Clues to the subsurface fault pattern of circum-Tharsis wrinkle ridges

Introduction

Wrinkle ridges are significant landforms on planetary bodies, and most of them occur in flood basalt units of large igneous provinces, [1]-[2]. On Mars, the circum-Tharsis wrinkle ridge system developed under compressional stresses associated with the response of the lithosphere due to the Tharsis volcanic load [1]. The morphology of ridges shows large variations and may reflect subsurface fault patterns [3]–[5]. Numerous studies on their physical dimensions [6]–[10], their accommodated horizontal strain (e.g., [11]–[12]), as well as a variety of conceptual formation models (e.g., [13]–[17]) have been performed to better understand the morphologies and geodynamic significance of wrinkle ridges. A variety of tectonic models including buckling, thrust/reverse faulting, fault-bend folding,  and fault propagation folding have been proposed to explain the formation of wrinkle ridges(e.g., [9]–[19]).

Even though there are many studies on wrinkle ridges, it is still uncertain what the subsurface of these structures looks like. To get insights into the subsurface we selected sites, where deep morphological incisions provide such exposures. Hence, we used steep escarpments formed by impact craters, collapse pits, and valleys. A prerequisite for this study is the availability of high-resolution remote sensing data and digital elevation models to investigate the fault patterns that exist in the subsurface of wrinkle ridges.  

Methodology

We used High-Resolution Imaging Science Experiment (HiRISE) (~0.25 m/px) [20], and Context Camera (CTX) (~6–7 m/px) [21] satellite imageries to generate high-resolution digital elevation models (DEMs) by using the Ames Stereo Pipeline [22] in combination with the Integrated System for Imagers and Spectrometers (ISIS) software [23]. CTX and HiRISE DEMs with the digital raster graphic (DRG) files were used to analyze and measure topographic offsets. We have selected twelve different study areas (with multiple outcrops from A to D) that all belong to the system of circum-Tharsis wrinkle ridges. Our area of interest includes regions at Solis Planum, at the borders of Nilus Dorsa, at the Coprates Chasma, at the south of Lunae Planum, and the Thaumasia Planum that shares significantly akin structures with a south of the Mela's Fossae (Fig. 1).

To measure the strike and dip of fault planes we used two methods: (i) we applied the LayerTools [24]  add-in for ArcGIS Software and (ii) we constructed manually the strike of faults by connecting points along the fault trace that have of the same elevation. The dip angle is determined perpendicular to the strike direction by recording intersection points of the fault trace with different elevation levels. We mapped all fault intersection lineations (red lines) on wrinkle ridges.

Results

Here, we present only two of twelve case study results. Fig. 2 (study area 1) shows that folding and faulting are intimately linked to each other. The outcrop sections show that the slopes of the wrinkle ridge are formed by the limbs of a vergent anticline. The dip of two subordinate thrust faults with NNW-SSE strike directions could be determined (38° and 46°). In Fig. 3 (study area 2), the western part of the flat crater floor is elevated by ~100 m with respect to the eastern crater floor. Along with this occurs a change in polarity of the fault with a dip direction to the east in the northern crater section and a westward dip in the southern crater section. The wrinkle ridge shows complex fault pattern north and south of the crater, where faults cut obliquely through the wrinkle ridge.

Discussion and Conclusion

Both reverse (>45°) and thrust (<45°) faults are frequent in the subsurface of wrinkle ridges and along with the anticlinal folding document that horizontal compression is the driver for their formation. A multitude of subsidiary and splay faults exist. Symmetric wrinkle ridges contain a conjugate system of thrusts or reverse faults. Asymmetric wrinkle ridges have one dominant reverse/thrust that reaches the surface at the base of the steeper slope. In such cases, additional antithetic faults are subordinate and merge into the main fault. A polarity change of wrinkle ridges can take place along strike and is associated with a change in the amount of displacement that is accommodated along the faults. The fault with the largest amount of slip is situated beneath the ridge crest and steeper slope. Several wrinkle ridges display the main thrust fault whose dip angle abruptly gets shallower at a depth of 500-1000 m beneath the surface. The application of fault-propagation fold models to wrinkle ridges [14]-[19] show conditionally the best match to observations.

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