Gneiss Domes in Orogeny
Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting
Published:January 01, 2004
Annia K. Fayon, Donna L. Whitney, Christian Teyssier, 2004. "Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting", Gneiss Domes in Orogeny, Donna L. Whitney, Christian Teyssier, Christine S. Siddoway
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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 two-dimensional 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 unroofing mechanism in metamorphic core complexes. Pressure-temperature-time 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 significant 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 final unroofing and juxtaposition of formerly deep rocks and upper crustal rocks.