Thermal control on post-orogenic extension in collision belts

Abstract

Intracontinental extension develops in two different ways. In the Rhine Graben, the Baikal or the East African Rifts, it is localized in a long and narrow zone, along a few normal faults which often delineate a single graben. In contrast, in the Basin and Range Province or the Tibetan Plateau, it is distributed over a much broader area, up to more than 1000 km across. This difference arises from differences in rheology and applied forces. Widespread extension probably reflects the thinning of a previously thickened crust. Geological observations indicate that only wide orogenic belts undergo late or post-orogenic thinning. A related fact is that anatectic granites seem to be most abundant in wide orogenic belts. The general correlation between belt width and occurrence of both extension and anatectic granites suggest that it reflects in part the characteristics of horizontal heat transfer between thickened crust and surroundings. In wide orogenic plateaux, the large amount of heat produced in the crust by radioactive decay and lack of erosion leads to high temperatures. In narrow mountain belts, horizontal heat transfer and high erosion rates concur to prevent temperatures from rising to values at which the crust may yield in extension. We have tested this hypothesis using transient two-dimensional numerical models of orogenic belts, focussing on the effects of the shortening rate, the final width of the belt and the rate of crustal radiogenic heat production. In the range of geological values, the shortening rate has little influence on the results. Crustal temperatures increase as a function of belt width. We find that the sensitivity of crustal temperatures to belt width is most pronounced in the 100–300 km width range and for large values of crustal heat generation. The pressure-temperature paths followed by geological formations depend on the belt width, which is consistent with available data. The integrated shear strength of the lithosphere decreases with time as the belt is being built and then eroded. This evolution is sensitive to belt width. If extension occurs when the integrated shear strength drops below a critical value, our results yield a framework for comparing different orogenic belts. We predict that extension starts late in the history of wide belts and never occurs in narrow belts. A final implication of this study is that the average rate of crustal heat production is probably close to 1.1 × 10 −6 W/m 3, which is in the upper range of possible values. We present a simple and convenient diagram summarizing the thermal characteristics of orogenic belts as a function of width and shortening rate.