Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting

Abstract

Many high-grade metamorphic terrains record isothermal decompression, implying rapid exhumation or heat input during decompression. These terrains commonly contain gneiss domes that are spatially and temporally associated with low-angle normal faults such as those bounding metamorphic core complexes. To understand the thermomechanical relationship of gneiss domes and core complexes, we use twodimensional numerical modeling to evaluate exhumation and cooling rates resulting from diapiric ascent of partially-molten crust, a proposed mechanism for gneiss dome formation, versus exhumation of orogenic crust by low-angle normal faulting, the primary unroofi ng mechanism in metamorphic core complexes. Pressure-temperaturetime paths calculated for vertical ascent rates of 2-20 km/m.y. show that isothermal decompression is possible for rocks within a diapir. In contrast, paths calculated for rocks in the footwall of low-angle normal faults show that cooling occurs as rocks are brought closer to Earth's surface, and the rate at which these rocks cool is controlled by fault dip, displacement rate, and amount of displacement. The amount of heat loss per unit time during decompression increases with fault dip. For exhumation of rocks in the footwall of a very low-angle fault (~10°), decompression paths occur with little cooling (quasi-isothermal), but the shallow dip of the fault does not allow signifi cant decompression. Low-angle normal faulting alone cannot result in isothermal decompression: The presence of gneiss domes in core complexes requires an additional exhumation process, such as diapiric ascent or a more structurally and thermally complex evolution of the detachment system. In the late stages of exhumation, once the rocks have risen to depths of <15 km and experience rapid cooling, detachment faulting may be the primary mechanism of the fi nal unroofi ng and juxtaposition of formerly deep rocks and upper crustal rocks.

Tectonics of the West Antarctic rift system: new light on the history and dynamics of distributed intracontinental extension

“…Overprinting textures of cordierite possibly record decompression of footwall rocks due to translation upon the detachment structure; and the late-tectonic melt-filled, normal-sense shear bands that cut all older structures may be an indication of a small volume of late leucogranite formed by decompression-induced melting (Siddoway et al, 2004b;Korhonen et al, 2007b). The very high rates of cooling recorded by 40 Ar/ 39 Ar mineral cooling data are comparable to rates of advective heat loss that arise from pluton emplacement in cold country rock (Fayon et al, 2004). These observations are possible indications of a component of upward, gravity-driven flow (diapirism) during emplacement of the dome (e.g., Teyssier and Whitney, 2002).…”

The South Tibetan detachment system facilitates ultra rapid cooling of granulite‐facies rocks in Sikkim Himalaya

“…Although the tectonic regimes for such detachment systems are quite distinct from those forming the STDS (crustal extension vs. crustal shortening), the fault geometry and relative contrasts in T and peak metamorphic conditions are comparable. Numerical models of such detachment system geometry suggest that it is not possible to isothermally decompress footwall rocks by normal fault displacement in the absence of an additional heat source [ Fayon et al ., ]. Fayon et al .…”

“…Fayon et al . [] found that relatively steep fault angles (30°) are required to achieve significant decompression in the footwall of a detachment by normal slip on the structure. This results in a juxtaposition of hot footwall rocks against colder hanging wall rocks and thus cooling of the footwall during decompression.…”

Geochemistry and tectonic development of Cenozoic magmatism in the Carpathian–Pannonian region

“…Buck, 1991;Wernike, 1992). The concept evolved mainly toward understanding crustal flow involving extension and thinning of thick and hot crust, leading to the formation of a large diversity of metamorphic core complexes cored by migmatite domes that are bounded by vertical or usually low angle faults (Fayon et al, 2004;Dunkl and Frisch, 2002). Models of crustal partial melting related with crustal flow have been also developed (e.g.…”