Mid-cretaceous crustal shortening: evidence from a regional-scale ductile shear zone in the Coastal Range of central Chile (32° S)

In the Coastal Range of central Chile, widespread Early Cretaceous volcanism associated with extensional volcanosedimetary intra- or back-arc basins and subsequent basin closures, uplift, and increased erosion/exhumation rates during the mid-Cretaceous suggests a major change from a mainly extensional tectonic regime to a relatively contractional regime and resultant crustal shortening. The author documents the contractional Silla del Gobernador shear zone (SGSZ), which developed at the western boundary of the Coastal Range in central Chile (32° S). This structure corresponds to a high-strain ductile and cataclastic shear zone that developed under low-grade (greenschist facies) metamorphic and fluid-present conditions, which indicates EW-NWW crustal shortening in a compressional (transpressional) regime. UVLAMP 40Ar/39Ar laserprobe dating of neoformed white mica during mylonitic deformation suggests a maximum age for the reverse ductile shearing of 109±11 Ma. An inverse isochron age of 97.8±1.5 Ma from biotite samples of a mylonitized granodiorite suggests the minimum age of deformation. These ages constrain the ductile deformation age to approximately 100 Ma (mid-Cretaceous), coeval with high exhumation/erosion rates that appear to represent uplift of the Coastal Range. The uplift and crustal shortening of the Coastal Range of central Chile has been associated with high spreading rates from the SE Pacific and southern Atlantic convergence during a change from an extensional regime developed during the Early Cretaceous to a more compressional regime that started during the mid-Cretaceous. In this sense, the SGSZ records this tectonic regime change.