Quantifying tectonic exhumation in an extensional orogen with thermochronology: examples from the southern Basin and Range Province

Abstract

This paper is part of the special publication No.154, Exhumation processes: normal faulting, ductile flow and erosion (eds: U.Ring, M.T. Brandon, G.S. Lister and S.D. Willett). The Colorado River extensional corridor is a region of large-scale crustal attenuation, where the integration of thermochronological techniques illuminates processes of extension and exhumation. Biotite and K-feldspar 40Ar- 39Ar, and zircon and apatite fission-track ages from metamorphic core complexes in the corridor consistently young in the displacement directions of detachment faults. The onset of rapid extension is denoted by an abrupt change, at c.22-21 Ma, in cooling rate derived from age-closure temperature plots, and in slope of age-distance gradients in footwalls. Linear age-distance trends with slopes of c.3-8 km Ma -1, for low-temperature thermochronometers, give time-averaged displacement rates on detachment faults. Gradients of mineral age across thick tilted hanging wall fault blocks reveal palaeotemperature gradients when the angle of tilting is known. Pre-extensional thermal gradients for tilted blocks in the southern Basin and Range province were c.20-25°C km -1, 17°±5°C km -1, and c.20-24°C km -1. Thermochronological data from the footwall of the Chemehuevi detachment fault reveal a trend of increasing temperature, in the known slip direction, from ≤200°to 350-400°C over an exposed distance of 23 km, at the onset of extension. The gradual increase in temperature constrains the Chemehueyi detachment fault to have had an original dip of c.15-30°, within the brittle upper crust.