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A tectonic model for the formation of the gridded plains on Guinevere Planitia, Venus: Implications for the thickness of the elastic lithosphere

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A tectonic model for the formation of the gridded plains on Guinevere Planitia, Venus: Implications for the thickness of the elastic lithosphere

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. Hie quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand corner and continuing from left to right in equal sections with small overlaps. 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A TECTONIC MODEL FOR THE FORMATION OF THE GRIDDED PLAINS ON GUINEVERE PLANITIA, VENUS: IMPLICATIONS FOR THE THICKNESS OF THE ELASTIC LITHOSPHERE by David Donald Bowman A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (Geological Sciences) August 1996 Copyright 1996 David Donald Bowman Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1381579 UMI Microform 1381579 Copyright 1996, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY O F SOUTHERN CALIFORNIA THE GRADUATE SCHOOL. UNIVERSITY PARK LOS ANGELES. CALIFORNIA 9 0 0 0 7 This thesis, written by David Donald Bowman____________________ under the direction of h i-M . Thesis Committee, and approved by all its members, has been pre' seated to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of Master o f S cie n c e Dtma August 8, 1996 THESIS COMMITTEE idG***a.. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgments This thesis is a great step forward in my academic growth at USC. I am deeply indebted to a great m any people, all of whom have contributed to my sanity (or lack thereof) during my tenure thus far at USC. I would specifically like to thank my thesis advisor, Dr. Charlie Sammis, who has been a stalwart mentor, supporter, and friend. I would also like to thank the other members of my committee, Dr. Steve Lund and Dr. Tom Henyey, for their help in the completion of this thesis. Dr. Bruce Banerdt at the Jet Propulsion Laboratories was also essential to the early development of this thesis. In addition, there are two professors who have had a profound influence on my intellectual development. Dr. Gibson Reeves in the Department of Physics and Astronomy was an invaluable friend and advisor during my undergraduate career, and his friendship has been inestimable during my graduate career, as well. Dr. Greg Davis was the person who inspired me to become a geologist, and his guidance has likewise been invaluable. My friends have given me constant inspiration, commiseration, and plenty of good times. A heartfelt thanks to Adam, Semele, Nick, Sam, Rahul, Julie, and all the rest of the gang. I would also like to thank Rene Kirby and the entire front office, without whom none of us would ever graduate. And finally, I would like to thank m y father, mother, sister and brother-in-law for their continual love and support. You made it all possible. I promise to call home this weekend! ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONTENTS Acknowledgements....................................................................................ii List Of Illustrations................................................................................... iii Abstract....................................................................................................... iv Introduction............................................................................................... 1 Fracture Patterns In Guinevere Planitia................................................ 6 Formation Of Regular Lineations........................................................... 12 Formation Of Irregular Fractures...............................................................14 Distribution Of Irregular Fracture Length Scales..................................19 Discussion.................................................................................................... 25 Conclusions...................................................................................................28 References.................................................................................................... 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Illustrations Figure Page la. The "gridded plains" of Guinevere Planitia........................... 8 lb. Magnification of the gridded plains......................................... 10 2. Localized volcanic flows in the gridded plains....................... 11 3. Shear fractures developed in a layer of clay............................. 15 4. Inferred tectonic history of Guinevere Planitia........................ 17 5. Fracture length distribution for the gridded p lain s............... 20 6. Hypothesized m odel of lithospheric fracture propagation. . 21 iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Abstract The "gridded plains" of Guinevere Planitia are comprised of a north-east trending set of extremely linear and regularly spaced lineations, and a north-west trending set of more irregularly spaced curvilinear radar- bright structures. The fainter NE set has been interpreted as cooling cracks in a thin but widespread basalt surface layer in frictional contact with its substrate (Banerdt and Sammis, 1992). The brighter NW trending set are interpreted as shear fractures in the lithosphere which have propagated up through the surface basalt flow after the more regular set of cooling cracks had formed. The length distribution of this set of lithospheric shear cracks has a sharp bend at a length of about 80 km, which we interpret as reflecting a transition from the 3-D growth of a penny-shaped crack in the lithosphere to the 2-D elongation of a crack which has penetrated the elastic lithosphere. This interpretation implies that the lithosphere is roughly 40 km thick, in agreement with other recent estimates based on flexure and analyses of gravity and topography. This interpretation also implies that the 500 Ma resurfacing event suggested by crater statistics was accompanied by the emplacement of extensive flood basalts in the plains regions, but that the global resurfacing event m ay still have been accomplished by global destruction of an older preexisting lithosphere. v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction In recent years, studies of the planet Venus have been greatly furthered by images returned by the Magellan spacecraft. Magellan, which m apped 99% of the planet at a minimum resolution of 220 m, and a significant portion at 75 m resolution, revealed a unique landscape. Some of the m ost remarkable features of the Magellan images are the intricate and varied deformation patterns on the volcanic plains (Solomon et al., 1991). To a first approximation, these patterns can be grouped into two categories: (1) large structures, developed at the 100- 1000 km scale, which include coronae, ovoids, and deformation belts, and (2) smaller-scale lineations and polygons which are ubiquitous in the less deformed areas between the larger structures and which have scale lengths on the order of kilometers. Properly interpreted, fracture patterns on both the large and small scales may be used to help unravel the tectonic history of the Venusian surface. The large scale deformation belts have been interpreted in term s of buckling or other instabilities of the lithosphere and have been used to constrain its thickness (Zuber, 1987; Banerdt and Golombek, 1988). The ovoid structures (coronae, arachnoids, and ticks) have been interpreted as resulting from m antle plumes and thus offer insight into the mode and structure of mantle convection (Stofan and Head, 1990; Squyers et al., 1991; Stofan et al., 1992; Bindschadler et al., 1993; Herrick and Phillips, 1992). While the large scale deformation patterns provide insight into the mantle dynamics of Venus, small scale deformation patterns can be used to 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. constrain m odels of the tectonic evolution of the lithosphere. For example, the interpretation of small scale lineations in the "gridded plains" region of Guinevere Planitia has provided im portant constraints on the tectonic evolution of the area (Banerdt and Sammis, 1992; Bowman and Sammis, 1994a,b). The purpose of this thesis is to construct a model for the development of fractures in the Guinevere Planitia region, and explore the implications of this model for geophysical models of the planet's evolution. While Venus has often been referred to as Earth's twin sister because of the many physical similarities between the planets (see table 1), the geologic processes and surface features of the sisters are very different. The most striking contrast between Venus and Earth is the apparent lack of plate tectonics on Venus. Studies of impact craters have revealed that the distribution of craters over the entire surface of the planet appears to be completely random and indistinguishable from a uniform distribution (Schaber et al., 1992; Strom et al., 1994), suggesting that the entire surface of the planet is the same age. Furthermore, only 935 craters have been recognized on Venus (Strom et al., 1994), in contrast to the thousands of recognized craters on other terrestrial bodies such as the Moon, Mercury, and Mars. The paucity of craters on Venus suggests that the planet was globally resurfaced approximately 500 Ma (Schaber et al., 1992), and has been relatively quiescent ever since. There are two major types of models to explain the 500 Ma global resurfacing event. In "uniform itarian" models (e.g. Solomon, 1993; for a detailed discussion, see Phillips and Hansen, 1994), the thermal budget of Venus is in a nearly steady state balance between secular cooling of the planet 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1 Bulk properties of Venus and Earth | Venus Earth Mass (kg) 14.87 x 1024 5.98 x 1024 M ean Planetary Radius (km) 6052 6371 Mean density (gm /cm 3) 5.25 5.52 Escape Velocity (km/sec) 110.4 11.2 Orbital Data Semi-major axis (AU) 0.723 1.0 Obliquity 178° 23.4° Inclination 3.39° 0° Eccentricity 0.007 0.017 Rotation period -243 Earth days (retrograde) 23.93 hours Revolution period (Earth days) 224.7 365.26 Atmospheric Data Mean surface temperature (K) 726 281 Surface pressure (atm) 90 1 96% C 02 78% N2 Bulk composition 3% N2 21% 0 2 0.1% H20 (vapor) 1% Ar Surface Data Highest point on surface Maxwell Montes M ount Everest (17 km above MPR) (8 km above MPR) Bulk crustal composition mafic rock mafic rock and silicic rock 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and radioactive heat production. These models essentially claim that tectonism on Venus is "running down". The apparent 500 Ma surface age of the planet represents a rapid cessation of tectonism as the secular cooling fell below some critical value. Heat loss in uniformitarian models occurs by thermal conduction through the lithosphere, lower-crustal and lithospheric delamination, and isolated mantle plumes. In contrast, "catastrophic" (or "episodic") models (e.g. Turcotte, 1993; Arkani-Hamed, 1994) call for repeated periods of intense tectonism followed by long times of quiescence. Turcotte (1993) has proposed the most well- known (and controversial) catastrophic resurfacing mechanism. He calls for a process similar to the vigorous thermal convection observed in terrestrial lava lakes (Turcotte, 1995). In this scheme, the thermal lithosphere thickens through time until it becomes gravitationally unstable. When this happens the lithosphere founders and is consumed in a episode of rapid global subduction. The vigorous convection which accompanies foundering of the old lithosphere cools the mantle sufficiently to allow the formation of a new therm al lithosphere. The new lithosphere will thicken through time, repeating the process. The above discussion demonstrates the importance of understanding the lithospheric structure of Venus, from a theoretical standpoint. Accurate determinations of the elastic thickness of the Venusian lithosphere provide important constraints on geophysical models of the planet's evolution. Early studies of Venus assumed that the planet had a relatively thin lithosphere [e.g. Phillips and Malin, 1983; Solomon and Head, 1984; and Banerdt and Golombeck, 1988]. This assumption was justified by the planet's high surface 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. tem perature, which was believed to elevate the geotherm, raising the elastic- plastic interface to shallower levels. More recently, a relatively thick elastic lithosphere has been called upon to explain such diverse problem s as the support of large volcanic edifices [e.g. Smrekar and Solomon, 1992] and the evolution of coronae [McGill, 1994]. Solomon et al. [1994] reported that the large positive free air gravity anomalies beneath large volcanoes on Venus imply an elastic thickness of 30-70 km. This result falls in the upper range of thicknesses found through studies of flexure around coronae. Sandwell and Schubert [1992] interpreted topographic profiles outboard of four large coronae in terms of the flexure of an elastic plate. They found that models for the topography were best matched by an elastic plate with thicknesses ranging from 30-60 km. In this paper, we develop a new m ethod for determ ining the elastic thickness of the lithosphere based on the length distribution of small-scale fractures on the volcanic plains. This technique gives an elastic thickness on the order of 40 km. The interpretation further implies that resurfacing of the volcanic plains involved a relatively thin (<1 km thick) surface layer widely deposited over a lithosphere which had undergone an earlier episode of fracturing. 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fracture patterns in Guinevere Planitia One of the first images returned by Magellan was of the "gridded plains" of Guinevere Planitia (Figure la), named for the nearly orthogonal set of lineations visible in the radar images (Solomon et al. ,1991). Although such small-scale lineations are common features of the volcanic plains, the orthogonal character at this location is unique (Bowman et al., 1994a). The fainter, regularly spaced north-east trending set have been interpreted as tensile cracks [Solomon et al., 1991; Banerdt and Sammis, 1992]. They are very narrow and show no internal structure (Figure lb), to the limit of resolution of Magellan images (75 m/pixel). There does not appear to be any lateral offset along the fractures, suggesting that they formed purely in tension. Furthermore, the reflectivity of the fractures appears to be constant regardless of illum ination direction, thus ruling out asymmetric fault scarps. It seems more likely that they are mode I (tension) fractures, having no lateral or vertical displacement beyond simple opening of the crack. At right angles to the parallel NE fractures is a set of more irregular radar-bright lineations. Because they are orthogonal to the set of tension fractures, these were originally interpreted by Sammis and Banerdt [1991] as compressional features. However, as Solomon et al. [1991] pointed out, many of the lineations become graben to the north. Thus, the irregular fracture set is also interpreted to have formed in a tensional strain regime. But as Banerdt and Sammis [1992] noted, a close examination of Figure lb reveals that the irregular fractures have a distinct curvilinear en echelon morphology. This indicates a component of shear in the stress field. Note that the morphology of the irregular set is completely 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure la. The "gridded plains" of Guinevere Planitia. Note that the image is dom inated by short fractures; very few fractures span the entire width of the region. The width of the image is ~490km. The inset shows the location of figure lb. (Portion of Magellan image C1-30N333) 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure la (continued) 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Figure lb. Magnification of the gridded plains. The north-east trending shear lag fractures are indicated by a black arrow. The white arrow shows the location of a small graben formed by local rotation of the stress field in a thin surficial layer. Note the distinctive en echelon nature of the northwest trending fractures. See text for details. different from that of the regularly spaced NE trending fracture set. Notably, the curvilinear shape, broader width, and inconsistent spacing of the NW trending set argues against a solely tensile stress field during their formation [Banerdt and Sammis, 1992]. The relative tim ing of the formation of the lineations can be determined from crosscutting relations between the fractures and several small volcanic constructs in the region. Figure 2 shows a part of the gridded plains with several small volcanoes. The irregular fractures cut the volcanoes, implying that they are younger than the age of the volcanism, whereas the regular lineations are generally obscured. However, a close inspection of the area circled in Figure 2 shows that, although most of the regular fractures are covered by the volcanism, a few of them cut through the flows. Thus, we interpret the formation of the regular lineations to be pre- to syn-volcanic. The irregular lineations generally cut through the flows, although there is a tendency for the smaller fractures to end at the apron of the flows. Thus, the irregular fractures are interpreted to be latest- syn to post-volcanic. 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2. Localized volcanic flows in the gridded plains. The arrows indicate small shields and their associated flows which have been dissected by the irregular north-west trending fractures. The regular lineations generally are covered by the lava flows. Although the regular lineations are generally obscured by the flows, they locally cut the flows, as in the circled region on the right side of the image. 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Formation of the regular lineations The formation of the NE trending fracture set was discussed in detail by Banerdt and Sammis [1992] and Bowman et al. [1994a], w ho interpreted the fractures to have formed according to a shear-lag model. While the formation of these fractures is not the focus of this work, it is instructive to review the implications of the shear-lag model. This mechanism is commonly used in materials science to describe the fracturing of thin brittle layers deposited on metal substrates (e.g. H u and Evans, 1989; and Zok and Spearing, 1992). According to conventional materials science applications, the substrate is assumed to be in tension. As the substrate extends, it exerts a traction on the base of the brittle surface layer causing it to fracture in tension, forming very regularly-spaced parallel fractures. Banerdt and Sammis [1992] pointed out that a uniform contraction of the surface layer is mechanically the same as a uniform extension of the substrate. Thus, the fractures predicted by the shear-lag model for an extending substrate can also be produced by a cooling, contracting surface layer. In relation to Venusian tectonics, the shear-lag model has im portant implications. The nature of the contact between the surface layer and the underlying lithosphere has an im portant influence on the developm ent of the fractures. As Banerdt and Sammis (1992) pointed out, the spacing of the fractures, Xc, should be related to the thickness of the surface layer, h, according to the relation where Ot is the tensile strength of the surface layer, and x is the basal shear stress. For the materials science example, x is the yield stress of the metal 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. substrate. However, for the geological case, the surface layer is allowed to detach from the substrate, in which case x is the frictional strength of the interface, given by X = ll<Jn (2) where (I is the coefficient of friction and < J n is the normal force across the interface due to the weight of the overlying rock. If we assume that the surface layer is homogeneous, then the shear stress can be rewritten as x = \ian = (ipgh (3) where p is the density of the layer and g is the acceleration of gravity on Venus. Substituting back into Eq. (1), we find that X .= — - (4) m Thus, the fracture spacing predicted by the shear-lag model should be independent of the thickness of the surface layer, a result supported by the observation that the spacing is constant over large distances (Banerdt and Sammis, 1992; Bowman et al., 1994a). 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Formation of the irregular fractures As Banerdt and Sammis [1992] pointed out, the shear-lag model can not be used to describe the irregular fractures. The shear-lag model requires a tensional stress field in the surface layer, w ith the resulting fractures forming as mode I cracks perpendicular to the tensional stress axis. However, the irregular shape of the fractures indicates that they are not simple mode I fractures, as required by the shear-lag model. As noted above, the morphology of the irregular fractures implies that the stress field during their formation included a component of shear. The general m orphology of the irregular fractures was reproduced in the laboratory by An (1995) by deforming a clay cake in simple shear. As show n in Figure 3, the resultant network of shear fractures is morphologically similar to the irregular fractures in the G ridded Plains, with one im portant difference: the laboratory fractures lacked the distinctive en echelon structure of the irregular fractures in Guinevere Planitia. In terrestrial settings, en echelon fracture segments such as the ones in the gridded plains are commonly observed in soil layers above a basement fault or fracture (e.g. Tchelenko, 1970; Engelder, 1987). As movement occurs on the basement structure, the fracture propagates upward through the soil layer. During upw ard propagation, the stress field of the fracture rotates to accommodate shear at the base of the soil. This rotation creates the observed en echelon m orphology (Engelder, 1987). Furthermore, the rotation of the stress field induces a tensile moment on the fracture, causing the formation of small graben w ithin each en echelon segment. These graben are clearly visible in Magellan images, and are 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3. Fractures developed in a layer of day deformed in distributed simple shear. Sense of shear is right-lateral. Note the similarities to the fractures in figure la. Photo courtesy of Linji An. 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. indicated w ith an arrow in Figure lb. Thus, to account for the formation of the en echelon irregular fracture set, a thin surface layer is again required - the same one required by the shear-lag model. However, in this case the observed en echelon fractures are controlled by a series of subparallel shear fractures in the lithosphere which predate the deposition of the surface laye The proposed fracture mechanisms and geologically constrained sequence of fracturing discussed above can be used to construct a tectonic history of the Gridded Plains as idealized in Figure 4. The first event in this proposed sequence was the development of fractures by simple shear deformation of the elastic lithosphere. The plains were then covered by flood basalt. As the flood basalt cooled, it contracted to form the regularly spaced shear-lag fractures. Immediately following the formation of the shear-lag fractures, localized volcanism occurred, obscuring segments of the regularly spaced parallel lineation sets. Finally, the older lithospheric fractures propagated upward through the surface flood basalt, dissecting the late-stage volcanism. 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4. Inferred tectonic history of Guinevere Planitia. First, through- going fractures form in the lithosphere (a). The plains are then overlain by a thin volcanic surface layer (b). As this layer cools (c), it decouples from the original lithosphere to form the shear-lag fractures. Fracturing occurs concurrent with localized volcanism. As localized volcanism continues (d), the original through-going shear fractures propagate to the surface, form ing the en echelon fracture set. 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Distribution of Irregular Fracture Length Scales An inspection of Figure la reveals that most of the irregular fractures in Guinevere Planitia are relatively short (<80 kilometers). However, there are also a num ber of very long (up to 1000 kilometers) fractures in the region. Figure 5 is the cumulative length distribution of irregular fractures in the gridded plains. In this plot, two populations of fractures are clearly visible as distinct line segments with different slopes. In this work, population I fractures are defined as the set of all fractures with lengths less than 80 kilometers, while population II fractures are those with lengths greater than 80 kilometers. Within the Gridded Plains, there are approximately 5600 population I fractures, which constitute approximately 99% of the fractures in the Gridded Plains. Although the 17 longer population H fractures visually dominate images of the region, they comprise less than 1% of the total num ber of fractures (see Table 2). The two populations of lineations are interpreted as representing fractures w ith two distinct propagation modes. W hen the fractures are short com pared to the thickness of the elastic lithosphere, they can be modeled as simple shear fractures in a semi-infinite elastic half-space. The fractures themselves can be approximated as semi-circular cracks centered on the surface of an elastic plate, as shown in Figure 6. To this approximation, the depth to which any given crack has propagated is equal to the radius of the semi-circle, whereas the surface length of the fracture is equal to its diameter. The length distribution such a system of cracks will produce can be qualitatively understood by simple damage mechanics. 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. £1000 100 1 10 100 1000 Fracture Length (km) Figure 5. Fracture length distribution for the gridded plains region of Guinevere Planitia. The points labelled I in the plot are fractures that have not yet broken through the lithosphere. The points labelled II are through-going fractures that have begun to "run-away" relative to the population I fractures. The sharp decrease in the number of population II fractures which occurs at a length of 300 km is a result of the limited spatial extent of the gridded plains. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. emeniT^” ' ff the fracture Figure 6. Hypothesized model of lithospheric fracture propagation. Ductile creep in the substrate increases the stress intensity factor at the shallower levels of the advancing crack front, causing the fracture to rapidly increase in length. 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2 Fracture Lengths in Guinevere Planitia Fracture Length (km) N um ber of Fractures Fracture Length (km) N um ber of Fractures 2.48 3477 44.64 1 3.72 564 45.88 1 4.96 388 47.12 3 6.2 262 52.08 2 7.44 182 53.32 1 8.68 133 54.56 3 9.92 98 55.8 4 11.16 68 58.28 1 12.4 62 60.76 2 13.64 55 62 1 14.88 43 64.48 1 16.12 32 66.34 1 17.36 23 69.44 2 18.6 27 71.92 1 19.84 19 74.4 1 21.08 16 75.64 1 22.32 14 78.12 1 23.56 6 88.04 1 24.8 7 95.48 1 26.04 13 104.16 1 27.28 7 114.08 1 28.52 10 132.68 1 29.76 5 158.72 1 31 5 163.68 1 32.24 7 226.92 1 33.48 7 259.16 1 34.72 4 282.72 1 35.96 2 288.92 1 37.2 5 347.2 1 38.44 2 350.92 1 39.68 2 381.92 1 40.92 1 409.2 1 42.16 4 1037.9 1 43.4 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. As an elastic plate undergoes progressive fracturing, the volume of intact (unfractured) material decreases. The decreased volume of intact material m ust still support the same external stress. The rem aining volume of rock will experience a higher local stress, which will cause the layer to nucleate fractures at a higher rate. The num ber of fractures nucleating in the plate will increase w ith time. At any given instant, the continually increasing nucleation rate will be observable as a uniform decrease in the number of fractures w ith increasing fracture length, thereby producing the negative slope observed for the population I fractures in Figure 5. However, the lithosphere of Venus is not a semi-infinite elastic half space. As the cracks propagate downward, they eventually penetrate deeply enough to reach the base of the elastic lithosphere. At this point, ductile shear displacement can occur at the base of the crack on a time scale comparable to that of crack growth. The effect is to increase the stress concentration at the vertical leading edges of the advancing crack, thus increasing its surface propagation velocity. Thus, a fracture which has penetrated the elastic lithosphere will have a markedly longer surface trace than might be expected if it had continued propagating in an elastic half space. In the cumulative length distribution plot, these fractures (Figure 5, population II) plot with a shallower slope than is observed for fractures which have not penetrated the lithosphere (i.e. the short, population I fractures). The specific fracture length at which the length distribution changes from population I to population II is a function of the elastic thickness of the 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lithosphere in the region. At this critical fracture length, the crack has penetrated to the base of the elastic lithosphere, but has not yet begun to "run away". Thus the penetration depth of these cracks can be used as an estimate of the elastic thickness of the lithosphere. Since we have assumed that the fractures propagate as semi-circles centered on the surface of the planet, our model predicts that the depth to which the crack has propagated is equal to half the surface fracture length. In the gridded plains of Guinevere Planitia, the change between population I and population II fractures occurs at a fracture length of 80 km, which corresponds to an elastic thickness of approximately 40 km. 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Discussion Any regional tectonic history for Venus must fit into the broader tectonic framework imposed by global resurfacing. However, models for resurfacing are themselves constrained by estimates of the thickness of the lithosphere. Unfortunately, the concept of the lithosphere has become somewhat clouded in recent years. As Anderson (1995) pointed out, there are many ways to define the lithosphere. To properly evaluate any geophysical or geological model, it is fundamentally important to understand the definition of the lithosphere being invoked. This is particularly true of models for the global resurfacing of Venus. Catastrophic models of global resurfacing (e.g. Turcotte, 1993) typically invoke a very thick lithosphere. However, this usage of the term lithosphere is actually referring to the conductive Thermal Boundary Layer overlying the convecting mantle. According to this definition of the lithosphere, the measured thickness m ight be expected to be much thicker than the elastic thickness. Indeed, a recent analysis by Moore and Schubert (1995) found the thickness of the thermal lithosphere to be 270 km. In contrast, estimates of the thickness of the lithosphere from flexural methods range from 15 - 60 kilometers (Johnson and Sand well, 1994). These values reflect the thickness of the elastic lithosphere, and agree well with the 40 kilometer value determined in this study. Unfortunately, the relationship between the thermal lithosphere and the elastic lithosphere is very complex. Anderson (1995) states that the thickness of the Thermal Boundary Layer on earth is approximately twice the thickness of the elastic lithosphere. However, this relationship should also vary with stress, local 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. therm al gradient, mineralogy, strain rate, and even the time since loading and duration of loading (Minster and Anderson, 1981). Indeed, different techniques for measuring the thickness of the elastic lithosphere might be expected to produce dissimilar results, depending on which of the factors m entioned above is dom inant in determining the elastic response of the plate. In the case of plate flexure models, the elastic thickness is determined by the response of the lithosphere on time scales determined by the strain rate in the Venusian lithosphere. The elastic thickness implied by fracture length distributions is determined by the depth at which the rate of plastic creep is faster than the rate of elastic fracture growth. Unfortunately, we do not know the growth rate of the fractures seen on Venus. However, the close correlation in elastic thicknesses on Venus as determined by fracture lengths and flexure models suggests that the observed fracturing occurred on a time scale comparable to that required to induce flexure in the lithosphere. If the time scales are indeed comparable, then fracture length distributions may yield a useful tool for verifying the elastic thicknesses predicted by flexural models. Unfortunately, the application of this method requires rather special geological circumstances. The formation of such large scale coherent fracture netw orks requires a relatively simple tectonic environment. A successful application of the technique requires that a region with dimensions significantly greater than twice the thickness of the lithosphere be modified only by fracturing due to a uniform regional stress field. For instance, in the case of Venus, the hypothesized 40 km thick lithosphere implies that the 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fractures will not begin to "run away" until they have reached a surface length of 80 km. To observe the different populations of fractures in Figure 5, the fractures m ust be allowed to propagate to lengths of several hundred kilometers. If other active geologic processes, such as the form ation of corona or deformation belts, occur in the same region, they w ould tend to obscure or modify existing fractures, thus changing the observed length distribution. 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Conclusions The well-developed, spatially extensive fracturing of the gridded plains of Guinevere Planitia provides a unique opportunity to understand the structure of the lithosphere in this region of Venus. This network of fractures may be interpreted as being produced by shear fracturing of the lithosphere, accompanied by extensive volcanism which covered the entire region with a thin (< 1 km) layer of basalt. A simple damage mechanics interpretation of the fracturing in the plains has allowed us to model, the distribution of fracture lengths in the region. This distribution implies that the elastic lithosphere of Guinevere Planitia is about 40 kilometers thick, which agrees well with values of the effective elastic thickness found through flexure and gravity studies. 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. References An, L., Fault development under simple shear: experimental studies, Ph.D Thesis, University of Southern California, 1995 Anderson, D. L., Lithosphere, asthenosphere, and perisphere, Rev. Geophys. 3 3 ,125-149,1995. Arkani-Hamed, J., On the thermal evolution of Venus, J. Geophys. Res., 99, 2019-2033,1994. Banerdt, W.B. and C.G. Sammis, Small scale fracture patterns on the volcanic plains of Venus, J. Geophys. Res., 97, 16,149-16,166, 1992. Banerdt, W.B. and M.P. Golombek, Deformational models of rifting and folding on Venus, J. Geophys. Res., 93, 4759-4772, 1988. Bindshadler, D., G. Schubert, and W. Kaula, Coldspots and hotspots: global tectonics and mantle dynamics of Venus, J. Geophys. Res., 97, 13,495- 13,532,1992. Bowman, D.D., C.G. Sammis, and W.B. Banerdt, Spacing distributions and intersection angles for kilometer scale lineations on the plains of Venus, Lunar and Planetary Science X X V , 155-156, 1994a. Bowman, D. D., and C. G. Sammis, Determining the thickness of Venus' elastic lithosphere using fracture length distributions, EOS Trans. AGU, 75, 413,1994b. Bowman, D.D., and C.G. Sammis, Implications of small-scale fracturing on Guinevere Planitia, Venus, Lunar and Planetary Science X X V I, 155- 156,1995. Engelder, T., Joints and shear fractures in rock, in Fracture Mechanics of Rock, edited by B.K. Atkinson, Academic Press, 1987. Herrick, R. R. and R. J. Phillips, Geological correlations with the interior density structure of Venus, J. Geophys. Res. 97, 16,017-16,034, 1992. Hu, M. S., and A. G. Evans, The cracking and decohesion of thin films on ductile substrates, Acta Metall., 37, 917-925, 1989. Johnson, D. and C. Sandwell, Lithospheric flexure on Venus, Geophys. Jour. Int., 119, 627-647,1994. 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. McGill, G. E., Hotspot evolution and Venusian tectonic style, /. Geophys. Res., 9 9 ,23149-23162,1994. Minster, J. B. and D. L. Anderson, A model of dislocation- controlled rheology for the mantle, Philos. Trans. R. Soc. London, 299, 319-356, 1981. Moore, W. B. and G. Schubert, Lithospheric thickness and m antle/lithosphere density contrast beneath Beta Regio, Venus, Geophys. Res. Lett., 22, 429-432, 1995. Phillips, R. J., and M. C. Malin, Tectonics of Venus, Anna. Rev. Earth. Planet. Sci.., 12, 411-443, 1983. Phillips, R. J. and V. L. Hansen, Tectonic and magmatic evolution of Venus, Anna. Rev. Earth Planet. Sci., 22, 597-654, 1994. Sammis, C.G., and W.B. Banerdt, Self organized critical faulting on Venus,(abst.) Lunar and Planetary Science XXII, 1163-1164, 1991. Sandwell, D. T., and G. Schubert, Flexural ridges, trenches, and outer rises around coronae on Venus, f. Geophys. Res., 97, 16069-16083, 1992. Schaber, G. G., R. G. Strom, H. J. Moore, L. A. Soderblom, R. L. Kirk, D. J. Chadwick, D. D. Dawson, L. R. Gaddis, J. M. Boyce, and J. Russel, The geology and distribution of impact craters on Venus: W hat are they telling us?, f. Geophys. Res., 97. 13,257-13,302, 1992. Smrekar, S. E., and S. C. Solomon, Gravitational spreading of high terrain in Ishtar Terra, Venus, f. Geophys. Res., 97, 16121-16148, 1992. Solomon, S. C., and J. W. Head, Venus banded terrain: Tectonic models for band formation and their relationship to lithosphere thermal structure, J. Geophys. Res., 84, 6885-6987, 1984. Solomon, S. C., J. W. Head, W. M. Kaula, D. McKenzie, B. Parsons, R. J. Phillips, G. Schubert, and M. Talwani, Venus tectonics: Initial analysis from Magellan, Science, 252, 297-312, 1991. Solomon, S. C., The geophysics of Venus, Physics Today, 46,48-55, 1993. 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Solomon, S. C., P. J. McGovern, M. Simons, and J. W. Head, Gravity anomalies over Volcanoes on Venus: Implications for lithospheric thickness and volcano history, (abst.) 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Spearing, Matrix crack spacing in brittle matrix composites, Acta metall. mater., 40, 2033-2043, 1992. Zuber, M. T., Constraints on the lithospheric rheology of Venus from mechanical m odels and tectonic surface features, Proc. Lunar Planet. Sci. Conf. 17th, Part 2, f. Geophys. Res., suppl., 92, E541-E551, 1987. 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Creator Bowman, David Donald (author)

Core Title A tectonic model for the formation of the gridded plains on Guinevere Planitia, Venus: Implications for the thickness of the elastic lithosphere

School Graduate School

Degree Master of Science

Degree Program Geological Sciences

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

Tag Geology,geophysics,OAI-PMH Harvest,Physics, Astronomy and Astrophysics

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Sammis, Charles G. (committee chair), Henyey, Thomas (committee member), Lund, Steve (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-3834

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