Fault and fold growth of the Amenthes uplift
Transcription
Fault and fold growth of the Amenthes uplift
Earth and Planetary Science Letters 408 (2014) 100–109 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Fault and fold growth of the Amenthes uplift: Implications for Late Noachian crustal rheology and heat flow on Mars Karl Mueller a,∗ , Arwen Vidal a , Stuart Robbins b , Matthew Golombek c , Colin West d a Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, United States c Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, United States d Department of Applied Mathematics, University of Colorado, Boulder, CO 80309, United States b a r t i c l e i n f o Article history: Received 30 November 2013 Received in revised form 20 September 2014 Accepted 29 September 2014 Available online xxxx Editor: C. Sotin Keywords: Amenthes Rupes thrust fault heat flow brittle–ductile transition MOLA a b s t r a c t Determining the rheologic conditions that control growth of compressive structures on Mars is an elusive problem, one limited by the lack of seismologic and other data commonly available for comparable active uplifts on Earth. In some instances however, the geometry of faults on Mars that offset impact craters can be deduced from surface topography alone. With this aim, construction of a balanced and restorable structural cross section across the Late Noachian Amenthes uplift, or fault-related fold suggest it forms above a deeply penetrating blind thrust with a gently curved or listric geometry that flattens downward. Preferred structural solutions suggest the thrust dips between 41.5◦ and 56.1◦ at the surface and flattens into a horizontal decollement at depths of ∼33–48 km. The range of values for depth to detachment are greater than previous estimates for the Amenthes thrust based on elastic modeling for a range of planar thrust geometries. Using the inference that the decollement corresponds to the onset of plasticity in the crust, the depth to detachment is consistent with surface heat flow of 24–33 mW m−2 based on average values for heat production and the temperature threshold for the transition from brittle faulting to ductile shear on Mars. This suggests the crust at Amenthes during the Late Noachian may be slightly cooler than previously thought, similar to recent estimates derived from studies of lithospheric strength. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The formation and structure of Amenthes Rupes has long been recognized as having important implications for the rheology of the Martian crust early in its history (Watters, 2003). Central to this idea is the interpretation that the depth the Amenthes thrust penetrates corresponds to a change in brittle to ductile behavior in the Martian crust (Schultz and Watters, 2001; Ruiz et al., 2008). Evidence that large thrusts may flatten into the brittle to ductile transition on Earth includes direct imaging from seismic reflection profiles (Smithson et al., 1978; Bloxsom Lynn et al., 1983; Allmendinger and Shaw, 2000; Munoz, 2008), focal mechanisms of large earthquakes and aftershock sequences (Pujol et al., 2006; Verges et al., 2007) and the temperature threshold for plasticity in quartz rich rocks (Coward, 1984; Scholtz, 1988; Munoz, 2008). * Corresponding author. Tel.: +1 303 552 7067. E-mail address: [email protected] (K. Mueller). http://dx.doi.org/10.1016/j.epsl.2014.09.047 0012-821X/© 2014 Elsevier B.V. All rights reserved. The onset of plasticity on faults is chiefly controlled by the mineralogy of rocks they deform, pressure and temperature conditions, and the presence of water (for a review of terrestrial examples see Burgmann and Dresen, 2008). Compressive structures such as Amenthes can thus yield insight into the physical conditions that control their development. This offers an opportunity to constrain the ambient conditions in the Martian crust by analysis of large compressive uplifts and inferences for appropriate mineralogy and flow laws (Schultz and Watters, 2001; Grott et al., 2007; Ruiz et al., 2008). This depends however, on knowledge of fault geometry and requires direct information of the dip of the fault near the surface, something that can be determined for Amenthes where it offsets a large impact crater, allowing both the vertical and horizontal components of shortening to be measured (Vidal, 2008). Armed with this information, solutions for the depth the Amenthes thrust penetrates and flattens can be constrained using fault-related fold theory, where mass balanced structural solutions of nonrecoverable strain can be restored to a starting condition (Suppe, 1983; Allmendinger, 1998; Seeber and Sorlien, 2000). Results of this K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 101 Fig. 1. Geologic map of the Amenthes uplift illustrating surface deposits, major thrust faults, impact craters and their ejecta. The main Amenthes Rupes lobate scarp extends across the center of the image. Line of dots denotes MOLA shot points used to construct topographic profile for structural analysis. Mapping was conducted on existing THEMIS global thermal mosaics, MOLA shaded relief maps and MOC imagery and comparisons to existing geologic mapping by Hiller (1979). 3. Mapping offset at Amenthes 3.1. Methods Several methods were used to characterize the structural geology of Amenthes, based on techniques developed to determine the geometry of thrust faults on Earth. These include basic tenets of fault-related fold theory that relate the shape of thrust faults to the geometry and style of folds formed simultaneously above them (Suppe, 1983; Allmendinger, 1998; Seeber and Sorlien, 2000; Johnson and Johnson, 2002). In addition, a large impact crater offset by the Amenthes thrust was exploited to determine the horizontal and vertical components of surface displacement, and hence the dip of the fault at the surface. This was then combined with area balancing of the Amenthes uplift to define the geometry of the thrust at deeper levels in the crust and to test the goodness of fit for an ensemble of restorations. Fig. 2. Oblique view of Amenthes scarp overlain with trace of thrust. Note discontinuous fault strands and rollover of forelimb as a 5 km wide fold. Arrows denote sense of rotation of forelimb. Mars digital image map (MDIM 2.1) draped over MOLA topography. paper thus build on previous studies that use elastic solutions to fit a wide range of possible planar fault orientations to the pattern of surface uplift (Schultz and Watters, 2001; Ruiz et al., 2008). 2. Regional setting Amenthes Rupes is a 380 km long lobate scarp or surface thrust rupture and fold with ∼1 km of vertical surface relief located in the equatorial eastern hemisphere of Mars (Fig. 1). The scarp is formed above a northeast dipping thrust fault and offsets heavily cratered highlands terrain of Late Noachian age (Fig. 2). The exact age of the Amenthes uplift is difficult to closely constrain, but must postdate the Late Noachian crust it displaces and predate Early Hesperian volcanic rocks deposited across its leading edge (Fig. 1). 3.2. Mapping surface topography Raw MOLA shot points were used from over 2000 orbit tracks to create a digital elevation model of the Amenthes region using the natural neighbor interpolation method (Vidal, 2008). The resulting DEM was then used to characterize the topography of the Amenthes region with the aim of identifying subtle changes in surface slope related to folding above the thrust. The approach is an offshoot of methods originally developed in terrestrial thrust belts for mapping the edges of folds with seismic reflection profiles (Shaw et al., 1994; Shaw and Suppe, 1996; Bergen and Shaw, 2010). Termed axial surface mapping, the method uses the morphology of fault-related folds to predict the geometry and distribution of slip on underlying blind thrusts. The method was adapted to map the edges of the Amenthes uplift as defined by topography by creating first derivative (slope) and second derivative (curvature) maps of the region. The derivative maps were created by calculating a bi-directional average slope for individual MOLA shot points relative to values for nearest neighbor points. MOLA orbit 102 K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 Fig. 3. Topographic profile and corresponding slope values for transect across Amenthes uplift. Location of transect shown in Fig. 1. Figure is highly vertically exaggerated. tracks are spaced unevenly across the Amenthes region, as illustrated by significant gaps between orbit tracks. This allowed the most ideal transect across the Amenthes uplift to be defined with the closest spacing between orbit tracks (see location of transect on Fig. 1). It was also located away from a large crater and associated ejecta blanket that altered the topography of Amenthes after it was formed (Fig. 1). 3.3. Morphology of the Amenthes uplift Maps of surface topography suggest the Amenthes uplift is inclined on average ∼1◦ to the northeast across its entire width. Closer examination of detrended topography suggests however, that the surface of the Amenthes uplift is comprised of two dip domains, one dipping 0.4◦ to the northeast that defines its broad, nearly flat top and a second 20 km-wide panel that dips 2.2◦ between the plateau-like crest and the base of the uplift to the northeast (Fig. 3). The dip of each panel was determined by regressions of the MOLA shotpoints and the axial surface mapping (Vidal, 2008; Fig. 3). The 58 km width of the Amenthes uplift is remarkably broad with respect to terrestrial structures with comparable fault displacement. This illustrates the importance of back limb width in the restorations, where the tradeoff between the dip of the thrust and the geometry and depth at which the thrust flattens is consistent with observed fold geometry at the surface. In contrast to the crest and backlimb of the Amenthes uplift, the forelimb of the structure (i.e. the lobate scarp itself) is much more steeply inclined, and typically dips between 5–15◦ (Fig. 3). Second derivative mapping suggests the scarp has a gentle S-shaped morphology along much of its length and is ∼5 km wide near the offset crater. 3.4. Fault dip and displacement from crater offset Craters have previously been used as finite strain markers to measure offset across faults on Mars (Golombek et al., 1996). This opportunity is also available at Amenthes where the lobate scarp, or surface thrust rupture and fold deforms a large, 33 km-diameter crater located at 110.19◦ E, 2.3◦ N (Figs. 1, 2). This allows the vertical and horizontal displacement across the crater to be measured and combined to estimate the dip of the Amenthes thrust at the surface. This is required to develop robust solutions of the geometry of the thrust and the depth at which it flattens in the brittle crust. The average elevation and offset of the rim of the displaced impact crater was first measured using 10 topographic profiles derived from the digital elevation model; this yielded an average value for vertical displacement of 1.00 ± 0.07 km. This is similar to the ∼920 m of vertical relief measured across the scarp using individual MOLA shot points along the transect used in the structural restoration (Fig. 3). Horizontal displacement across the crater was determined by measuring how the shape of the originally circular crater was deformed (i.e., shortened) across the Amenthes scarp (Fig. 4). This was first undertaken by manually mapping the rim of the crater on both sides of the thrust using THEMIS imagery. The crater rim is marked by shadows on the footwall side of the crater wall (illuminated from the west) and conversely is brightened on the opposite side of the crater (Fig. 4). Whereas the portion of the offset crater rim (i.e. about one half of the original) in the footwall of the thrust is well preserved, about 40% of the crater rim on the hanging wall of the thrust has been modified by an ejecta blanket produced by a subsequent large impact located immediately towards the northeast (Figs. 1, 4). After manually picking well defined points on the crater rim, semicircles on both sides of the thrust were fitted to the points to define two circles using a crater mapping algorithm. The center of each circle was then calculated and the horizontal displacement and slip direction were determined by measuring the difference in their locations. Results of this indicate about 1040 m of horizontal offset along a slip vector at 23◦ azimuth, a direction oriented 74◦ from the strike of the thrust at the location of the transect (Fig. 4). While significant error exists in the measurement, the calculated slip direction is oriented within 16◦ of that expected for pure dip slip across the thrust. In addition to mapping in the THEMIS image, offset across the crater was measured by mapping the crater rim using topographic profiles on individual MOLA orbits (Fig. 4). Using the 30 orbit tracks that cross the crater, and fitting to circles, 720 m of horizontal displacement was determined along an azimuth of 27◦ . The azimuth of the resulting transport direction is 78◦ from the strike of the thrust – comparable to the results from mapping on the THEMIS image. The circle-fitting code used to determine the centers of circles fit to each half of the crater also outputs formal χ 2 uncertainties. The difference in latitude between fits to the two halves of the crater is 0.032◦ , while the difference in longitude is 0.014◦ . K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 103 Fig. 4. MOLA orbit tracks used to map the offset crater. Curved line denotes rim of crater mapped on the underlying THEMIS image. Circles mark locations of offset crater rim as mapped on individual orbit tracks. Straight gray segment shows area of offset crater rim obscured by younger crater ejecta. Table 1 Values for combinations of vertical and horizontal offset, slip at the surface, fault dip at surface and radii of curvature used to construct cross section. Vertical offset Horizontal offset Y/X Slip 920 920 1000 1000 720 1040 720 1040 1.28 0.89 1.39 0.96 1168 1389 1232 1442 m m m m m m m m m m m m However, the χ 2 uncertainties are approximately 0.05◦ and 0.15◦ , respectively. The large formal uncertainties are due to a large degree of scalloping on the southwest side and the lesser amount of the original rim remaining on the northeast side (due to being obscured by the large younger crater). These 1σ uncertainties were alternatively added to and subtracted from the crater centers and new offset distances calculated. When doing this, the standard deviation from the mean of the various offsets was ±0.73 km. Using vertical displacements and shortening determined across the offset crater, the dip of the thrust at the surface can be calculated using: θ = tan−1 (μv /μh ) (1) where θ is the dip of the fault at the surface, μv is the vertical separation and μh is the horizontal separation. Similarly, dip slip across the fault was calculated using: U= μ2v + μ2h 1/2 (2) Fault dip Radius of curvature upper ramp Radius of curvature lower ramp Depth to Detach. 52.0◦ 41.5◦ 54.3◦ 43.8◦ 167 198 176 206 30.4 36.2 32.0 37.5 43.1 32.6 47.8 36.2 km km km km km km km km km km km km where U is the total slip along the fault. Based on 920 and 1000 m of vertical separation across the scarp, and shortening of 720 and 1040 m, the Amenthes thrust dips between 41.5–54.3◦ at the surface and accommodated between 1168 to 1442 m of slip (Table 1). Preferred values of 41.5◦ to 52.0◦ for fault dip at the surface based on 920 m of vertical separation measured on the topographic profile across Amenthes yields 1168 and 1389 m of slip. 4. Structural analysis The structural analysis presented here is based on basic tenets of fault-related fold theory for compressive structures on Earth using first order assumptions for conservation of mass and nonrecoverable (i.e. non-elastic) strain. These include a rigid footwall block that does not deform during shortening and translation of rocks in the hangingwall, parallel to the surface of the thrust. Internal strain in fault-related folds on Earth has been shown to occur by a number of physical deformation mechanisms (Suppe, 1983; 104 K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 Fig. 5. Balanced cross section through Amenthes uplift. Four fault geometries are based on combinations of vertical and horizontal separation across offset crater (920 and 1000 m vertical, 720 and 1040 m horizontal) through which fault dip at the surface is determined. Curvature of each fault solution equals slip at the surface divided by inclination of the broad crest and backlimb of the fold (0.4◦ and 2.2◦ or 0.007 and 0.0384 radians). See Table 1 with values for these solutions and Seeber and Sorlien (2000) for methods. Slip is conserved through both curved segments on each model and equals displacement at surface. Change in curvature in each fault surface occurs at upper axial surface (dashed line) at top of backlimb. Solid line defines base of ramp on all four models and lies directly beneath foot of backlimb at the surface. Stars denote predicted location of base of backlimb for each fault model. MOLA topography defined by open circles (i.e. individual data points along line of transect). Line of cross section shown on Fig. 1. Surface topography exaggerated 500%. Fig. 6. Cross section of the Wind Rivers thrust, a deeply penetrating thrust fault in the Laramide province of Wyoming. Simplified from Stone (1993). Erslev, 1986), but for Mars it is assumed that rocks are penetratively deformed by widely distributed brittle fracturing, producing a broad fold above a dipping fault (Seeber and Sorlien, 2000). Fracturing is likely to be concentrated within the forelimb of the fold in response to increased shear along the blind thrust. While other deformation mechanisms have been shown to occur in layered sedimentary rocks in thin-skinned thrust belts on Earth such as layer parallel shear, this is inappropriate for deformation of Noachian age crust on Mars, which is assumed to be largely igneous in origin. Detrended topography across Amenthes suggests the base of its forelimb (i.e. the lobate scarp) is at the same elevation as the base of its backlimb (Fig. 5). This implies the width of the curved thrust ramp is contained between these two points and that the base of the back limb marks the point where the thrust flattens into a horizontal detachment. Examination of the Amenthes uplift from MOLA topography suggests its most likely terrestrial analogues are isolated, basement-cored structures, such as the Wind River Mountains in the Laramide province of the western USA (Fig. 6; Erslev, 1986; Mitra, 1990; Stone, 1993). These structures, or fault-propagation folds are typified by steeply dipping or overturned forelimbs where shear is localized in a downward tightening, triangular envelope (Erslev, 1986; Allmendinger, 1998). Numerous studies suggest that shear in the forelimbs of these structures is accommodated by early folding followed by displacement on a thrust that propagates upward from depth. MOLA topography suggests the forelimb of Amenthes is about 5 km wide and dips 5–15◦ (Figs. 3, 5). This and the detailed morphology of the forelimb or lobate scarp as shown on THEMIS and other imagery (Fig. 2) both suggest it is a broad flexure offset by short discontinuous fault ruptures, consistent with an origin as a fault propagation, or trishear fold formed above a blind thrust. Alternatively a simple thrust rupture at the surface without broad prior folding would require a much narrower and steeper forelimb where limb width is comparable to fault slip and limb dip is greater than fault dip. Here the observed limb width is 5 km versus less than 1.5 km (i.e. fault slip) as might be expected for a simple rupture. Similarly for a simple thrust rupture with no prior folding, forelimb dip would be greater than the ∼40–55◦ dip of the thrust as opposed to 5–15◦ as observed from MOLA topography. Further, at larger scales, these asymmetric structures have been shown to form by displacement on gently curved, moderatelydipping thrust ramps (Fig. 6; Erslev, 1986; Stone, 1993) where fault slip is a small fraction of the width of their backlimbs. This is an K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 important distinction, one that indicates Amenthes did not form above a thrust with abrupt changes in its dip, as would be associated with sharp, kink-like folds whose limbs matched fault slip (Suppe, 1983). This includes an abrupt step up from a flat-lying decollement onto the base of a planar thrust ramp, as has been previously modeled for Amenthes (Schultz and Watters, 2001; Ruiz et al., 2008). Additionally, while such ramp and flat thrusts are common in thin-skinned thrust belts that deform sedimentary basins on Earth (Suppe, 1983) and in regions characterized by volcanic rocks on Venus (Suppe and Connors, 1992), they typically accommodate shortening at shallow levels in layered sedimentary or volcanic strata as opposed to more isotropic (i.e. less well bedded) rocks as might be expected in late Noachian highlands crust on Mars. Given the range of fault offset estimated at the surface, and the broad width of the backlimb of Amenthes, any structural model must therefore include a listric geometry for the thrust ramp beneath the uplift. The broad, nearly flat top of the Amenthes uplift (i.e. the panel dipping 0.4◦ ) can be related to a very gently curved and wide thrust ramp, one on which rocks are translated upward. In contrast, the 2.2◦ panel marking the backlimb located immediately to the east implies a relatively greater amount of uplift produced by folding over a shorter distance (i.e. the width of the backlimb) and greater fault curvature at depth. As a basis for comparison, either a shorter radius of curvature that describes a ramp segment or greater fault displacement can both act to produce a more steeply inclined fold limb above a listric fault. In addition, the width of a thrust ramp segment can be directly related to the edge of associated fold limbs at the surface because a change in the dip of one corresponds with a change in the other. Active examples of fault propagation folds have been shown to grow upward from the base of the seismogenic crust, suggesting a link between the depth to detachment and the brittle ductile transition (Allmendinger and Shaw, 2000). This has previously been invoked for Amenthes, namely the assumption that the depth the thrust flattens corresponds with the rheological threshold for plasticity in the Martian crust (Schultz and Watters, 2001; Ruiz et al., 2008). 4.1. Balanced cross section construction Studies of well constrained listric thrusts on Earth have derived a set of relationships that allow fault geometry to be related to fault displacement, radius of curvature and the dip of backlimbs on the folds produced above them (Seeber and Sorlien, 2000; Amos et al., 2007). For a thrust ramp of constant curvature, a circular listric fault is connected tangentially to a horizontal detachment at depth, (i.e. the depth to detachment) which we relate to the brittle–ductile transition. In this case, the hanging wall above the thrust rotates about a horizontal axis, parallel to the strike of the fault and requires uplift at the base of the ramp where it starts to steepen upward. For this case, fault slip S = R α , where R is the radius of curvature of the fault and α is the cumulative rotation angle (in radians) of the hanging wall block (i.e. the dip of the backlimb, Erslev, 1986). The trajectory, or shape of four fault solutions derived from combinations of fault slip and dip at the surface and the dip of the broad crest and backlimb of the Amenthes uplift (Fig. 5) was determined as follows. The upper tip of the thrust was pinned at the midpoint of the forelimb or lobate scarp, consistent with a trishear envelope centered symmetrically across an upwardly propagating fault. Radii of curvature for sections of the underlying thrust were then calculated for the two dip panels (2.2◦ backlimb = 0.0384 rad; 0.4◦ broad crest = 0.007 rad; Table 1) and four possible combinations of vertical (920 and 1000 m) and horizontal (720 and 1040 m) offset measured across the scarp and offset crater 105 (i.e. fault slip). The resulting circular arcs were then used to define overall fault geometry in each of the four models. The upper circular arc in each model was constructed for the shape of the thrust beneath the broad crest that ended at the axial surface that marks the top of the backlimb (Fig. 5). These were then fit to a lower circular arc consistent with similar fault slip and the dip of the backlimb. The dip of the thrust was similar where the two circular arcs in each model meet, producing a smooth transition where curvature changes at the vertical axial surface at the top of the backlimb. It follows that continuity of slip is required through synclinal axial surfaces to conserve mass, a basic tenet for constructing balanced cross sections of curved or listric thrust faults. The predicted location of the base of the fault ramp for each model (i.e. where they flatten to horizontal; see stars on Fig. 5) was then plotted and compared to the location of the synclinal axial surface at the base of the backlimb. Results of the fault modeling suggest that solutions for fault geometry are relatively insensitive to fault dip as shown by all four cases where predicted fold geometry can be closely fit to surface topography (Fig. 5). Note that the balancing method solves for surface dips that exactly match the 0.4 and 2.2◦ dip of the panels at the surface that are themselves based on regressions of MOLA shotpoints (Vidal, 2008). These solutions thus easily match MOLA topography within limits posed by the inherent roughness of the cratered late Noachian surface and spacing of MOLA shotpoints in the cross section. The resulting area balancing thus lends credence to the fault models, and indicates a curved thrust at depth that steepens above a horizontal decollement as depicted in the cross sections (Fig. 5). Further, the depth to detachment in the models can be related to slip at the surface where uncertainty is dictated by measurements of fault displacement, in particular horizontal shortening across the offset crater (Fig. 4). For the four models, shallower depths of detachment correspond to estimates with the greater value for horizontal shortening (i.e. 1040 m) and range from ∼32.6–36.2 km in depth based on fault dips at the surface of 41.5–43.8◦ (Fig. 5). Alternatively smaller values for horizontal shortening yield greater depths to detachment that range from 43.1–47.8 km. Curvature is greatest at the deepest part of the thrust ramp, where models for thrust geometry suggest the ramp steepens between 26.9 and 33.5◦ over 3.7–5.7 km of the crust. The goodness of fit of the solutions can be defined by comparing the horizontal distance between the predicted location of the base of the thrust ramp with the synclinal axial surface marked by the solid line on Fig. 5. These misfits range from 0.2–0.3 km for values with 920 m of vertical displacement across the scarp, and 2.1–2.2 km for values with 1000 m of vertical displacement. 4.2. Previous results from fault modeling Results of the modeling presented here differ from previous studies that also sought to determine the geometry of the Amenthes thrust (Schultz and Watters, 2001; Ruiz et al., 2008). These studies used the elastic dislocation method with the program, Coulomb (Toda et al., 1998), to test whether an optimally oriented, planar thrust fault could produce surface folding similar to that measured by MOLA topography. Coulomb is typically used in studies of active faults on Earth to predict instantaneous changes in an elastic stress field following a large earthquake (for a review see Freed, 2005). Schultz and Watters (2001) assumed a planar fault for modeling the Amenthes thrust. Their model consisted of a fault embedded in the upper crust, one that was not linked with a horizontal decollement at depth. The term depth of penetration is used here to describe the lower limit of the thrust modeled in their analysis and that of Ruiz et al. (2008). In contrast, the term depth of detachment 106 K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 is used in our model to describe the level where faults in the models flatten to horizontal as defined by area balancing techniques (Fig. 5). Schultz and Watters (2001) considered an ensemble of models with a broad range of assumed values for fault dip, displacement and depth of penetration in order to match surface topography. This analysis was based on a topographic profile from a single orbit acquired soon after MOLA data were first acquired as part of the Mars Global Surveyor program, before a gridded DEM was available. The profile used in the study was oriented about 61◦ from the average strike of the Amenthes scarp. Results of the analysis argue for a depth of penetration ranging from 25–30 km for Amenthes, overlapping with, or less than the 32.7–47.8 km determined here for the depth to detachment using area balancing techniques (Fig. 5). In a subsequent test of the depth of penetration of the Amenthes thrust, Grott et al. (2007) also used the elastic dislocation method to model surface topography across the uplift. Using increments for 1 km for depth of penetration and 2◦ for fault dip, their solutions suggested a higher range of values for the thickness of the brittle crust, about 32–40 km, corresponding with fault dips of 30–35◦ . Subsequent work by Ruiz et al. (2008) again used Coulomb elastic modeling to constrain the depth of penetration of a planar thrust, using the same methods and assumptions as Schultz and Watters (2001) and Grott et al. (2007). The topographic profile used in this analysis was oriented perpendicular to the Amenthes uplift and derived from the MOLA 1/128◦ digital elevation model. Comparison of the set of parameters used in their analysis was based on determining RMS error for the goodness of fit between surface topography from MOLA data and that calculated from the elastic models. RMS were lowest for the range of values that include 19–24◦ for the dip of the thrust, 1900–2300 m of displacement on it and 27–35 km for the depth it penetrates. While Ruiz et al. (2008) calculate a range of temperatures for the onset of plasticity in the crust based on estimations of strength at this depth, the brittle–ductile transition is determined here using a different method of structural analysis. This refinement offers the advantage of directly determining the dip of the thrust and slip across it by measuring the vertical and horizontal displacement across a large crater it offsets. These provide a starting point for constructing mass balanced models based on well established relationships between fault and fold shape (Fig. 5). The resulting balanced structural models are thus internally consistent, where fault dip and curvature match more detailed surface slope, displacement and most importantly depth to detachment. Similar to previous studies, it is then argued that the flat decollement of the thrust formed at the brittle to ductile transition and surface heat flow calculated accordingly. In contrast, previous studies of Amenthes use elastic dislocation theory to explore a broad set of models, each of which is comprised of some combination of values for fault dip, displacement and depth of penetration. Each model produces surface deformation that is compared to MOLA topography. These are then assessed for goodness of fit between the geometry of the fold predicted by the model versus surface topography. This thus narrows the most likely combinations of fault orientation, displacement and depth of penetration, forming the basis for determining a range of values for heat flow. Elastic dislocation theory generally predicts slip on thrust faults to increase towards the surface (see model setup in Fig. 4 of Schultz and Watters, 2001), whereas models based on fault-related fold theory conserve slip on the fault through the entire brittle crust (i.e. displacement is constant with depth). Although values obtained for horizontal displacement across the thrust have some uncertainty from crater rim mapping (Fig. 4), results in this study predict an average depth to detachment equal to or significantly greater than values derived from the previous studies. This has implications for estimates of Late Noachian heat flow, which are outlined in the following section. 5. Depth to detachment and the brittle–ductile transition Previous studies of Amenthes have all sought to relate the depth to detachment, or penetration of the Amenthes thrust to the temperature of the brittle–ductile transition, to estimate surface heat flow during the Late Noachian in this part of Mars (Schultz and Watters, 2001; Grott et al., 2007; Ruiz et al., 2008). Whereas uncertainty in the depth of detachment of the Amenthes thrust is considered in the previous sections, estimations of the temperature of the brittle–ductile transition, and it’s likely variation depend largely on the composition and thermal properties of the Martian crust, which have been assumed for Mars. It is beyond the scope of this paper to comment on the suitability of assumptions for rheology and strain rate used to calculate surface heat flow at Amenthes and the reader is referred instead to the comprehensive consideration of heat flow in Ruiz et al. (2008) and methods and parameters used in that analysis, which are adopted here. For the purposes of comparison, the temperature of the brittle to ductile transition, surface heat flow and thermal gradients are recalculated using the range of depth to detachment determined from the structural analysis. These are based on a range of values for thermal conductivity and heat production in the crust, strain rate and crustal density (Ruiz et al., 2008). 5.1. Surface heat flow Ruiz et al. (2008) first estimate the temperature of the brittle– ductile transition using the thermal dependence of ductile strength where brittle failure transitions to plastic creep. See equations and solutions in their Section 4. Heat flow from the depth of the Amenthes Rupes-related thrust fault (Ruiz et al., 2008). The temperature of the brittle ductile transition is determined for zero pore fluid pressure, a stress coefficient for compression, a range of crustal densities (2700–3100 kg m−3 ) and strain rates (10−16 –10−19 s−1 ), gravity on Mars and empirically derived values for the activation energy of creep for flow of wet diabase. The resulting values for the temperature of the brittle ductile transition range from 530–610 K. This then permits surface heat flow to be estimated using the previously determined temperature of the brittle–ductile transition and surface temperature as well as a range of values for volumetric heat production (0.46–0.60 μW m−3 ), and thermal conductivity in the crust (2.0 W m−1 K−1 ; Ruiz et al., 2008). For a depth range of 27–35 km to the brittle ductile transition, Ruiz et al. (2008) calculate surface heat flow of 26–37 mW m−2 and thermal gradients of 9–14 K km−1 . They also determine surface heat flow for zero crustal heat production with resulting values of 18–29 mW m−2 . Using the assumption that the height of the lower ramp in the four models (3.7–5.7 km) represents the depth interval over which the full onset of plasticity occurs, it is possible to determine the corresponding difference in temperature over this thickness of the crust. For a temperature range of 530–610 K at the brittle ductile transition (Ruiz et al., 2008), this indicates the change in temperature ranges between ∼26–47 K (i.e. over the depth range of 3.7–5.7 km). For comparative purposes, we also plot surface heat flow as bounded by the same range of parameters listed above with respect to our work and that of the previous studies for the depth of the brittle ductile transition at Amenthes (Fig. 7). The plot illustrates how surface heat flow varies as a function of the depth to detachment, using the range of values for thermal conductivity, heat production, strain rate, crustal density, etc. used by K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 107 Fig. 7. Curves that plot surface heat flow versus depth of detachment or depth of penetration of the Amenthes thrust. Range of values from the previous studies shown by arrows. Area between the two curves contains solutions for surface heat flow using all possible combinations of volumetric heat production, strain rate and density of the crust listed in text box. Combination of upper set of values yield maximum possible surface heat flow (upper curve). The lower set of values yield minimum possible heat flow, shown as the lower curve. See Ruiz et al. (2008) for flow law constants and other variables. Ruiz et al. (2008); Fig. 7. The plot considers a range from estimates of the brittle–ductile transition, including that of Schultz and Watters (2001) of 25–30 km, Ruiz et al. (2008) of 27–35 km, and our results of 32.6–47.8 km (Fig. 7). Although the rate of change in surface heat flow decreases with increasing depth (i.e. the curve shown in Fig. 7 flattens with increasing depth to detachment), our results yield lower heat flow estimates than previous studies. For example, surface heat flow predicted by three previous studies range from 28–38 mW m−2 at 25–30 km (Schultz and Watters, 2001) and 26–37 mW m−2 for depths of 27–35 km (Ruiz et al., 2008; Grott et al., 2007; Figs. 7). In contrast, the greater depth to the brittle ductile transition estimated in this study yields shifted, generally lower surface heat flows that range from 24–33 mW m−2 (Fig. 7). 6. Discussion MOLA surface topography indicates the Amenthes uplift is an asymmetric fold with a broad, gently inclined backlimb formed above a wide (58 km), moderately to steeply dipping listric thrust ramp. The absence of narrow dip panels on the uplift, combined with values for fault slip indicates the thrust is not comprised of ramp and flat segments, as is common in thin-skinned fold belts that deform layered deposits with large strength contrasts on Earth (Suppe, 1983). In addition no correlation exists between the shape of the thrust and a shallow weak layer between the surface and 1–3 km depth, as might be expected for the thickness of megaregolith in Noachian age crust (Hartmann et al., 2001; Ward, 2002). It can therefore be argued the upper crust in the Amenthes region is largely homogeneous with regards to rock strength and that crustal scale layering with widely different mechanical properties is not present within the upper 30–40 km. Balanced cross sections are robust and can be restored such that the predicted location of the base of the thrust ramp closely matches its location as inferred by surface topography (i.e. the base of the back limb). The area uplifted above the thrust in the fault models similarly closely matches detrended topography, within the limits of ambient surface roughness. The balanced cross section and structural reconstruction argues that the fault that produced Amenthes penetrated to the base of the brittle lithosphere and that the structure is most closely analogous to basement cored uplifts of the Rocky Mountains that formed via fault-propagation folding (Fig. 6; Erslev, 1986; Mitra, 1990; Stone, 1993). Interpretations of the subsurface structure and depth of penetration of faults beneath wrinkle ridges has been argued to be thin-skinned in which faults only extend several kilometers deep or thick-skinned in which the faults extend to the brittle ductile transition (e.g., see review in Golombek and Phillips, 2010 and references therein). We do note, however, that the broad backlimbs of wrinkle ridges (similar to the backlimb of Amenthes) also argues that faults beneath wrinkle ridges penetrate deeply and may extend to the brittle ductile transition analogous to large fault-propagation folds on Earth (Fig. 6; Golombek et al., 2001; Mueller and Golombek, 2004). A significant change in the otherwise gently listric geometry of the Amenthes thrust inferred from the structural models is the more sharply curved 3.7–5.7 km-high and 16 km-wide section of the thrust that forms the base of the ramp above the basal detachment. This smaller ramp is required to balance uplift above the backlimb and marks the transition from the flat detachment to the main thrust ramp. As in many compressive orogens, thrust ramps step upward at the base of stronger units, especially in fold belts that deform rocks of widely different strength. We attribute the sharper curvature at the base of the thrust ramp to form in response to displacement across the brittle to ductile transition. The thickness over which the thrust curves sharply upward must therefore be related to a gradual increase in fault strength, over a thickness of ∼ 4–6 km in the crust. While the transition from brittle to plastic deformation mechanisms can be affected by a variety of rheological conditions (see previous discussion), an increase in temperature is the most likely cause over this thickness of the crust, in particular as it might affect rocks of appropriate composition for Mars. The middle crust on Mars is most likely mafic in composition, perhaps similar to Nahklite meteorites that consist of clinopyroxene, minor olivine and interstitial mesostatic plagioclase 108 K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 Our results are similar to recent estimates by Ruiz et al. (2011) who consider the thermal evolution of Mars using lithospheric strength as derived by estimates in the literature of the depth of penetration of large thrust faults and the effective elastic thickness of the lithosphere. Their work suggests that surface heat flow as implied by lithospheric strength in many regions on Mars is lower than those derived from radioactive heat production. This is interpreted to suggest that the contribution of secular cooling to heat flow on Mars is lower than previously thought, consistent with presently high effective elastic thickness shown by the lack of deflection from loading of its north polar ice cap and extensive recent volcanism. Further, work by Ruedas et al. (2013a, 2013b) suggests low values for surface heat flow early in Mars history as defined by models of the thermal and compositional evolution of its mantle. These results are comparable to ours. Taken as a whole, these results suggest Mars is cooling more slowly than previously thought, perhaps as a result of a more stagnant convective mantle and more inefficient heat loss (Ruiz et al., 2011). This further suggests that Mars may have had higher internal temperatures throughout much of its history. Fig. 8. Graph of surface heat flow curves for Mars from the previous studies. Results shown as follows. Coupled thermal-magmatic model (solid line; Hauck and Phillips, 2002), radiogenic heat production balanced with chemistry (dashed lines, Montesi and Zuber, 2003; Laul et al., 1986; Treiman et al., 1986), lithospheric cooling (Montesi and Zuber, 2003). Boxes are results from analysis of buckling instability (dashed) and gravity and topography (solid) by Montesi and Zuber (2003). Light gray envelope in background is from admittance analysis in many regions of Mars (McGovern et al., 2002; 2004; see Solomon et al., 2005 and Golombek and Phillips, 2010 for reviews) whereas dots are values from McKenzie et al. (2002). Results from this study shown as small, dark gray rectangle labeled “This Study”; other studies at Amenthes are labeled “Prev. Work”. Results from Ruiz et al. (2011) shown as star. Values for all studies that relate depth to detachment, or depth to penetration of the thrust at Amenthes are shown in the insert at the upper right and include the work of Schultz and Watters (2001), Grott et al. (2007) and Ruiz et al. (2008). (Berkeley et al., 1980). Of these minerals plagioclase is the phase that controls the onset of plasticity (Kirby and Kronenberg, 1984; Stunitz, 1993; Dimanov and Dresen, 2005). The 33–48 km depth to detachment determined for Amenthes is greater than the 30 km depth estimated by Schultz and Watters (2001) and mostly greater than the 27–35 km depth derived by Ruiz et al. (2008). Given the range of applicable temperatures for the brittle–ductile transition, results in this study imply correspondingly lower heat flow. It is thus useful to consider other independent estimates of heat flow early in Mars history as determined from other methods. A recent compilation of elastic lithosphere thickness from a variety of tectonic models suggest a thin lithosphere (10–35 km) in the Noachian and a progressively thicker lithosphere in the Hesperian (10–65 km) and Amazonian (20–300 km) (e.g., see Golombek and Phillips, 2010). As an example, in an analysis of local conditions that lead to the development of younger, Early Hesperian wrinkle ridges on Solis Planum, Montesi and Zuber (2003) used modeling of buckling instabilities to argue for surface heat flow of 30–48 mW m−2 , a geotherm of 12 ± 3 K km−1 and a brittle–ductile transition as deep at 60 km. Depth to the brittle–ductile transition is greater in this work than is presented here, although the structures on Solis Planum may be considerably younger than Amenthes and formed under a correspondingly lower thermal gradient in the crust. Studies based on lithospheric cooling models (Montesi and Zuber, 2003) are generally consistent with estimates of heat flow presented here (Fig. 8). However estimates here are considerably lower than most other previous studies including coupled thermalmagmatic models (Hauck and Phillips, 2002) and models of radiogenic heat production balanced with chemistry (Laul et al., 1986; Treiman et al., 1986; Fig. 8). 7. Summary We use a large impact crater offset by the Amenthes Rupes thrust fault and the shape of the region uplifted above it to determine its geometry and the depth it flattens in Late Noachian crust on Mars. Measurements of vertical displacement across the fault scarp and horizontal shortening across the crater define the dip of the fault at the surface, which ranges from 42–54◦ with slip of ∼1170–1440 m. MOLA surface topography suggests the region uplifted above the thrust is similar to fault propagation folds formed above curved or listric faults, like those associated with basement cored uplifts developed in continental crust on Earth. As defined by detrended MOLA topography the area uplifted above the thrust is marked by a moderately dipping forelimb formed above the fault tip, a wide and flat crest, and a gently inclined backlimb. Structural modeling based on fault-related fold theory suggests that surface topography is best fit by a deeply penetrating thrust ramp that flattens into a horizontal detachment at depths of 33–48 km. Solutions for fault dip, curvature and slip are fully balanced and restorable, meaning that the uplifted area predicted in the models can be closely matched to the width and inclination of the back limb as seen in surface topography. The depth range over which the thrust steepens above the detachment can be related to a gradual change in brittle to plastic behavior over a depth of ∼ 4–6 km, and the most likely cause for this change is temperature for a range of reasonable geothermal gradients. Calculations of surface heat flow based on an applicable range of temperatures for the brittle–ductile transition and the structural modeling yield values similar to some estimates of lithospheric strength and modeling of the thermal and compositional evolution of the martian mantle. Acknowledgements Support for this work was provided by NASA’s Planetary Geology and Geophysics Program to M. Golombek and K. Mueller (grant 1241698). Work at the Jet Propulsion Laboratory, California Institute of Technology was done under a contract with NASA. References Allmendinger, R.W., 1998. Inverse and forward numerical modeling of trishear faultpropagation folds. Tectonics 17, 640–656. Allmendinger, R.W., Shaw, J.H., 2000. Estimation of fault propagation distance from fold shape: implications for earthquake hazard assessment. Geology 28, 1099–1102. K. Mueller et al. / Earth and Planetary Science Letters 408 (2014) 100–109 Amos, C.B., Burbank, D.W., Nobes, D.C., Read, S.A.L., 2007. Geomorphic constraints on listric thrust faulting: implications for active deformation in the Mackenzie Basin, South Island, New Zealand. J. Geophys. Res. 112, B03S11. http:// dx.doi.org/10.1029/2006JB004291. Bergen, K.J., Shaw, J.H., 2010. Displacement profiles and displacement-length scaling relationships of thrust faults constrained by seismic-reflection data. Geol. Soc. Am. Bull. 122, 1209–1219. http://dx.doi.org/10.1130/B26373.1. Berkeley, J.L., Keil, K., Prinz, M., 1980. Comparative petrology and origin of Governador Valadares and other nahklites. In: Proc. 11th Lunar and Planet. Sci., pp. 1089–1102. Bloxsom Lynn, H., Quam, S., Thompson, G.A., 1983. Depth migration and interpretation of the COCORP Wind River, Wyoming seismic reflection data. Geology 11, 462–469. Burgmann, R., Dresen, G., 2008. Rheology of the lower crust and upper mantle: evidence from rock mechanics, geodesy, and field observations. Annu. Rev. Earth Planet. Sci. 36, 531–567. Coward, M.P., 1984. Major shear zones in the Precambrian crust: examples from NW Scotland and southern Africa and their significance. In: Kröner, A., Greiling, R. (Eds.), Precambrian Tectonics Illustrated. Schweizerbart’sche Verlagsbuchhandlung (Nagele U. Obermiller), Stuttgart, pp. 207–235. Dimanov, G., Dresen, G., 2005. Rheology of synthetic anorthite–diopside aggregates: implications for ductile shear zones. J. Geophys. Res. 110. http://dx.doi.org/ 10.1029/2004JB003431. Erslev, E., 1986. Basement balancing of Rocky-Mountain foreland uplifts. Geology 14, 259–262. Freed, A.M., 2005. Earthquake triggering by static, dynamic and postseismic stress transfer. Annu. Rev. Earth Planet. Sci. 33, 335–367. Golombek, M.P., Phillips, R.J., 2010. Mars tectonics. Chapter 5 In: Watters, T.R., Schultz, R.A. (Eds.), Planetary Tectonics. Cambridge University Press, pp. 183–232. Golombek, M.P., Tanaka, K.L., Franklin, B.J., 1996. Extension across Tempe Terra, Mars, from measurements of fault scarp widths and deformed craters. J. Geophys. Res. 101, 26,119–26,130. http://dx.doi.org/10.1029/96JE02709. Golombek, M.P., Anderson, F.S., Zuber, M.T., 2001. Martian wrinkle ridge topography: evidence for subsurface faults from MOLA. J. Geophys. Res. 106, 23811–23821. Grott, M., Hauber, E., Werner, S.C., Kronberg, P., Neukum, G., 2007. Mechanical modeling of thrust faults in the Thaumasia region, Mars, and implications for Noachian heat flux. Icarus 186, 517–526. Hartmann, W.K.J., Anguita, M.A., de la Casa, D.C., Berman, C., Ryan, E.V., 2001. Martian cratering 7. The role of impact cratering. Icarus 149, 37–53. Hauck II, S.A., Phillips, R.J., 2002. Thermal and crustal evolution of Mars. J. Geophys. Res. 107. http://dx.doi.org/10.1029/2001JE001801. Hiller, K.H., 1979. Geologic map of the Amenthes quadrangle of Mars: USGS Map I-1110 (MC-14). United States Geological Survey. Johnson, K.M., Johnson, A.M., 2002. Mechanical models of trishear-like folds. J. Struct. Geol. 24, 277–287. Kirby, S.H., Kronenberg, A.K., 1984. Deformation of clinopyroxenite: evidence for a transition in flow mechanisms and semibrittle behavior. J. Geophys. Res. 89, 3177–3192. Laul, J.C., Smith, M.R., Waenke, H., Jagoutz, E., Dreibus, G., Palme, H., Spettel, B., Burghele, A., Lipschutz, M.E., Verkouteren, R.M., 1986. Chemical systematics of the Shergotty Meteorite and the composition of its parent body (Mars). Geochim. Cosmochim. Acta 50, 909–926. McGovern, P.J., Solomon, S.C., Smith, D.E., Zuber, M.T., Simmons, M., Wieczorek, M.A., Phillips, R.J., Neumann, G.A., Aharonson, O., Head, J.W., 2002. Localized gravity/topography admittance and correlation spectra on Mars: implications for regional and global evolution. J. Geophys. Res. 107. http://dx.doi.org/ 10.1029/2002JE001854. McGovern, P.J., Solomon, S.C., Smith, D.E., Zuber, M.T., Simmons, M., Wieczorek, M.A., Phillips, R.J., Neumann, G.A., Aharonson, O., Head, J.H., 2004. Correction to “Localized gravity/topography admittance and correlation spectra on Mars: implications for regional and global evolution”. J. Geophys. Res. 109. http:// dx.doi.org/10.1029/2004JE002286. McKenzie, D., Barnett, D.N., Yuan, D.N., 2002. The relationship between Martian gravity and topography. Earth Planet. Sci. Lett. 195, 1–16. Mitra, S., 1990. Fault-propagation folds: geometry, kinematic evolution, and hydrocarbon traps. Am. Assoc. Pet. Geol. Bull. 74, 921–945. Montesi, L.G.J., Zuber, M.T., 2003. Clues to the lithospheric structure of Mars from wrinkle ridge sets and localization instability. J. Geophys. Res. 108. http://dx. doi.org/10.1029/2002JE001974. 109 Mueller, K., Golombek, M.P., 2004. Compressional structures on Mars. Annu. Rev. Earth Planet. Sci. 32, 435–464. http://dx.doi.org/10.1146/annurev.earth.32. 101802.120553. Munoz, M., 2008. The brittle/ductile transition in the lithosphere of the Andes region and its relationship with seismogenesis. In: 7th International Symposium of Andean Geodynamics. ISAG 2008, Nice, pp. 361–364. Pujol, J., Mueller, K., Shen, P., Chitupolu, V., 2006. High-resolution 3-D P-wave velocity model for the East Ventura – San Fernando basin, California, and relocation of events in the Northridge and San Fernando aftershock sequences. Bull. Seismol. Soc. Am. 96, 2269–2280. Ruedas, T., Tackley, P.J., Solomon, S.C., 2013a. Thermal and compositional evolution of the martian mantle: effects of phase transitions and melting. Phys. Earth Planet. Inter. 216, 32–58. Ruedas, T., Tackley, P.J., Solomon, S.C., 2013b. Thermal and compositional evolution of the martian mantle: effects of water. Phys. Earth Planet. Inter. 220, 50–72. Ruiz, J., Fernandez, C., Gomez-Ortiz, D., Dohm, J.M., Lopez, V., Tejero, R., 2008. Ancient heat flow, crustal thickness, and lithospheric mantle rheology in the Amenthes region, Mars. Earth Planet. Sci. Lett. 270, 1–12. http://dx.doi.org/ 10.1016/j.epsl.2008.02.015. Ruiz, J., McGovern, P.J., Jimenez-Diaz, A., Williams, J., Hahn, B.C., Tejero, R., 2011. The thermal evolution of Mars as constrained by paleo-heat flows. Icarus 2115, 508–517. Scholtz, C.H., 1988. The brittle–plastic transition and the depth of seismic faulting. Geol. Rundsch. 77, 319–328. Schultz, R.A., Watters, T.R., 2001. Forward mechanical modeling of Amenthes Rupes thrust fault on Mars. Geophys. Res. Lett. 28, 4659–4662. Seeber, L., Sorlien, C., 2000. Listric thrusts in the western Transverse Ranges, California. Geol. Soc. Am. Bull. 112, 1067–1079. Shaw, J.H., Suppe, J., 1996. Earthquake hazards of active blind-thrust faults under the central Los Angeles basin, California. J. Geophys. Res. 101, 8623–8642. http:// dx.doi.org/10.1029/95JB03453. Shaw, J.H., Hook, S., Suppe, J., 1994. Structural trend analysis by axial surface mapping. Am. Assoc. Pet. Geol. Bull. 78, 700–721. Smithson, S.B., Brewer, J., Kaufman, S., Oliver, J., 1978. Nature of the Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data. Geology 6, 648–652. Solomon, S.C., Aharonson, O., Aurou, J.M., Banerdt, W.B., Carr, M.H., Dombard, A.J., Frey, H.V., Golombek, M.P., Hauck, S.A., Head, J.W., Jakosky, B.M., Johnson, C.J., McGovern, P.J., Neumann, G.A., Phillips, R.J., Smith, D.E., Zuber, M.T., 2005. New perspectives on ancient Mars. Science 307, 1214–1219. Stone, D.S., 1993. Basement-involved thrust-generated folds as seismically imaged in the subsurface of the central Rocky Mountain foreland. Spec. Pap., Geol. Soc. Am. 280, 271–318. Stunitz, H., 1993. Transition from fracturing to viscous flow in a naturally deformed metagabbro. In: Boland, J.N., Fitz Gerald, J.D. (Eds.), Defects and Processes in the Solid State: Geosciences Applications, pp. 121–149. Suppe, J., 1983. Geometry and kinematics of fault-bend folding. Am. J. Sci. 283, 648–721. Suppe, J., Connors, C., 1992. Critical taper wedge mechanics of fold-and-thrust belts on Venus: initial results from Magellan. J. Geophys. Res. 97, 13,545–13,561. Toda, S., Stein, R.S., Reasenberg, P.A., Dieterich, J.H., Yoshido, A., 1998. Stress transferred by the 1995 Mw = 6.9 Kobe, Japan, shock: effects on aftershocks and future earthquake probabilities. J. Geophys. Res. 103 (24), 543–24566. Treiman, A.H., Drake, M.J., Janssens, M.J., Wolf, R., Ebihara, M., 1986. Core formation in the Earth and shergottite parent body (SPB): chemical evidence from basalts. Geochim. Cosmochim. Acta 50, 1071–1091. Verges, J., Ramos, V.A., Meigs, A., Cristallini, E., Bettini, F.H., Cortes, J.M., 2007. Crustal wedging triggering recent deformation in the Andean thrust front between 31◦ S and 33◦ S: Sierras Pampeanas–Precordillera interaction. J. Geophys. Res. 112, B03S15. http://dx.doi.org/10.1029/2006JB004287. Vidal, A., 2008. Thrust faulting on Mars: implications for early Martian heat flux. PhD. University of Colorado, Boulder, Colorado, p. 121. Ward, S.N., 2002. Planetary cratering: a probabilistic approach. J. Geophys. Res. 107 (E4), 5023. http://dx.doi.org/10.1029/2000JE001343. Watters, T.R., 2003. Thrust faults along the dichotomy boundary in the eastern hemisphere of Mars. J. Geophys. Res. 108. http://dx.doi.org/10.1029/2002JE001934.