Limits on the brittle strength of planetary lithospheres undergoing global contraction

Abstract

The degree and depth of fracturing of the lithospheres of Mars, Mercury, and the Moon remain poorly known. It is these two properties, however, that govern the mechanical behavior of a planetary lithosphere. Considering the lithosphere as a cohesive rock mass that consists of small and large blocky, interlocked rock fragments, as opposed to an intact body or a body entirely lacking cohesion, provides insight into the effect of lithospheric fracturing on tectonic processes on these bodies. Characterization of the near‐surface lithospheric brittle strength that incorporates the degree of fracturing is necessary for a realistic assessment of the lithospheric response to global contraction resulting from interior secular cooling. Such an assessment shows that all of these bodies could have accommodated substantial amounts of global contraction prior to the formation of thrust fault‐related landforms. In fact, their lithospheres were sufficiently strong so as to experience changes in radius of as much as 2.2 ± 0.4 km (Mars), 2.1 ± 0.4 km (Mercury), and 1.4 ± 0.3 km (the Moon) prior to the onset of widespread thrust faulting. These values imply that the process of global contraction begins before any evidence of it is established in the geologic record, requiring an earlier onset, and for Mars and Mercury a faster initial strain rate, of global contraction than previously thought. Such results add a heretofore unrecognized component of planetary radial decrease with implications for timing and strain rate to studies of global contraction on Mars, Mercury, and the Moon, in particular, and to planetary bodies, in general.

Widespread effusive volcanism on Mercury likely ended by about 3.5 Ga

“…This collocation of many of the youngest effusive volcanic units on Mercury with impact structures is consistent with predictions for a planet undergoing contraction from secular interior cooling [Solomon, 1978]. Although the importance to volcanism of impacts on Earth has been controversial Geophysical Research Letters 10.1002/2016GL069412 [Ivanov and Melosh, 2003;Glickson, 2004;Elkins-Tanton and Hager, 2005], the impact process removes overburden, promotes subsurface uplift, relaxes preexisting stresses, substantially fractures the lithosphere and thus provides pathways along which magma may ascend [e.g., Klimczak, 2015], and introduces heat into the mantle that may trigger the production of melt [Roberts and Barnouin, 2012]. Impact structures are therefore prime sites for late-stage eruptions in a tectonic regime otherwise generally unfavorable to extrusive activity.…”

“…In a tectonic regime governed by global contraction, the least compressive stresses act vertically and are governed by the overburden, whereas the most compressive stresses act in the horizontal plane. Such a stress field is compatible with the formation of thrust faults, once the brittle strength of the lithosphere is reached [e.g., Klimczak , ]. Yet a globally compressive stress state where the magnitude of the horizontal stress component exceeds that of the vertical component inhibits the vertical ascent of magma [e.g., Glazner , ; Hamilton , ; Watanabe et al ., ] and so is not readily conducive to widespread effusive volcanism [e.g., Solomon , ; Marrett and Emmerman , ].…”

Impact-induced changes in source depth and volume of magmatism on Mercury and their observational signatures

“…However, due to unfavorable illumination geometry 7 km is likely an underestimate of the total contraction 36 . In addition, part of the radius decrease is accomodated without any manifestation in the geologic record 37 . This “invisible” component can be as large as 2.5 km for Mercury, potentially bringing the total inferred contraction to 9.5 km 37 .…”

“…In addition, part of the radius decrease is accomodated without any manifestation in the geologic record 37 . This “invisible” component can be as large as 2.5 km for Mercury, potentially bringing the total inferred contraction to 9.5 km 37 . The set of parameters characterizing the baseline model can be regarded as representative of Mercury.…”

A rock-mechanical assessment of Mercury's global tectonic fabric

“…It follows, then, that the minimum amount of global contraction required to produce the observed faulting is that recorded by observed faults, in addition to the portion of global contraction that Mercury's lithosphere could resist prior to faulting. As shown by Klimczak et al (2014), Mercury's lithosphere was sufficiently strong to withstand a radius change of 0.29 km (RMR 45) to 1.59 km (RMR 75) prior to the initiation of brittle failure.…”