistent
along the length of the hinge line throughout the unit, and the half-wavelength of the
fold is approximately 1-2 km. The cleavage associated with cycle 2 transposition is
axial planar (sensu lato) to folded bedding. However, the hinge line of this fold is
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oblique to bedding-cleavage intersection lineations (L;, figure 2-7), indicating a
cleavage-transected fold (see Johnson, 1991; Johnson and Woodcock, 1991) and
suggesting a syn-folding change in the strain field. The fold orientation differs
markedly from that observed in regional-scale folds in the Chiwaukum Schist, where
fold hinge lines plunge gently to the NW (see Miller et al., 2003; Paterson et al., in
prep.). Unconstrained rigid rotation of the Tonga Formation by motion along its
bounding faults prevents extensive kinematic analysis with respect to regional
folding. A lineation defined by microcrenulations (L2, figure 2-9) of the S2 cleavage
also formed during cycle 2 transposition. This phase of deformation included an
extensional component approximately parallel to the regional-scale fold hinge line as
evidenced by local extensional crenulation cleavage development, high values of
stretch from quartz clasts, and boudinage of staurolite porphyroblasts. Stretching
lineations, however, are not well developed in the Tonga Formation. This may
simply be due to the fine-grained, phyllitic nature of much of the unit. Both plutonic
bodies that intrude the Tonga Formation are elongate parallel to the regional fold
hinge line, suggesting that they were emplaced in the hinge region of the fold or are
sheets exposed in the core of the anticline. Folding alone is an unlikely emplacement
mechanism, however, due to the relatively slow rates of saddle reef development
relative to magma emplacement rates (Stevens et al., 2001; Paterson and Tobisch,
1992). S2+ structures are gentle to open folds of S2, and indicate an incipient
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LEGEND
H
T onga F o rm atio n
□
Beckler Peak G ran o d io rite
m
Excelsior Ridge G ran o d io rite
i
G o blin C reek G ran o d io rite
Equal Area
Figure 2-4. Structural map of the Tonga Formation with legend showing orientation of
the S 2 cleavage and L2 microcrenulation lineations from field measurements. Also shown
is an equal-area, lower-hemisphere projection of poles to cleavage planes and calculated
contours (n = 110). Average trend and plunge are indicated by the triangle.
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transposition cycle that never went to completion. These structures correspond well
with the 2+ transposition cycle structures observed in the Chiwaukum Schist (see
Miller et al., 2003; Paterson et al., in prep.). The 2+ transposition cycle also folds
microcrenulation lineations (L2) into a weak small circle distribution, demonstrating
a regional folding mechanism with a strong component of flexural slip (Ramsay,
1967) or that the orientation of compositional layering relative to the bulk strain
ellipsoid produced local variations in small-scale folding.
The youngest deformation in the Tonga Formation is related to strike-slip and
oblique-slip displacement along the Fraser-Straight Creek and Evergreen faults,
respectively, which postdate transposition and metamorphism. Distributed
deformation in multiple fault-parallel shear zones and boudinaged dikes is common
to the north along the Evergreen fault, whereas it is a more discrete brittle feature to
the south. The Excelsior Ridge pluton displays both locally intense, subsolidus
ductile deformation and brittle deformation, although this deformation is not
penetrative through the body. Conversely, the Beckler Peak pluton displays
extensive cataclasis, which is also correlative with late brittle deformation associated
with faulting. These contrasting features along the length of the Evergreen fault are
attributable to deeper exposure and slightly more ductile deformation at the northern
end of the Tonga Formation.
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iiLa Cleavage surface
st ■ : i &
Bedding surface
4 S k
Figure 2-5. Field photo showing bedding-cleavage relationship.
View of the overturned limb, looking south. Bedding (S0 ) is
steeper than the axial-planar cleavage (S2) related to folding.
Hammer for scale.
Bedding surface
Cleavage surface
Figure 2-6. Field photo showing bedding-cleavage relationship.
View of the upright limb, looking west. The axial-planar cleavage
(S2) is steeper than bedding (S0 ). Hammer for scale.
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Equal Area
Equal Area
Figure 2-7. Structural map of the Tonga Formation showing orientation of deformed
bedding (S0 ), trend and plunge of overturned anticline, and S2 cleavage-bedding intersection
lineations (Li). Also shown are equal-area, lower-hemisphere projections of poles to
bedding (above, n = 27), and of intersection lineations (n = 20) and calculated contours.
17
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Elevation ( x TO 3 feet)
5
E vergreen
Fault ^
.0 0 0 ft
4
S traigh t C reek |
Fault v
3
2
1
Tertiary
V olcan ics
(u n d iffere n tia ted )
T onga
F orm ation 5tu art
0
Figure 2-8. E-W cross-section across the central Tonga Formation (see figure 2-7 for
line of cross-section) showing regional folding of bedding and cleavage-bedding
relationships. Bedding is shown in black (long dashes) and cleavage in gray (short
dashes). No vertical exaggeration.
Figure 2-9. Photograph of steeply plunging microcrenulations (L2 ) in the field. Pen
for scale is parallel to the hinge lines of these microcrenulations.
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Intrusion and Metamorphism
The Tonga Formation was intruded between 96 and 91 Ma by two felsic
plutonic bodies, the Beckler Peak granodiorite to the south, and the Excelsior Ridge
granodiorite, to the north. The pluton ages and compositions are similar to the
plutons that intrude the rest of the Wenatchee block. The Beckler Peak pluton yields
K-Ar cooling ages from hornblende and biotite of 92 ± 3 Ma (Yeats and Engels,
1971). No intact zircons for U-Pb dating for the Beckler Peak pluton could be
extracted from collected samples. Courtesy of Jenny Matzel (MIT), a new, high-
precision, U-Pb (zircon) date for the central region of the Excelsior Ridge pluton
yields a concordant age of 96.0 ± 0.4 (figure 2-10), which corresponds to some of the
oldest pulses of felsic magmatism within the Wenatchee block, including the NW
portion of the Mt. Stuart batholith and the Sulphur Mountain pluton. This matches
well with a U-Pb (zircon) age of 95.0 ± 0.6 Ma from a dacite sill at the northernmost
tip of the Tonga Formation (Duggan and Brown, 1994), implying that the sill may be
related to the same pulse as the Excelsior Ridge tonalite.
The Tonga Formation experienced at least three distinct episodes of
metamorphism. As is also true in the Chiwaukum Schist, the Mi event is poorly
preserved and is recorded primarily by structures such as remnant isoclinal rootless
folds with high-temperature quartz recrystallization microstructures (Plummer,
1980). This relationship suggests the coeval development of Mi and Si.
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Excelsior Ridge pluton: C oncordia diagram
0.0153
' FRO-61
0 .0 1 5 2
0 . 0 1 5 1
3
00
_q 0 .0 150
Q _
v O
O
W e ig h t e d M e a n A g e = 9 6 .0 + /- 0 .4
(M SW D = 0 .6 )
0.0148
0.0147
0.0971 0.0976 0.0981 0.0986 0.0991 0.0996 0.1001 0.1006 0.1011
2 0 7 Pb / 2 3 5 U
Figure 2-10. Concordia diagram showing results of high-precision U-Pb dating
of zircons from the central Excelsior Ridge granodiorite. Courtesy of Jenny
Matzel (MIT).
Intrusion of both of Beckler Peak and Excelsior Ridge plutons generated
Buchan-type metamorphic assemblages (M2) of andalusite, garnet, and biotite in
relatively narrow (<100 m) aureoles within the Tonga Formation. Cordierite is not
abundant but was observed by Yeats (1958) adjacent to quartz diorite sills along
Tonga Ridge southeast of Skykomish. Aluminosilicate formation was limited to
andalusite growth, whereas sillimanite is absent, probably due to the smaller size and
consequent lower heat generation of the plutons. Andalusite porphyroblasts around
the Beckler Peak pluton are wrapped by and in some cases “rotated” with respect to
the S2 cleavage, and pressure shadows along their margins display a crenulation
20
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cleavage that preserves Si. Inclusion trails are poorly preserved in these
porphyroblasts, and they are largely retrogressed (?) to fine-grained muscovite, but
the wrapping and, where they are preserved, instances of continuous Sj and Se
indicate that growth was syn-kinematic with respect to the formation of S2 .
The Buchan assemblages developed by contact metamorphism are
overprinted by amphibolite-grade Barrovian assemblages (M3) of staurolite, garnet,
and biotite in pelites. Units of more psammitic composition commonly contain calcic
amphibole. Euhedral staurolite and garnet are wrapped by the S2 cleavage, although
not as pervasively as andalusite within the contact aureoles. However, prisms of
staurolite show little to no alignment in the cleavage plane. Symplectites of staurolite
and quartz replacing andalusite are present within the Beckler Peak pluton aureole
(figure 2-11). These worm-like quartz intergrowths (see Hollister, 1969) with
staurolite, which indicate simultaneous reaction products, are conspicuously similar
to those observed to the east in the neighboring Chiwaukum Schist at Tunnel Creek,
and are cited by Evans and Davidson (1999) as evidence for a regional overprint
following contact metamorphism. If this locality is analogous, staurolite occurrence
near the Beckler Peak pluton is related to regional (Barrovian) overprinting rather
than contact (Buchan) metamorphism. Although all these observations indicate that
the development of Barrovian minerals is younger than andalusite growth, both
episodes are syn-kinematic and grew during the period of S2 development and
therefore regional folding.
21
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Figure 2-11. Plane-polarized photomicrograph of staurolite
(center and right) surrounded by altered andalusite (clear, high
relief). Note the worm-like quartz intergrowths of the
symplectite at the left boundary of staurolite with andalusite,
indicating a coeval reaction product. Base of photo is 1.0 mm.
The distribution of metamorphic minerals in the Tonga Formation (figure 2-
12) does not fit neatly within the isograds constructed by Tabor (1993) or Duggan
and Brown (1994). The occurrence or absence of many “index” minerals could
instead be explained by heterogeneous protolith composition. If this proves correct, a
continuous regional staurolite isograd could be constructed from the Chiwaukum
Schist into the Tonga Formation across a restored Evergreen Fault, near the town of
Skykomish, Washington.
The portion of the Tonga Formation south of this point is anomalous relative
to the rest of the unit. The southern area is truncated by both the Evergreen fault,
which separates the Tonga Formation from the Swauk Formation sediments to the
east, and an enigmatic fault (?) that separates the Tonga Formation from sandstones
22
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of the Barlow Pass Volcanics to the west. The southern region is highly chloritized
and sericitized, although this phenomenon appears to be secondary in nature. Fluid
migration along this fault, volatiles from the Beckler Peak pluton, or meteoric waters
may have contributed to retrograde metamorphism of this southern area.
At its northern end, the Tonga Formation approaches the metamorphic grade
of the adjacent Chiwaukum Schist to the east, although cordierite, sillimanite, and
kyanite do not occur. The peak assemblage contains porphyroblasts of garnet and
staurolite, and in local zones of more calcic composition, actinolite and hornblende.
Some workers (e.g. Duggan, 1992) noted the rare presence of cordierite, but I have
not observed it, despite rigorous sampling and petrographic analysis. There is also no
sign of “cryptic” metamorphism in veins or microscopically as is found in the
Chiwaukum Schist west of the Mt. Stuart batholith (e.g. Evans and Davidson, 1999).
Thermobarometric data determined using GAHB, GAHP, and GABI techniques on
samples from the unit’s northern quarter yield peak pressures and temperatures on
the order of 7.0-7.1 ±1.0 kbar and 520°-550° ± 50° C, respectively (Duggan and
Brown, 1994). These results corroborate the similarity of peak metamorphic
conditions in this region of the Tonga Formation and the adjacent Chiwaukum
Schist. No thermobarometric data are available for the southern Tonga Formation to
indicate the actual steepness of its metamorphic gradient, and abundant alteration in
this region precludes simple thermobarometric analysis. Using the original Rb-Sr
samples Duggan & Brown (1994) obtained for their analyses, which were donated by
23
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Metamorphic Assemblages
r ( h lo rile
it B iotile
a C a r n e t
s S ta u ro lite
A A n d a lu site (co m m o n ly a lte re d )
C d C o rd ie rite
lib H o rn b le n d e
Ac A ctin o lite
M in e ra l o c c u re n c e s co m p iled from :
-C u rre n t siti«l>
-D u iatan (1992)
-Heath (1*>7||
-Y eats (1958)
Figure 2-12. Map of compiled spatial porphyroblast distribution in the Tonga
Formation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
E. H. Brown (Western Washington University), Tim Johnson (University of
Maryland) determined the bulk composition of the unit and used this data to
construct a pseudosection in P-T space (figure 2-13). The pseudosection, which
outlines subsolidus phase relations of the Tonga Formation metapelites in the
MnNCKFMASH system, was constructed using THERMOCALC v. 3.1 (Powell and
Holland, 1988) and based on the thermodynamic data set of Holland and Powell
(1998; updated 05/14/01). The aluminosilicate triple point, which is adjusted for a
higher enthalpy of sillimanite, is based on that of Pattison (1992) as discussed by
Johnson et al. (2003). Bulk composition of the Tonga Formation is slightly
anomalous relative to most subaluminous pelites, as is the Chiwaukum Schist
(Tinkham, 2002). Chemical analysis indicates high amounts of Ca and Na, and a low
K/Na ratio (<1). The assemblage within the Beckler Peak pluton contact zone (Chi +
Crd + Grt + And, not including staurolite) yields a contact metamorphic temperature
of -550° C at a pressure of ~3 kbar, indicating a relatively cool and shallow
emplacement depth (-10 km) during M2. To the north, the assemblage (Ms + Chi +
Grt + St) suggests a peak temperature for M 3 of 550°-600° C and peak pressure of 4-
7 kbar. These results are compatible within the errors of the thermobarometric
techniques used by Duggan and Brown (1994), but the discrepancies may also reflect
minor element substitution reactions or compositional heterogeneity within the unit
that the pseudosection does not incorporate. On the other hand, temperature
estimates from geothermometry techniques may be low due to their high sensitivity
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to iron and magnesium exchange. According to the pseudosection, growth of
staurolite after andalusite in the contact aureole is consistent with increased pressure
on the contact assemblage.
MnNCKFMASH (+ Qtz, PI, Bt; H20 in subsolidus)
P
(kbar)
V
L Ms Grt Ky
L Grt Ky
Ms Grt Ky
Ms Pg Chi Grt St L Grt Sil
fjP g Chi \ ■ >
Grt St > ^ Grt Sil '.I
Grt S t Sil
Ms C h Grt Chi Grt L Grt Crd Sil
f i
_ G r t St
/ Crd Sil
Grt Crd
L Grt
Crd Kfs
L Grt Crd
//
/ / Grt St Crd
Bulk (mol%)
Chi Grt
St And
S i02 70.59
Al20 3 11.08
CaO 2.77
MgO 5.23
FeO 5.81
K20 1.44
Na20 2.97
MnO 0.11
Grt Crd
Chi Grt
500 550 600 650 700
T(°C)
750
Figure 2-13. Subsolidus MnNCKFMASH P-T pseudosection showing stable phase
assemblages. An average Tonga Formation metapelite composition acquired from
geochemical analysis is modeled using the Holland and Powell (1998) data set
with a recalculated aluminosilicate triple point that is modified by raising the
enthalpy of sillimanite (see Pattison, 1992, Johnson et al., 2003). Shading changes
between stability fields indicate a change in variance. Abbreviations are after
Kretz (1983). Courtesy of Tim Johnson (University of Maryland).
26
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Discussion
The recent findings in the Tonga Formation highlight the importance of
combining structural and metamorphic studies in complex terranes such as those
found in the Cascades crystalline core. The Cascades provide an ideal crustal section
in which to study contraction and related deformation preserved in a syn-magmatic
setting. Only by pairing a detailed study of structures, on regional, outcrop, and
microscopic scales, with that of phase(s) of intrusion, loading, and related
metamorphism, however, is a more complete understanding of the orogenic
processes operating on the region realized. In particular, recognizing transposition
cycles and using porphyroblast-matrix relationships provide ways to relate the
relative timing of deformation and metamorphism.
What appears at first-observation to be a single, through-going cleavage in
the Tonga Formation reflects a more complex strain history of regional folding of
bedding and multiple transposition cycles. What sets the Tonga Formation apart
from the Chiwaukum Schist and other units in the Cascades is that bedding and
primary structures are preserved, which allow a perspective and structural tool not
available throughout much of region. Despite a multifaceted deformation history,
cleavage-bedding intersection lineations are remarkably consistent in orientation,
with local variations due only to small-scale folds and heterogeneity in the strain
field. The regional anticline and fold profile are consistent and can be mapped out
along the full length of the unit. Strain partitioning is observed, especially at the
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northern end of the unit in fold limbs, where relatively little deformation is evident in
coarse-grained metasandstones, in contrast with interbedded, highly sheared
metashales that possess a strong, almost bedding-parallel cleavage.
Due to the heterogeneous composition of the protolith, there is no simple way
to construct traditional metamorphic isograds to fit the distribution of metamorphic
minerals in the Tonga Formation. Therefore, attempting to assign classic isograds in
multiply deformed and metamorphosed domains is not a simple task, and needs to be
done carefully lest it convey misinformation. The symplectites of staurolite and
quartz replacing andalusite near the Beckler Peak pluton further link the tectono-
metamorphic histories of the Tonga Formation and Chiwaukum Schist, and suggest
that crustal loading to produce amphibolite-grade conditions extended south to
include at least the northern portion of the Beckler Peak pluton. Large-scale
contraction, magmatism, and crustal loading occurred over a relatively short duration
according to structural and metamorphic timing relationships.
Conclusions
The Tonga Formation is unique within the Wenatchee block in that it
preserves primary (So) features, which provide an additional constraint on timing of
intrusion, deformation, and metamorphism. The Tonga Formation and Chiwaukum
Schist have similar depositional ages and formed in related accretionary prism
settings, although they were not necessarily juxtaposed during subsequent
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deformation and metamorphism. The Tonga Formation has experienced a complex
tectono-metamorphic history, similar, but not identical to that of the Chiwaukum
Schist. An early, poorly constrained episode of syn-kinematic (cycle 1 transposition)
metamorphism (Mi) was followed by a second phase of deformation that generated
regional folds of bedding (So), an axial-planar cleavage (S2), formation of
microcrenulation lineations (L2), and was accompanied by pluton intrusion between
96 Ma and 91 Ma. The intrusive event was succeeded rapidly and syn-kinematically
by loading that generated Barrovian assemblages (M3). Plutonic bodies in the Tonga
Formation are relatively small (<10% area), and therefore do not support exclusive
magmatic loading of the Wenatchee block as suggested by Brown and Walker
(1993). An incipient transposition cycle (2+), indicated by minor, gentle to open
folding of S2 and flexural folding of the L 2 microcrenulation lineations, followed.
Both low- and high-temperature deformation associated with movement along the
Fraser-Straight Creek fault and Evergreen fault is also evident in the unit.
The distribution of metamorphic minerals in the Tonga Formation does not fit
systematically within previously constructed isograds (Tabor et al., 1993; Duggan
and Brown, 1994). This distribution is misleading due to the heterogeneous nature of
the protolith. The presence of staurolite-quartz symplectites after andalusite in the
contact aureole of the Beckler Peak granodiorite indicate that both the northern and
portions of the southern regions of the unit were loaded after growth of the Buchan
assemblage. However, the southernmost portion of the Tonga Formation is
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anomalous and inconsistent with the rest the unit in that it is highly chloritized and
sericitized, potentially due to volatilization by the nearby pluton or faulting (e.g.
Yeats, 1958). It is clear that the Evergreen fault offsets the Tonga Formation and
Chiwaukum Schist and that the units have distinct metamorphic, compositional, and
textural differences, although the magnitude of this displacement, and therefore jump
in metamorphic grade, remains uncertain.
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CHAPTER III: INTERPRETATION OF MICROSTRUCTURES AND
PORPHYROBLAST-MATRIX RELATIONSHIPS
Introduction
Due to their rapid growth, porphyroblasts overtake and incorporate growing
grains of other minerals and/or pre-existing stable or metastable minerals. Any initial
alignment is commonly preserved, so that a shape (dimensional) preferred
orientation of elongate inclusions can be seen in the porphyroblast as inclusion trails.
Inherited foliations, including sedimentary bedding, can also be revealed by different
concentrations of inclusions. Because alignment of elongate minerals typically
results from growth in a tectonic foliation, inclusion trails potentially allow the
timing of growth of porphyroblastic minerals relative to the formation of
deformation of a specific foliation. Patterns of foliations and microfolds outlined by
inclusion trails preserve evidence of the earlier stages of development of a foliation
or even generations of structures that have been obliterated from the adjacent matrix
by subsequent deformation, recrystallization and neocrystallization. In fact, in some
rocks, porphyroblasts may preserve the only evidence of previous deformational
events. In this way, porphyroblasts can preserve evidence of structural history of
metamorphic terrains, even if only of local significance. Provided suitable inclusion
trails are present, porphyroblast-matrix microstructural relationships may be used to
infer the relative timing of metamorphic and foliation-forming events. For example,
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Vernon (1988) used porphyroblast-matrix microstructural relationships to infer a
prograde metamorphic reaction, as well as indicating the presence of a previously
unrecognized crenulation foliation, in schists of the Cooma Complex, south-east
Australia.
Conceivably, metamorphic minerals can grow before, during or after a
particular foliation-forming or foliation-folding deformation event. Thus, a
porphyroblast may be classified as being pre-kinematic (pre-deformational), syn-
kinematic (syn-deformational) or post-kinematic (post-deformational) with respect to
that event. However, as emphasized by Vernon (1977, 1978), these terms should not
be applied without specifying a particular S-surface or fold set in the matrix.
Furthermore, it is necessary to specify which of the following matrix features is
being considered: (1) initiation of an S-surface, (2) flattening or folding of an
existing S-surface, or (3) growth of the minerals defining an S-surface. A “standard”
set of microstructural criteria was suggested by Zwart (1960a, 1960b, 1962), using
geometrical relationships between Si (the “internal” foliation outlined by inclusion
trails in the porphyroblast) and Se (the “external” foliation in the matrix). Subsequent
work has shown that, though these criteria are broadly applicable to the
determination of relative time relationships between growth of porphyroblasts and
deformation in regional metamorphic rocks, complications may occur, so that great
care must be taken in their application (Vernon, 1978). Many porphyroblast-matrix
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microstructural relationships are ambiguous, and so only clear, critical relationships
should be used. Jamieson (1988) has provided a summary of the approach.
Microstructures and inclusion trail patterns in porphyroblasts provide
additional evidence of the timing of deformation relative to metamorphism, and the
interaction between porphyroblasts and a deformed matrix in the Tonga Formation
and Chiwaukum Schist of the Cascades core. Several specific examples are
considered from both units. The observation that porphyroblasts are not limited to
growth in Q domains throughout much of the core, along with inconsistent
orientations between porphyroblast inclusion trails do not support a Bell model of
porphyroblast nucleation and growth (Bell, 1985).
Sample TF-97 was taken from the nearest exposed outcrop of Tonga
Formation west of the contact (-100 m) with the Beckler Peak granodiorite (see
figure 2-4). Though this raises some concerns that structures in the sample may
relate only to pluton emplacement, similar structures occur elsewhere throughout the
Tonga Formation, although the mineral assemblage indicates that this sample is
within the metamorphic aureole of the Becker Peak pluton. TF-97 contains quartz,
biotite, muscovite, and graphite in addition to porphyroblasts of andalusite, garnet,
and staurolite. The majority of andalusite porphyroblasts are altered to fine-grained
sericite. Recrystallization, neocrystallization, pressure solution, and minor fracturing
are the primary deformation mechanisms in this sample. The mineral assemblage,
before alteration, and recorded microstructures suggest a close to equilibrium state.
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Section TF-97 preserves and illustrates three distinct phases of deformation
(figure 3-1). Si is parallel to the z-axis of the section, and may or may not have been
bedding-parallel. Si is folded with maximum shortening parallel to the z-axis,
creating the isoclinal fold brightly visible in the center of the section. The formation
of S2 is accompanied by generation of a zonal and locally discrete crenulation
cleavage and banding parallel to fold axial planes. S2+ is an incipient foliation
discernible due to gentle folding of the crenulations and banding normal to S2 and
parallel to the y-axis of the section. S2+ is only observed locally throughout the
Tonga. These events counter the suggestion that deformation in the Tonga Formation
is simple and involves only one deformational event that developed a
I
f
s.
Figure 3-1. Plane-polarized light thin-section of TF-97 (YZ) showing
schematic orientations of maximum contractional strains during different
phases of deformation. Base of photo is 2.0 cm. See text for explanation.
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predominant single foliation (Duggan and Brown, 1994). At map scale and in cross
section, the Tonga Formation does have a moderate to steeply northeast-dipping
foliation, but careful microstructural study reveals a more complex history.
Inclusion Trail Patterns: Timing of Deformation and Porphyroblast Growth
In most thin sections of the Tonga Formation, garnet and staurolite
porphyroblasts do not contain abundant inclusion trails. However, recrystallized
biotite porphyroblasts preserve opaque inclusion trails quite well. The biotite shows
evidence of several stages of growth, the first (Mi) being syn-tectonic with respect to
generation of Si. Inclusion trails commonly record opaque crenulations or portions
of larger folds. Sj is slightly curved, although it is not highly deformed, and is not
usually continuous with Se. Biotite also shows evidence of growth during M 2 and
M 3, although these biotite porphyroblasts have less abundant inclusion trails. Though
not containing abundant inclusion trails, some andalusite and staurolite
porphyroblasts have a Si at their margins that is continuous with the matrix foliation.
These porphyroblasts may or may not have experienced rotation (relative to a matrix
reference frame) or neighboring Se transposition, and they are syn-tectonic with S2.
Samples from the Chiwaukum Schist collected along Icicle Creek display
outstanding porphyroblasts and folds, and present an opportunity to study the
relationship between metamorphic conditions and folding. Samples from this area
display refolded folds in a type 3 interference pattern geometry, with recumbent,
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isoclinal folds with their axial planes now parallel to the dominant cleavage; and a
younger, more open generation of folds, in some places with a minor axial planar,
zonal crenulation cleavage that is not evident at mesoscopic scale. However, for the
most part the later generation of folds lacks a strong axial-planar cleavage. These
samples, which contain abundant quartz and biotite in addition to the porphyroblasts
discussed below, underwent recrystallization, neocrystallization, and pressure
solution, and record a near-equilibrium mineral assemblage and stable
microstructures, as in the Tonga Formation.
Porphyroblast nucleation and growth in these samples are not limited to Q
domains; porphyroblasts appear throughout many compositional domains. Small
staurolite porphyroblasts and recrystallized biotite porphyroblasts preserve graphitic
inclusion trails that are continuous with the folded matrix foliation. Inclusion trails
display little or no deflection from the through-going fold patterns. This observation
presents two possibilities: ( 1 ) the porphyroblasts and matrix were strongly coupled
and folded together after inclusions of an initial, sub-planar foliation formed; or (2 )
porphyroblasts grew during the late stages of folding and experienced little or no
subsequent deformation. Garnet and small staurolite porphyroblasts, which have rare
inclusion trails in this region, are also compatible with these histories as they are not
strongly wrapped by matrix folia. Since porphyroblast vorticity rarely approaches or
exceeds fold limb spin (see Williams & Jiang, 1999; Jiang, 2001), porphyroblasts
from the Icicle Creek region are syn-to-post-tectonic.
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Sample CS-49, from the Tunnel Creek locality where two lobes of the Mt.
Stuart batholith surround a narrow septum of Chiwaukum Schist, displays evidence
of a different tectono-metamorphic history. The sample contains quartz, muscovite,
biotite, tourmaline, and plagioclase in addition to andalusite and cordierite
porphyroblasts with fibrous sillimanite that replaced andalusite (figure 3-2).
Figure 3-2. Plane-polarized light photomicrograph of CS-49 displaying fibrous
sillimanite (left of center) replacement in fractures within andalusite porphyroblast
(clear, high relief). Tourmaline (dark gray), biotite (medium gray), and quartz (clear,
low relief) are also present. Base of photo is 1.0 mm.
CS-49 experienced recrystallization, neocrystallization, and pressure
solution; and possesses roughly equilibrium microstructures. The mineral
assemblage contains sillimanite that grew after andalusite due to an increase in
temperature within the thermal aureole of the Mt. Stuart batholith. Since this process
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did not go to completion, overall the sample is in disequilibrium. An initial, sub-
planar foliation preceded folding. Subsequent folding was accompanied by zonal to
locally discrete crenulation cleavage development. Andalusite inclusion trails, which
are weak, are sub-planar and the porphyroblasts have been wrapped by the matrix
foliation. Slight curvature of inclusion trails at andalusite margins suggests that they
are syn-tectonic with respect to the initial stages of matrix foliation crenulation.
Cordierite porphyroblasts appear to have formed during the folding event and have a
folded Si with more complex deformation at their margins (figure 3-3).
Figure 3-3. Photomicrograph of CS-49 showing a cordierite porphyroblast (center,
gray, inclusion-rich) in cross-polarized light that preserves a crenulated S; composed
of muscovite and quartz. Chlorite (dark gray to black) appears to the lower right of
the porphyroblast. Base of photo is 4.0 mm.
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Porphyroblast-matrix Micro-mapping
Porphyroblast inclusion trails and their matrix relationships, despite their
inherent complexity, have been used for decades by geologists to interpret a variety
of deformational and metamorphic processes, including timing of nucleation and
growth, foliation development, pluton emplacement, finite longitudinal strain
determination, and folding mechanism (e.g. Johnson, 1999). Although these diverse
applications make porphyroblasts a valuable tool, considerable uncertainty still exists
surrounding how inclusion trails develop and what processes they record. A major
problem is that geologists are restricted to interpreting what is observed in the field,
or the resultant state, but are not able to observe cleavage and porphyroblast
development directly and therefore lack information about the processes’ temporal
evolution. One’s interpretations are only as accurate as his or her understanding of
the natural process involved upon which they are based. In response to these
ambiguities, a systematic approach is taken to evaluate porphyroblast dynamics and
help constrain their uses. In order to objectively determine whether porphyroblast
inclusion trails record sequential episodes of “accordion-style” orthogonal
deformation, as suggested by the Bell model (e.g. Bell, 1985), or rather local
heterogeneous strain and resulting structures (e.g. Paterson and Vernon, 2001),
detailed micro-mapping was undertaken. The Cascades crystalline core was chosen
as an ideal setting to pursue a study of porphyroblast-matrix relationships, given an
abundance of coarse-grained metamorphic minerals in a variety of arc-related
39
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tectonic settings. Porphyroblasts were examined in the specific contexts of regional
metamorphism, aureole metamorphism, and folding. Si/Se relationships were
examined relative to each of these environments and with respect to associated well-
constrained regional and local deformation.
A suite of ten samples were processed from the Tonga Formation and
Chiwaukum Schist. Each sample was cut to produce three mutually perpendicular
thin sections: one parallel to both the dominant cleavage and lineation (XY), one
perpendicular to the dominant cleavage and parallel to the dominant lineation (XZ),
and one perpendicular to both the dominant cleavage and lineation (YZ). Future
work will involve cutting thin sections in multiple, non-perpendicular orientations to
confront the problem of whether any interpretations are influenced by cut effects.
One set of micro-maps from the Tonga Formation and one from the Chiwaukum
Schist are presented here (figures 3-4, 3-5), although results are preliminary and
detailed statistical analysis on shapes and orientations has yet to be conducted.
High-resolution images of each thin section were captured using a Leica Wild
M420 macroscope equipped with an apochromatic zoom lens and fitted with a Spot
RT real-time digital camera. The matrix cleavage (Se), porphyroblast outlines, and
inclusion trails (Si) for each porphyroblast population were traced using a vector
graphics program to produce each micro-map. The result is one of the most detailed
data sets of porphyroblast-matrix relationships ever obtained. On a large scale, both
thin sections record structures related to regional deformation in an arc setting:
40
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development of a dominant cleavage that has experienced multiple transposition
cycles. However, detailed analysis shows that local heterogeneities control deflection
of the matrix cleavage as well as the inclusion trail patterns. Though inclusion trails
are locally curved, no evidence exists for truncations of Si that would indicate the
presence of multiple foliation intersection/inflexion axes (FIA’s; e.g. Bell et al.,
1995, 1998; Bell and Hickey 1997, 1999; Hickey and Bell 1999).
The sample from the Tonga Formation (TF-14) is from west of the Beckler
River road, at about the midpoint between the northern and southern extremities of
the Tonga Formation. This locality displays strong compositional layering with some
areas containing a highly graphitic matrix and abundant large (up to 5 cm) staurolite
prisms. The equilibrium assemblage, generated by regional loading and consequent
metamorphism, contains staurolite, garnet, and biotite. Aligned biotite grains define
the dominant cleavage, and are generally too small to be traced by micromapping at
the scale of the entire thin section. In some instances, staurolite is boudinaged,
evidence for which is visible at both outcrop and thin-section scales, and boudin
necks are occupied by quartz and muscovite. Micromapping shows that, although
staurolites and garnet porphyroblasts have relatively poor inclusion trails, where it is
visible, Sj is roughly parallel to and continuous with the matrix cleavage, Se. Any
curvature of Sj is related to growth of the porphyroblasts over a heterogeneously
strained and folded matrix cleavage. Deformation has also occurred after growth of
the staurolite porphyroblasts as they are strongly wrapped by the
41
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Sample: TF-14XY
O '
Figure 3-4a. Fligh-resolution photomicrograph (above) and micro-map (below) of sample TF-14XY
containing porphyroblasts of staurolite (elongate) and garnet (equant). Base of photo is 7.5 mm.
42
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Sample: TF-14YZ
— O'
o o
’lan e o f se ctio n : 0 8 4 , 77
Figure 3-4b. High-resolution photomicrograph (above) and micro-map (below) of sample TF-14YZ
containing porphyroblasts of staurolite (elongate) and garnet (equant). Base of photo is 7.5 mm.
43
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■ * ’
Sample: TF- 14XZ
Figure 3-4c. High-resolution photomicrograph (above) and micro-map (below) of sample TF-14XZ
containing porphyroblasts of staurolite (elongate) and garnet (equant). Base of photo is 7.5 mm.
44
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Sample: CS-1I7XY
Figure 3-5a. High-resolution photomicrograph (above) and micro-map (below) of
sample CS-117XY containing garnet (large) and sillimanite (small) porphyroblasts.
Base of photo is 7.5 mm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3-5b. High-resolution photomicrograph (above) and micro-map (below) of sample CS-
117XZ containing garnet (large) and sillimanite (small) porphyroblasts. Base of photo is 7.5 mm.
46
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Sample: CS-117YZ
o
o
= p
Figure 3-5c. High-resolution photomicrograph (above) and micro-map (below) of sample CS-
117YZ containing garnet (large) and sillimanite (small) porphyroblasts. Base of photo is 7.5 mm.
47
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matrix cleavage. Staurolites, however, are not aligned on thin-section or outcrop
scale, indicating that their growth was not fabric controlled. Apparent porphyroblast
rotation with respect to the preserved matrix cleavage does not appear to have
occurred.
The Chiwaukum Schist sample (CS-117) was taken from near a localized
shear zone in the recently named Nason Ridge migmatitic gneiss (Miller et al.,
2003), north of the Chiwaukum Schist and northeast of the regional sillimanite
isograd. This area was metamorphosed regionally at moderate- to high-temperature
and pressure. The equilibrium assemblage contains garnet, sillimanite (which is
largely altered to sericite), fine-grained staurolite, and biotite. The cleavage is
defined by recrystallized and aligned biotite aggregates. Sillimanite porphyroblast
long-axes tend to be well-aligned with respect to the matrix cleavage. Larger garnet
porphyroblasts preserve inclusion trails (Si), and although they are commonly curved
and oblique to the matrix cleavage (Se), they are generally continuous with it. The
garnet appears to record two episodes of growth, one expressed by relatively
inclusion-free cores, and a second by the inclusion-rich rims. An episode of
deformation must have occurred between these two growth phases, as the cores are
wrapped by “pinched” inclusion trails in many porphyroblasts. The garnet
porphyroblasts show little evidence for a consistent sense of offset or rotation or for
a simple spatial relationship between them. This suggests some amount of
porphyroblast rotation, with respect to the matrix, in a heterogeneous strain field.
48
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The matrix itself is heterogeneous as indicated by small-scale folds as well as
compositional and therefore Theological variation. Se was deformed further after
garnet growth, as some of the porphyroblasts are weakly wrapped by the matrix
cleavage.
Conclusions
Comparison of Tonga Formation and Chiwaukum Schist microstructures
agrees with previous work suggesting that the units are correlative, although the
latter appears to have undergone more episodes of deformation and higher grade
metamorphism, confirming previous work on transposition cycles (e.g. Miller et al.,
2003). Both units are highly recrystallized and neocrystallized, and experienced
pressure solution. Whether true porphyroblast rotation, with respect to geographic
reference frame, transpired cannot be discerned, although it is clear that matrix
transposition occurred in several localities and that it may or may not have been
accompanied by rotation. Porphyroblasts, however, are not limited to Q domains and
formed without control by Q or M domain structures, posing problems for a Bell
model of nucleation and growth. Many porphyroblasts appear to be late relative to
dominant folding. Examination of Si/S e relationships suggests that porphyroblasts in
the Cascades record heterogeneous deformation within the matrix rather than
multiple episodes of regional, orthogonal contraction. Subsequent quantitative
analysis of S;, Se, and porphyroblast grain shape orientation will yield additional
49
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results and help further constrain their use as a tool to help further unravel tectono-
metamorphic terrains.
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CHAPTER IV: COMPARISON OF FOLDING UNDER DIFFERENT CRUST AL
CONDITIONS
Introduction
A temperature-based approach to folding is outlined and initial numerical
forward modeling results are presented to show how temperature serves as a primary
control of fold formation and addresses several issues fundamental to the folding
process. Geometric classification, formation mechanism interpretation, and strain
analysis of outcrop-scale and micro-folds are used to examine whether and, if so,
how temperature and therefore varying rheology affect resulting fold structures in
both the Tonga Formation and the Chiwaukum Schist. The youngest folds (T2 + ) in
both units are compared to examine differences in folding at different crustal
conditions, as well as mechanisms responsible for controlling and generating these
variations, and their implications for crustal rheology.
Temperature as a Primary Control of Fold Formation
Laboratory studies of rock deformation show that the viscosity of most
materials is strongly temperature dependent, and many flow laws incorporate a
temperature term directly or through a temperature-dependent variable. Temperature
therefore affects the overall rheology of a system. Changes in temperature may even
reverse competence contrasts if materials have different sensitivities to temperature
51
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(Ramsay and Huber, 1987, p. 400-401; figure 4-1). Parrish et al. (1976) incorporated
temperature into finite-element models of similar folds, although they apply
unrealistic jumps in temperature to produce similar geometries that are
uncorroborated by field or laboratory data. Temperature also controls the dominant
deformation mechanisms on the microstructural and folding scale (figure 4-2),
metamorphic reactions, which modify features such as grain size, and general
material behavior of the deforming media (Jensen and Paterson, 2003).
Figure 4-1. Temperature-induced competence contrast modifications southwest of the Adamello
pluton, Italian Alps (from Ramsay and Huber, 1987): (a) Folded layers (light gray = limestone;
dark gray = marl) outside the thermal effects of the aureole (no scale provided), (b) Folds within
the thermal aureole (light gray = marble; dark gray = idocrase, pyroxene) exhibiting competence
contrast reversal and a switch in the sense of cuspate-lobate structures.
Through forward numerical modeling, I have begun to explore the role of
temperature in folding and present one example to illustrate its effects. A
temperature-dependent viscosity constitutive equation was incorporated into a
numerical, finite-difference model (FLAC/2D, Itasca) in order to analyze its effect
on fold development. A Maxwell elasto-viscous layer oriented parallel to the
52
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maximum shortening direction and containing small initial perturbations based on a
sine function was embedded in a similar matrix and deformed by pure shear at a
Undeform ed layer with strain, spin, and vorticity markers
A xial-plane parallel slip/flow
■ '' I
I * '■
i!:
I
H om ogeneous layer-parallel shortening
Figure 4-2. Four theoretical, end-member fold deformation mechanisms
showing the possible strain patterns and senses of spin and vorticity (after
Stallard and Hickey, 2001).
strain rate of 1042 s4 to 40% bulk shortening. Both elastic and viscous material
properties were chosen to match those of a psammite layer within a pelitic matrix.
The ratios of elastic and viscous parameters (i.e. the R value in the sense of Zhang et
al., 1996) are not identical between layers, as may be appropriate for naturally
deformed folds. Three test-cases were computed at 300°C increments, and
temperature was held constant throughout the deforming process. By effectively
53
a
ium cntial lonuiludinal strain
F I
f lexural si ip/How
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changing rheology and competence contrasts, temperature was found to determine
whether fold behavior was elastic- or viscous-dominated (see Schmalholz and
Podladchikov, 1999), at what bulk shortening fold amplification began, what
dominant wavelengths developed, and what strain patterns and folding mechanisms
resulted (Jensen et al., 2003). Temperature-induced changes in rheology, rather than
relatively small changes in layer-matrix competence contrast, are responsible for the
major changes in fold geometry.
At low temperatures (400-700°C; figure 4-3a,b), an interval of initial
perturbation amplification due to homogeneous shortening occurred, but a second
dominant wavelength, corresponding to Biot’s predictions developed. At high
temperatures (1000-1300°C; figure 4-3c), layer thickening is greater than at lower
temperatures, and relatively passive amplification of initial perturbations occurred
due to bulk shortening. Accordingly, deformation at lower temperatures was
dominated by more rapid amplification of instabilities and subsequent buckling.
However, both wavelengths were preserved until the final straining stages in some
model runs. This indicates that parasitic folds may be related to fold formation via
different scale deformation mechanisms operating in the same system (Jensen et al.,
2003). Though the two wavelengths do not develop simultaneously from initial
random noise in this example, the role of initial perturbations and competing
wavelength growth and decay rates may still be usefully evaluated using such
numerical modeling.
54
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: ?; :40.Q °C ' •
40% shortening
400°C
No shortening
400 C
20‘ i shortening
(b)
- • A ^ . 1
"
v % r / V - /
700°C ■ 700=C
No shortening shortening 40% shortening
£' x 1 r‘ 1 l'0 0 d °C ^
40% shortening
1000-C
20f i shortening
1000°C
No shortening
Figure 4-3. Modeled temperature effects on elasto-viscous folding. The dark-colored single
layer is more competent than the lighter-colored embedding material, which is also elasto-
viscous. Only a small part of the actual computational domain is shown. Competence contrasts
vary depending on temperature and range from approximately 31:1 (400°C) to 18:1 (1000°C).
(a) Low temperature folds start out with amplification of initial asperities, but rapid selection of
another wavelength for fold amplification, accompanied by the decay of initial perturbation
wavelengths, occurs, (b) Intermediate temperature folds showing preliminary amplification of
initial perturbations, but eventual Biot wavelength amplification, (c) High temperature folds are
dominated by layer thickening and amplification of initial perturbations by bulk shortening.
55
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Comparison of Naturally Deformed Folds
Folds are prevalent throughout both the Tonga Formation and Chiwaukum
Schist from a macroscopic to microscopic scale. Comparing folds at different
metamorphic grades is one way to correlate or distinguish between the two units.
The Cascades crustal section presents a region where it is possible to address
whether and how temperature controls resulting fold geometries and characteristics.
Grid-mapping in the field and analysis of micro-scale folds were carried out to study
these effects. I discuss fold geometry, deformation mechanisms, and strain values to
compare and contrast folds at both grades.
Folds related to S2+ were examined in both units, as these are the most readily
comparable folds and occur on approximately the same scale and in the same
orientation. These folds are dominantly class 1C, with weakly convergent dip
isogons (Ramsay & Huber, 1987), in both the Tonga Formation and Chiwaukum
Schist (figures 4-4, 4-5). The Tonga Formation folds are gentle to close, whereas
Chiwaukum Schist folds are commonly open to tight. Strong metamorphic layering
exists in both units.
Asymmetric shears and Z or S parasitic folds occur in the limbs of folds in
both units, while symmetric M or W folds appear in the hinges. This suggests that
the folds are mechanically active and formed by a flexural slip or flow mechanism.
In the Chiwaukum, folds are more complex and show multiple wavelengths and
layer thickness variations. The fact that folds are asymmetric and do not display
56
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Figure 4-4. Schematic examples of field grid maps in the (a) Tonga Formation (TF-65) and
(b) Chiwaukum Schist (CS-56) folds.
Y w f f
. / ? '• I 1 ~ '-'V X.
§ l ./ " J\ *'V
Jf tfc ! l) / / -
f t
.''.() mm S I p ;,r '
MTUTFIr */»/??** '< ^ ^ • #
* ■ -./'H "' ' '-V^V
/ / ‘f > f
Figure 4-5. Plane-polarized light photos of folds in the (a) Tonga Formation (TF-65) and
(b) Chiwaukum Schist (CS-25). See text for geometric comparison and analyses.
simple patterns of shear or parasitic folding within their limbs makes a formation
mechanism difficult to determine. They are probably the result of both flexural slip
or flow and bulk shortening mechanisms (Jiang, 2001; Miller et al., 2003). Thinning
57
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and pressure solution along limbs are common in both units. This may indicate a
strong component of bulk shortening operating either during or after folding, but also
complicates the quantification of fold shape and total strain accumulation.
i
M is i '.i h l'i. i.i i 1 1 .
FI / I
Figure 4-6. Strain contour map (base after Schmalholz and
Podladchikov, 2001) showing the relationship between layer
thickness, wavelength, amplitude, total percent strain due to folding,
and layer competence contrast. Results from Tonga Formation and
Chiwaukum Schist are shown. See text for discussion.
Total strain due to folding is calculated using the method outlined by
Schmalholz and Podladchikov (2001). Strain in a quartz-rich layer of average
thickness (H) is calculated using fold wavelength (k) and amplitude (A) in a variety
of outcrop grid maps and thin sections of the same scale (n = 19). Results are
presented in graphical form with each unit plotted within a range on a strain contour
map (figure 4-6).
58
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Tonga Formation folds have an estimated average strain of 48-54%; whereas
folds in the Chiwaukum Schist have higher average strains of 63-66% and larger
amplitude to wavelength ratios. The results indicate that a large competence contrast
exists between layers in both units, emphasizing the role that competence contrast
plays in fold development, which disagrees with recent suggestions that competence
contrasts in rocks are of one order of magnitude or less (e.g. Treagus and Treagus,
2002). A lower viscosity contrast (albeit much higher than predicted in the initial
numerical models) is observed in the higher-temperature Chiwaukum Schist than in
the lower-temperature Tonga Formation, fitting with general model predictions. Fold
shapes are quite similar in the two units, however. It is important to recognize that
this model does not take into account initial perturbations or anisotropies in or
contrasts in these properties between layers. If these features existed before folding
initiated, which is likely in the case of these metasediments, they would drastically
affect resulting fold magnitudes and geometries (e.g. Williams and Jiang, 2001) and
make this technique of folding strain analysis and its exclusive dependence on
competence contrasts invalid. This is also the first time the Schmalholz and
Podladchikov (2001) method has been applied rigorously in a field study, and overall
the process worked well and produced reasonable results. However, the application
to a relatively small scale, and in particular to multilayers, may introduce additional
errors into the strain estimates.
59
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Conclusions
Temperature is an important aspect that, in a broad sense, dictates fold
development through overall rheology, deformation mechanisms, and metamorphic
processes. In the Tonga Formation and Chiwaukum Schist, noticeable differences
occur between folds at lower grades and higher grades, but most folds are of class 1C
in both cases. Flexural folding is the dominant mechanism for fold formation, but
folding also involved an element of bulk shortening. Strain analysis, if its limitations
are accepted in this instance, reveals that high competence contrast played a
dominant role in fold formation and resulting geometry in both units. However, the
role of initial perturbations and mechanical anisotropies in and between layers may
reduce folding dependence on competence contrast alone. More work is needed to
decipher temperature effects with respect to folding in naturally deformed rocks,
additional careful numerical modeling being an important next step in unraveling the
specific role of temperature.
60
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Creator Jensen, Luke Anthony (author)

Core Title Reinterpreting the tectono-metamorphic evolution of the Tonga Formation, North Cascades: A new perspective from multiple episodes of folding and metamorphism

Degree Master of Science

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-317550

Unique identifier UC11327428

Identifier 1422392.pdf (filename),usctheses-c16-317550 (legacy record id)

Legacy Identifier 1422392.pdf

Dmrecord 317550

Document Type Thesis

Rights Jensen, Luke Anthony

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

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