Thermal modeling of extensional tectonics: Application to pressure‐temperature‐time histories of metamorphic rocks

One‐ and two‐dimensional finite difference models are used to generate theoretical pressure‐temperature‐time (PTt) paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere (“pure shear”) and via movement of crustal blocks along rooted large‐scale low‐angle normal faults (“simple shear”). The temperature‐time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies, although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear, and experience a greater amount of posttectonic cooling. The depth‐temperature paths of rocks uplifted from deep crustal levels (30 km depth) via pure shear extension are often characterized by a period of nearly isothermal (high dP/dT) uplift during the early stages of extension. A modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces PTt paths characterized by a more protracted period of nearly isothermal uplift. In contrast, simple shear uplift along low‐angle normal faults produces almost linear depth‐temperature paths with moderate dP/dT, and isothermal uplift is not observed. For a given initial geotherm, the single factor controlling the PTt paths of rocks unroofed below gently dipping normal faults that dip less than 30° is the rate at which unroofing takes place (unroofing rate is defined as the rate of uplift relative to the surface), and the syntectonic PTt paths are insensitive to fault dip or horizontal displacement rate. Current geothermometric and geobarometric techniques yield typical uncertainties in real pressure‐temperature and temperature‐time data of ±1 kbar and ±50°C. Thus it will be difficult to distinguish between these various extensional mechanisms on the basis of PTt data derived from a single sample or from a single structural level, although use of data from samples collected from a broad range of structural levels may improve resolution. Ultimately, the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to ±0.5 kbar and ±25°C.