Potential Sources of Hesperian Contractional Tectonics on Mars

Introduction: The formation of wrinkle ridges on early Hesperian volcanic plains constituted the most significant episode of contractional deformation in the history of Mars. Current hypotheses for the source of this deformation, based largely on photogeological mapping and basic physiographic relations, run the gamut from local and regional [e.g., 1, 2] to global mechanisms [e.g., 3]. Here we review the potential contribution to contractional strain of a variety of mechanisms in order to exclude those processes unlikely to be important and to isolate those worthy of further study. Background: Volcanic plains materials of early Hesperian age occupy ~30% of the surface of Mars [e.g., 4]. Subsequent contractional deformation formed wrinkle ridges on these units [1-6]. While the characteristics of these ridges have formed the basis for studies of Martian lithospheric structure [1, 5, 7, 8], the source(s) of contractional deformation remains an open question [1-3]. Most of the ridges are generally concentric to Tharsis, implying a contribution from the long-wavelength stress field associated with support of the Tharsis rise, but the relative importance of Tharsis loading to local loading by the plains themselves, and global effects has not yet been quantified. Potential Mechanisms: We divide sources for contractional tectonic features on Mars broadly into two categories: global and local/regional. Examples of global processes include: (i) global contraction due to long-term cooling of the interior [e.g., 9], (ii) cooling of the mantle via extraction of heat during an episode of high volcanic flux, and (iii) a large excursion in surface temperature [e.g., 10]. Possible local and regional sources include: (iv) loading of the lithosphere by the volcanic plains [e.g., 1], (v) response to loading by bodies of standing water [e.g., 11], and (vi) a response to loading by ice during a large change in obliquity [e.g., 12. ]. A process with both global and regional attributes is (vii) response to loading of the lithosphere by the Tharsis rise [e.g., 2]. Analysis: (i) Global Thermal Contraction.  With a parameterized model of convection [13] for Martian thermal evolution that is coupled to a tectonic model of an elastic shell overlying an inviscid core [14] we calculate the amount of surface horizontal strain and strain rates through time due to planetary heat loss and inner core growth. Results from a suite of model runs that include a temperatureand pressure-dependent, wetolivine mantle rheology, the effects of crustal generation, and a range in core sulfur contents of 0-15 wt % suggest maximum secular strain rates of order 10 s in the Hesperian. This rate gives ~0.003% strain in 100 Myr. The effects of solid-solid phase changes in the mantle (e.g., basalt-eclogite) are less by two orders of magnitude. Turning the global contraction problem around: if the 0.3% strain recorded in Lunae Planum wrinkle ridges [5] is equivalent to the global average contractional strain at the time of formation and were due solely to inner core growth, that would imply an 18% change in core volume (e.g., change in fractional inner core radius from 0 to 0.56) would be required during the Hesperian. (ii) Contraction Due to an Hesperian Volcanic Pulse.  The substantial volcanism experienced in the Hesperian [e.g., 4, 15] suggests that secular cooling of the interior may have been punctuated by distinct events. An approximate calculation of the average drop in mantle temperature due to the extraction of partial melt can be made from: