Earth and Planetary Physics (Mar 2023)
Formation and modification of wrinkle ridges in the central Tharsis region of Mars as constrained by detailed geomorphological mapping and landsystem analysis
Abstract
Wrinkle ridges are common landforms documented on all rocky planets and the Moon in the inner solar system. Despite the long research history, their formation mechanisms remain debated. A key unresolved issue is whether the wrinkle-ridge formation is related to igneous processes. This is because wrinkle ridges are mostly associated in space and possibly in time with the occurrence of flood-basalt volcanism in all cases in the inner solar system. To address this issue, we conducted geomorphological mapping, a topographic-data analysis, and a detailed landform and landsystem analysis of satellite images at a resolution of 25 cm/pixel to 6 m/pixel in the central Tharsis region of Mars. The main results of this work are in the form of (1) a regional geomorphological map at a resolution of 6 m/pixel and (2) a local geomorphological map at a resolution of 50 cm/pixel. Our work suggests the following older-to-younger sequence of geological events in the study area: (1) formation of a northeast-trending mountain range (i.e., the Thaumasia plateau) along the eastern margin of the Tharsis rise that was created by the Himalayan-style crustal-scale thrusting; (2) coeval volcanic-plateau construction west of the thrusting-induced rising mountain range; (3) eastward-flowing lavas that were sourced from a volcanic plateau to the west terminated at the rising Thaumasia plateau to the east; (4) wrinkle-ridge development by decollement folding of recently emplaced warm, ductile volcanic-lava piles; (5) emplacement of a regionally extensive ice sheet over the central Tharsis region that produced extensive boulder-bearing materials, striated surfaces, and boulder-bearing dendritic-ridge networks possibly representing subglacial eskers; and (6) local deposition of highly concentrated glacial flours resulted in the formation of mantled terrain on plains between wrinkle ridges. Our work supports the early suggestion that the Tharsis wrinkle ridges were created by horizontal shortening induced by crustal-scale tectonic processes. In detail, however, the occurrence of flow-front-like fold margins associated with many mapped wrinkle ridges suggests the involvement of ductile-flow deformation during ridge formation. We attribute the flow-like fold fronts to ductile deformation of thermally weakened lava piles that were emplaced during or immediately before the folding event. Our compression-induced wrinkle-ridge model also differs from the early hypotheses in that the thin-skinned folding is associated with basement subduction, which explains the lack of coeval and parallel folding and extensional faulting associated with wrinkle ridge formation in the study area. Post-folding glacial modification means that the present wrinkle-ridge morphologies may differ significantly from the original fold shapes, which prevents the utility of using topographic profiles across wrinkle ridges for inverting the underlying thrust geometries.
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