Appearance
Peering Beneath Mars's Wrinkles: What Hidden Faults Tell Us About the Red Planet's Geology
From orbit a wrinkle ridge is a gentle bump. What makes it is a fault, and the fault is underground, which is the awkward part: we cannot drill into Mars to look at it.
Occasionally something else has done the drilling. Impact craters, collapse pits and canyon walls cut straight through wrinkle ridges and leave the subsurface exposed in section. This study went looking for those. Twelve sites across the Tharsis region, with cuts reaching 500 to 1,000 metres below the surface, each one a natural cross-section through a structure that is otherwise invisible.
What the exposures show
The structures underneath are thrust and reverse faults: one block pushed up and over another. The two are the same mechanism at different angles, thrust below 45°, reverse above, and both turn up beneath wrinkle ridges, often in the same structure.
The most consistent observation is that an asymmetric ridge has one dominant fault system, and the main thrust reaches the surface at the foot of the steeper flank. Where a fault plane is exposed cleanly enough to measure, the primary thrusts dip at 37° ± 2° and the backthrusts dipping the other way come in a little shallower, at 28° ± 2°.
At nine of the twelve sites the faults do not keep one angle all the way down. They start steep near the surface and bend to shallower angles somewhere between 500 and 1,000 metres depth. That listric shape is the useful part: a fault that keeps flattening is usually a fault on its way to joining something horizontal.
Few of these ridges are the work of a single clean fault. Most carry a crowd of subsidiary and splay faults branching off the main structure, which is what produces the wrinkled surface the features are named for.
Folding and faulting together
At one exposure in the wall of Ophir Chasma individual rock layers can be traced right through a ridge. They are folded into a lopsided arch, and small thrusts cut across the folded layers, each one matching a step in the topography above it. Wrinkle ridges are not faulting or folding. They are both, at once.
Some ridges also change polarity along their length: a steep eastern flank in one stretch, a steep western flank further on. The asymmetry flips where the dominant fault system changes direction.
Which formation models survive
Several models for wrinkle ridges have been in circulation for decades. Seeing the subsurface lets us discard some of them.
Simple buckling does not survive. Major thrust faults cut through these structures, which rules out elastic bending of an intact layer.
Fault-bend folding is unlikely. It requires the main fault to be shallow where it reaches the surface, and we keep measuring faults breaking the surface at around 38°.
Fault-propagation folding fits. A thrust climbing from a deeper detachment loses displacement as it rises, and the shortfall is taken up by folding of the rock above. That produces an asymmetric fold with a steep forelimb and a steep reverse fault emerging at the base of the ridge, which is what the exposures show. The two-ramp variant of Chester and Chester (1990) also reproduces the stair-stepped profile common here, where a narrow ridge sits offset on a broader swell.
Continuum models are consistent with the same observations: the elastic dislocation model of Watters (2004) and the boundary element model of Okubo and Schultz (2004) both predict blind thrusts, backthrusts forming in weak zones, bimodal topography, and the pairing of a thrust with the steeper flank.
Thin-skinned or thick-skinned
The long-running question is whether these faults connect to a shallow horizontal detachment, with the upper crust sliding more or less independently, or whether they run deep into the crust with nothing to flatten onto.
The listric geometry points to the first. Faults that steepen upward from shallower angles at depth are usually joining something near-horizontal below. Earlier work at Lunae Planum put that detachment at the base of the permafrost, where trapped water under pressure would make a weak layer. These observations are consistent with that, and they do not prove it: the exposures bottom out at 500 to 1,000 metres and the detachment should be at 2.5 to 4 kilometres. We are looking at the top of the structure and inferring the bottom.
At one site, Area 12, there is a conjugate system instead: two fault sets dipping toward each other and meeting at depth, building a pop-up with comparable slopes on both sides. That confirms a configuration proposed by Allemand and Thomas (1992) and Mangold et al. (1998), and it is the exception. Where both flanks of a ridge are faulted, one system almost always dominates.
Are crater exposures representative
It is fair to ask whether a ridge seen in a crater wall has been modified by the impact. In most cases the ridge is clearly younger: crater floors have been uplifted, so faulting continued well after the impact. The craters are windows onto pre-existing structures, not events that rewrote them.
A few ridges are slightly deflected where they cross a crater, and thinner overburden inside a cavity may allow a little more slip, but the fault geometry is what these structures show everywhere else.
The terrestrial comparison
The closest analogue on Earth is the Yakima fold belt in Washington State, where asymmetric anticlinal ridges grew by fault-propagation folding of the Columbia River Basalts. That is a flood basalt province, which is also the setting of most Martian wrinkle ridges. The Yakima folds show the same inventory: steep thrusts at the surface, folded basalt, a detachment at depth, and subsidiary faulting complicating the surface. The detachment there is deeper, but the mechanics are the same, which is part of why the fault-propagation models can be carried across.
What it says about the Martian crust
The crust went through real horizontal shortening, varying with position: about 116 metres nearer the Tharsis centre and 56 metres further out, from the Lunae Planum measurements.
Thin-skinned deformation requires mechanical layering. You cannot decouple the upper crust from the lower crust without a weak layer between them, so the geometry is itself evidence that one existed.
Brittle faulting and ductile folding occurring side by side constrains the conditions in the upper few kilometres during the Hesperian, roughly 3.6 to 3.9 billion years ago. And if the detachment really does sit at the base of a permafrost layer, the depth at which these faults flatten is an indirect measurement of where liquid water was.
Variability is the result, not the noise
The through-line of the study is that wrinkle ridges are structurally diverse. They share a mechanism and very little else. Some are symmetric, most are not. A few have conjugate faults, most have one dominant system. Dips run from 20° to 60°. Some faults are clearly listric and others look planar.
That spread tracks differences in the material being deformed, the depth and character of the weak layer, the local stress, how much shortening was taken up, and whether there were older structures to reuse. Forcing every ridge into one formation model loses that information rather than explaining it.
Limits
The exposures reach 500 to 1,000 metres. The question that matters most, whether there is a regional detachment at three to four kilometres, is below that and stays out of reach. Settling it needs ground-penetrating radar or seismic coverage. InSight has given a first direct look at the interior, and more of that from more regions is what would turn this inference into a measurement.
There is also a selection problem worth naming: a fault visible near the surface may be a subsidiary structure rather than the main one. We worked with the most prominent through-going faults we could identify, and some uncertainty about which are primary remains.
The paper. Karagoz, O., Kenkmann, T., and Hergarten, S. (2022). Insights into the subsurface structure of wrinkle ridges on Mars. Earth and Planetary Science Letters. doi.org/10.1016/j.epsl.2022.117759
Study areas: Solis Planum, Nilus Dorsa, Coprates Chasma, Lunae Planum and Thaumasia Planum, all part of the circum-Tharsis system. Imagery from HiRISE and CTX on Mars Reconnaissance Orbiter, with elevation data from several Mars orbiters.
The morphometric study this builds on is What Ancient Ridge Formations Tell Us About the Red Planet's Crust, and the regional reconstruction that followed is Unraveling Mars's Hidden Plume History.