Amenthes Rupes is the topographic expression of a main fault belonging to a thrust fault system located parallel to the martian dichotomy boundary. A 3D forward model has been applied to the Amenthes thrust fault system, constraining fault geometries at depth, variations of slip along strike, and structural parameters controlling the formation of fault propagation folds. Our results provide a complex 3D view of the tectonic framework of the area, with implications for tectonic evolution, regional shortening distribution, and the main mechanical discontinuities in the lithosphere. The modeled fault surfaces show planar morphologies combined with listric geometries at depth. The obtained depths of faulting for the major faults of this fault system suggest a depth of the brittle‐ductile transition (at the time of formation) of 20–24 km, somewhat shallower than previous estimates for this area. A possible mechanical discontinuity located at 10.5–13 km deep can be deduced from the faulting depths of the secondary faults. The listric geometries at depth imply that slip is transmitted from the decollement, which, together with the inclusion in the model of secondary and subsidiary faults, allow us to estimate the horizontal shortening recorded in this area ranging from 2–3 km up to ~5.5 km in the southeastern part of the fault system. This range increases the previous shortening estimates in this area between ~60% and ~200%. Consequently, global shortening estimates based on global fault maps are biased by the detail of mapping, and shortening would substantially increase if secondary faults were included.
<p><strong><span lang="en-US">Introduction</span></strong></p> <p><span lang="en-US">The structural modeling of lobate scarps, the topographic effect of surface breaking large thrust faults, have been accomplished by different techniques. 2D Mechanical forward dislocation models (</span><span lang="en-US">Grott et al., 2007; </span><span lang="en-US">Ruiz et al., 2008;</span> <span lang="en-US">Egea-Gonzalez et al., 2017</span><span lang="en-US">, </span><span lang="en-US">Herrero-Gil et al., 2019</span><span lang="en-US">) and 2D balance area models (</span><span lang="en-US">Mueller et al. 2014, </span><span lang="en-US">Herrero-Gil et al., 2019</span><span lang="en-US">) were applied to Martian lobate scarps. </span><span lang="en-US">The r</span><span lang="en-US">ecent development of robust 3D </span><span lang="en-US">structural </span><span lang="en-US">modeling software (</span><span lang="en-US">MOVE</span><sup><span lang="en-US">TM</span></sup><span lang="en-US"> software suite, Petroleum Experts Ltd.;</span> <span lang="en-US">used for this study through the </span><span lang="en-US">Academic </span><span lang="en-US">L</span><span lang="en-US">icence </span><span lang="en-US">of the</span><span lang="en-US"> Universidad Complutense de Madrid) have allowe</span><span lang="en-US">d</span><span lang="en-US"> a more detailed 3D modeling approach that was successfully applied to Og</span><span lang="en-US">y</span><span lang="en-US">gis Rupes (</span><span lang="en-US">Herrero-Gil et al., 20</span><span lang="en-US">20a</span><span lang="en-US">) and the Amenthes Thrust Fault System (</span><span lang="en-US">Herrero-Gil et al., 20</span><span lang="en-US">20b</span><span lang="en-US">). Here we show the results of the </span><span lang="en-US">obtained</span><span lang="en-US"> 3D structural model</span><span lang="en-US">s</span> <span lang="en-US">for</span><span lang="en-US"> 3 thrust faults </span><span lang="en-US">at</span><span lang="en-US"> the southern </span><span lang="en-US">boundary of the Warrego Rise </span><span lang="en-US">(</span><span lang="en-US">i.e. the boundary between the Thaumasia highlands and the adjacent Aonia Terra to the South</span><span lang="en-US">)</span><span lang="en-US"> and the two main faults that make up Phrixi Rupes (</span><span lang="en-US">located in </span><span lang="en-US">Aonia Terra) </span><span lang="en-US">(Fig.1)</span><span lang="en-US">.</span></p> <p><img src="" alt="" width="474" height="308" /></p> <p><strong>Figure 1.</strong> Location of the studied thrust fault</p> <p>&#160;</p> <p><strong><span lang="en-US">Method</span></strong></p> <p><span lang="en-US">The structural modeling procedure consists in a pure geometrical approach assuming volume conservation. </span><span lang="en-US">T</span><span lang="en-US">he deformation of the hanging wall is </span><span lang="en-US">reproduced</span> <span lang="en-US">by fault parallel flow (</span><span lang="en-US">Egan et al., 1997; Ziesch et al., 2014</span><span lang="en-US">) and the propagation folding is modeled following the trishear algorithm (</span><span lang="en-US">Erslev, 1991; Allmendinger, 1998</span><span lang="en-US">). Full details of the modeling procedure can be found in </span><span lang="en-US">Herrero-Gil et al. </span><span lang="en-US">(</span><span lang="en-US">20</span><span lang="en-US">20</span><span lang="en-US">a,</span><span lang="en-US">b</span><span lang="en-US">)</span><span lang="en-US">.</span></p> <p>&#160;</p> <p><strong><span lang="en-US">Structural </span><span lang="en-US">m</span><span lang="en-US">odeling of thrust faults </span><span lang="en-US">at</span><span lang="en-US"> the southern boundary of Warrego Rise</span></strong></p> <p><span lang="en-US">Three faults, producing the main thrusting of Thaumasia's Noachian basement over Aonia Terra, have been modeled in this area </span><span lang="en-US">(Fig. 2)</span><span lang="en-US">. The obtained </span><span lang="en-US">maximum fault slips range between 3366-2431 m, the fault dips of the upper planar part range from 33&#186; to 40&#186;, and the depths of faulting from 13 to 27 km. All the studied faults show listric geometries at depth indicating that they root at a detachment level.</span></p> <p>&#160;</p> <p><strong><img src="" alt="" width="811" height="775" /></strong></p> <p><strong>Figure 2.</strong> 3D model of the Warrego Rise thrust faults</p> <p>&#160;</p> <p><strong><span lang="en-US">Structural mod</span><span lang="en-US">e</span><span lang="en-US">ling of the thrust faults of Phrixi Rupes</span></strong></p> <p><span lang="en-US">Two </span><span lang="en-US">large</span><span lang="en-US"> faults generating Phrixi Rupes have been modeled </span><span lang="en-US">(Fig.3)</span><span lang="en-US">. Their modeled maximum slips are 2847 m and 801 m, with common dip angles for the upper planar part of 33&#186;-33,6&#186; and depths of faulting ranging from 16 </span><span lang="en-US">to </span><span lang="en-US">19 km.</span></p> <p><strong><img src="" alt="" width="832" height="415" /></strong></p> <p><strong>Figure 3.</strong> 3D model of Phrixi Rupes thrust faults</p> <p>&#160;</p> <p><strong><span lang="en-US">I</span><span lang="en-US">mplications for the Circum-Tharsis tectonic contraction</span></strong></p> <p><span lang="en-US">The modeling results of this study together with the model for Ogygis Rupes </span><span lang="en-US">(</span><span lang="en-US">Herrero-Gil et al., 20</span><span lang="en-US">20a</span><span lang="en-US">) </span><span lang="en-US">show some remarkable characteristics common to all the studied faults in this area: (1) all the main faults (excluding two subsidiary backthrusts </span><span lang="en-US">in </span><span lang="en-US">Ogygis Rupes) show an out-of-Tharsis vergence, (2) all of them have listric geometries at depth suggesting that they root at a detachment </span><span lang="en-US">plane</span><span lang="en-US">, (3) the depth of faulting of the main faults </span><span lang="en-US">gets shallower </span><span lang="en-US">in an out-of-Tharsis direction. </span><span lang="en-US">Thus, o</span><span lang="en-US">ur 3D structural modeling results support </span><span lang="en-US">a commo</span><span lang="en-US">n</span><span lang="en-US"> detachment level for </span><span lang="en-US">all the modeled structures</span> <span lang="en-US">controlling a thick-skin </span><span lang="en-US">tectonic style</span><span lang="en-US">.</span></p> <p>&#160;</p> <p><span lang="en-US"><strong>References</strong></span></p> <p><span lang="en-US">Allmendinger, R. </span><span lang="en-US">(</span><span lang="en-US">1998</span><span lang="en-US">)</span><span lang="en-US">. Inverse and forward numerical modeling of trishear fault-propagation folds. Tectonics 17 (4), 640&#8211;656.</span></p> <p><span lang="en-US">Egan, S.S., Buddin, T.S., Kane, S.J., Williams, G.D. </span><span lang="en-US">(</span><span lang="en-US">1997</span><span lang="en-US">)</span><span lang="en-US">. Three-dimensional modelling and visualization in structural geology: new techniques for the restoration and balancing of volumes. In: Proceedings of the 1996 Geoscience Information Group Conference on Geological Visualization. In: Electron Geology, 1, 67&#8211;82.</span></p> <p>Egea-Gonz&#225;lez, I., Jim&#233;nez-D&#237;az, A., Parro, L. M., L&#243;pez, V., Williams, J.-P., Ruiz, J. (2017). Thrust fault modeling and Late-Noachian lithospheric structure of the circum-Hellas region, Mars. Icarus, 288, 53&#8211;68.</p> <p><span lang="en-US">Erslev, E. </span><span lang="en-US">(</span><span lang="en-US">1991</span><span lang="en-US">)</span><span lang="en-US">. Trishear fault-propagation folding. Geology 19, 617&#8211;620.</span></p> <p>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 the Noachian heat flux. Icarus, 186(2), 517&#8211;526.</p> <p><span lang="en-US">Herrero-Gil, A., Egea-Gonz&#225;lez, I., Ruiz, J., Romeo, I. </span><span lang="en-US">(</span><span lang="en-US">2019</span><span lang="en-US">)</span><span lang="en-US">. Structural modeling of lobate scarps in the NW margin of Argyre impact basin, Mars. Icarus 319, 367&#8211;380.</span></p> <p><span lang="en-US">Herrero-Gil, A., Ruiz, J., Romeo, I. (2020</span><span lang="en-US">a</span><span lang="en-US">). 3D modeling of planetary lobate scarps: The case of Ogygis Rupes, Mars. Earth and Planetary Science Letters, 532, 116004.</span></p> <p><span lang="en-US">Herrero&#8208;Gil, A., Ruiz, J., Romeo, I. (2020</span><span lang="en-US">b</span><span lang="en-US">). Lithospheric contraction on Mars: a 3D model of the </span><span lang="en-US">A</span><span lang="en-US">menthes thrust fault system. Journal of Geophysical Research: Planets, 125(3), e2019JE006201.</span></p> <p>Mueller, K., Vidal, A., Robbins, S., Golombek, M., West, C. (2014). Fault and fold growth of the Amenthes uplift: Implications for Late Noachian crustal rheology and heat flow on Mars. Earth and Planetary Science Letters, 408, 100&#8211;109.</p> <p><span lang="en-US">Ruiz, J., Fernandez, C., Gomez-Ortiz, D., Dohm, J.M., Lopez, V., Tejero, R. </span><span lang="en-US">(</span><span lang="en-US">2008</span><span lang="en-US">)</span><span lang="en-US">. Ancient heat flow, crustal thickness, and lithospheric mantle rheology in the Amenthes region, Mars. Earth Planet. Sci. Lett. 270, 1&#8211;12.</span></p> <p><span lang="en-US">Ziesch, J., Tanner, D.C., Krawczyk, C.M. </span><span lang="en-US">(</span><span lang="en-US">2014</span><span lang="en-US">)</span><span lang="en-US">. Strain associated with the Fault-Parallel </span>Flow algorithm during kinematic fault displacement. Math. Geosci. 46, 59&#8211;73.</p>
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