Controls on the geometry of fold-thrust belt salients

Abstract

Salients, convex-to-the-foreland curves in the map traces of orogens, typically involve fold-thrust belts formed along the foreland margin of orogens. An examination of the map-view shape, degree of asymmetry, dimensions, and structural trend-line patterns (i.e., the variation in fold trend and fault strike along the length of the belt) of 20 fold-thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed. In order to understand these relationships, we used simple sandbox models to simulate the formation of three of the several types of salients. Specifically, we examined indenter-controlled salients (the geometry of which reflects interaction of a fold-thrust belt with a hinterland indenter), basin-controlled salients (the geometry of which reflects lateral variations in the thickness of predeformational basin fill), and detachment-controlled salients (the geometry of which reflects lateral variations in the strength of a detachment surface). Our models demonstrate that thrusts in these three types of salients initiate with a curved shape, and that rocks within thrust sheets on the limbs of salients can undergo a vertical-axis rotation in response to simple shear, without requiring major rotation of the fault trace. Furthermore, trend-line patterns distinguish basin-controlled from indenter-controlled salients. Specifically, structural trend lines in basin-controlled salients converge at the end points of the curve, while in indenter-controlled salients, they diverge at the end points of the curve. The asymmetry of a salient correlates with the asymmetry of the predeformational basin and/or of the indenter, and with the convergence direction. Thus, our results explain geometric characteristics of individual salients, and address controversies concerning paleomagnetic data in salients. They also may help prioritize petroleum-exploration targets in salients (foreland-basin oil fields concentrate at the apex of basin-controlled salients) because the geologic factors that generate the salient also contribute to localizing oil fields.