Does shallow dike intrusion and widening remain a possible mechanism for graben formation on Mars?

Shallow dike intrusion and widening was proposed several decades ago as a cause of surface faulting and graben formation on Earth. This hypothesis was subsequently applied to the spectacular linear and/or radial graben systems visible on Mars. However, a recent study has suggested that shallow dike intrusion on Mars results in contractional folding and uplift adjacent to dike walls rather than extensional faulting and subsidence above the dike tip, even in the presence of concurrent regional extension. Here, discrete element numerical modeling is used to re-examine the typical style of deformation above the shallow tips of widening dikes on Mars. The dikes are embedded in a frictional, cohesive material representing the Martian crust. In the experiments presented here, subsidence and extensional faulting (graben formation) are produced above the dike tips, even with modest amounts of widening. For suggested depths to dike tips on Mars, an overlying graben is produced in all cases; no upright detachment-style folds are produced. Results indicate that dike widening does indeed remain a possible mechanism for graben formation on Mars. Implications for the interpretation of deformation associated with shallow dike intrusion on Earth and distant planetary surfaces are discussed. INTRODUCTION Shallow intrusion of magmatic dikes has long been associated with deformation at the Earth’s surface. At a regional scale, dike intrusion has been shown to be intimately associated with extensional faulting in active rift settings (e.g., Belachew et al., 2013). On a more local scale, observational and geophysical data, together with analogue and numerical modeling studies, have supported the link between dike intrusion and faulting, graben formation, and limited uplift at the Earth’s surface (e.g., Mastin and Pollard, 1988; Rubin, 1992; Rowland et al., 2007; Hollingsworth et al., 2013). This hypothesis has subsequently been applied to the spectacular linear and/or radial graben systems seen on both Mars and Venus (e.g., Scott et al., 2002; Wilson and Head, 2002; Grindrod et al., 2005; Pollard and Fletcher, 2005; Goudy and Schultz, 2005; cf. Fig. 1A). In the Tharsis region of Mars, the radial graben systems are characterized by a simple morphology: long narrow graben with pit craters, bounded by normal faults, with a downthrown flat floor unbroken by antithetic faults (Wyrick and Smart, 2009a; Fig. 1B). However, a recent discrete element study has suggested that this hypothesis is not sufficient to explain these features on Mars (Wyrick and Smart, 2009a), even in the presence of concurrent regional extension (Wyrick and Smart, 2009b). This assertion has been repeated in subsequent papers (Smart et al., 2011; Wyrick et al., 2015). Somewhat surprisingly, in these discrete element models, cover deformation took the form of uplift and contractional (detachment-style) folding adjacent to the dike walls rather than faulting and subsidence above the dike tip (cf. Pollard and Fletcher, 2005; Figs. 1A and 1C). Furthermore, these models required the growth of extremely wide, kilometric-scale dikes to produce significant deformation in the cover. These results seem to contradict physical, analytical, and boundary element modeling studies that predict a zone of horizontal stretching and associated faulting above the tip of a shallow, widening dike (e.g., Mastin and Pollard, 1988; Rubin, 1992; Pollard and Fletcher, 2005; Fig. 1A). The question then arises as to *E-mail: [email protected] GEOLOGY, February 2016; v. 44; no. 2; p. 107–110 | doi:10.1130/G37285.1 | Published online 22 December 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. D C A