RIFT PROCESSES AT THE VALLES-MARINERIS, MARS - CONSTRAINTS FROM GRAVITY ON NECKING AND RATE-DEPENDENT STRENGTH EVOLUTION

Recent spherical harmonic representations of the gravity field of Mars have sufficient resolution to examine the regional crustal and lithos pheric structure of the Valles Marineris and are used as constraints f or rift modeling. While gravity signatures of individual troughs are n ot evident, a broad 190-260 mgal low indicates that the central chasma ta as a whole are compensated at a depth of 30-80 km, representing the thickness of crust surrounding the troughs. Furthermore, at the time large scale relief was established, the effective thickness of the ela stic lithosphere was <30 km, corresponding to heat flow >20 mW m(-2). The calculated range of crustal thickness and heat flow can be used to test two semianalytic models of rift formation. In the first model, t he distinction between single (''narrow rift'') and multiple (''wide r ift'') troughs is determined by the wavelength of necking instabilitie s. In the second model, narrow-versus-wide morphology is controlled by the evolution of lithospheric strength during rifting. The large-scal e, parallel, multiple troughs of the central Valles Marineris are morp hologically similar to terrestrial wide rifts, but a lack of distinct faulting in some of the chasmata could imply that the faulted troughs may have formed as isolated narrow rifts. Our results are only margina lly consistent with necking leading to a wide rift; allowable combinat ions of crustal thickness and heat now lead to decoupling within the l ithosphere resulting in a second principal necking wavelength that is smaller than the main trough spacing, for which evidence is equivocal. At high heat flow (>40 mW m(-2)), however, necking of the strong uppe r crust alone can yield a shorter wavelength characteristic of a singl e trough, allowing narrow-rift origins of faulted chasmata. In contras t, the inferred range of crustal thickness and heat flow poorly match the narrow-rift regime of the strength-evolution model but are in good agreement with its predictions for wide rifting. Furthermore, the dis tinction between wide rift and core complex in this model places an up per bound on heat flow of 70 mW m(-2).