Appearance
Unraveling Mars's Hidden Plume History Through Wrinkle Ridges
Tharsis is hard to miss. It is a volcanic bulge roughly 6,000 kilometres across, standing more than 20 kilometres above the surrounding plains, built up over something like four billion years. The question people have asked for decades is what put it there. The question I find more interesting is whether whatever put it there stayed in one place.
Wrinkle ridges are a way to ask that. They are the long, wave-like features that run across much of the Martian surface, and each one is a record of the compression that was acting on the crust at the moment it formed. If the compression came from something rising underneath Tharsis, the ridges should be arranged around it.
Mapping 77,000 kilometres of ridges
We mapped 650 individual wrinkle ridges across the western hemisphere, 77,294 kilometres of them end to end, which is close to twice the circumference of Earth. Those were then cut into 34,741 segments of about two kilometres each, and every segment was measured against high-resolution imagery and elevation data.
The logic is straightforward. A ridge driven by stress from a single source should curve around that source, the way ripples curve around the point where a stone went into the water. Measure how far each segment departs from a perfect circle and you can work backwards to where the stress was coming from.
Five centres, not one
The geometry does not resolve one centre. It resolves five, spread across Tharsis:
- C1 near the southern edge of the Alba Mons caldera, in the north
- C2 between Jovis Tholus and Ceraunius Fossae
- C3 between Ulysses Patera and Pavonis Chasma
- C4 at Phoenicis Lacus, near Noctis Labyrinthus
- C5 at Claritas Rupes, in the south
They span 4,518 kilometres from the northernmost to the southernmost. A single fixed hotspot does not produce that.
Which came first
Five centres raise an obvious follow-up: in what order were they active? Where one ridge cuts across another, the one doing the cutting is the younger of the two. Working through enough of those intersections gives a sequence:
C2 → C3 → C1 → C4 → C3 again → C5 → C4 again
It is not a march from north to south. The activity jumps, and two of the centres switch back on after a gap. C4 appears to have been the last one active.
A very gentle wedge
Mapping where the ridges are is one thing. What is underneath them is another. Ridge heights and widths give an estimate of how much the crust shortened, between 1.5 and 3.8 kilometres depending on the profile, and of how deep the faults beneath them reach, between 2.9 and 8.8 kilometres.
Plot those detachment depths against distance from each centre and both the surface and the buried faults turn out to form a wedge, tapering at between 1.2° and 2.2°. That is very shallow. Earth's mountain belts are much steeper.
The wedge shape is the interesting part, because it is what critical taper theory describes, and that theory was built for fold-and-thrust belts on Earth. Tharsis appears to have behaved like an enormous accretionary wedge, with material pushed outward and upward by whatever was rising beneath it.
Something was lubricating the faults
A taper that gentle requires the detachment to be far weaker than rock sliding on rock. Our numbers put the friction coefficient between 0.055 and 0.093. Ordinary rock friction is 0.6 to 0.85. Something was reducing it by close to an order of magnitude.
The explanation that fits best is liquid water held beneath an impermeable layer of frozen ground. The detachment depths we recover, three to nine kilometres, are where Mars's internal heat would have kept water liquid despite the surface temperature. Permafrost above would have sealed it in. With nowhere to drain, the weight of the overlying rock drives the fluid pressure up until the upper crust is very nearly floating, and faults at that level slip easily.
Clay minerals and salt deposits can do a similar job, and both are known on Mars. The evaporites in Valles Marineris, not far away, make that worth keeping on the table.
What this says about Tharsis
The volcanism was not anchored to one spot. It moved across the region over hundreds of millions of years, between the Late Noachian and the Early Hesperian, roughly 3.7 to 3.1 billion years ago. Individual centres also reactivated, which suggests either that the plume branched or that it moved back and forth rather than tracking steadily in one direction.
There is a broad drift from north to south across the sequence, and the most likely reason is the crust itself. Tharsis sits near the dichotomy, where the thin northern lowlands meet the thick southern highlands. A plume running into crust of a very different thickness is liable to be deflected or stalled by it, and that is one way to get branching and reversal out of a single source.
Tharsis matters beyond its own geology: it dominates Mars's topography, it shifted the planet's rotation, and it has been implicated in its climate and in the water that once ran across the surface. The ridges are a way to read all of that off a surface that has sat essentially undisturbed for three billion years, which is a luxury Earth does not offer.
The paper. Karagoz, O., Kenkmann, T., and Hergarten, S. (2024). Unraveling the Tectonic History of the Tharsis Rise on Mars: Plume Migration and Critical Taper Dome. Journal of Geophysical Research: Planets. doi.org/10.1029/2023JE007965
The companion study on the subsurface structure of these ridges is Insights into the subsurface structure of wrinkle ridges on Mars, and the morphometric work that preceded both is What Ancient Ridge Formations Tell Us About the Red Planet's Crust.