The presence of a stable “Megablock” in the southwestern North American Proterozoic craton in northern Mexico

Abstract

The present study, based on combined geophysical, geochronological and isotopic data identifies a cratonic block which we call the Western Chihuahua-Mesa Central megablock (WCMB) in northern Mexico. New Bouguer and isostatic gravity anomaly maps revealed a conspicuously distinct, spatially-extensive gravity minima over a region extending from north western Chihuahua to as far south as the Mesa Central (MC). This region can be differentiated from the surrounding mobile belts with significantly higher gravity anomaly values. Gravity modeling combined with sparse seismically determined crustal thickness estimates indicate that the proposed WCMB region has crustal thicknesses ranging from 38 to 42km. The regions surrounding WCMB are the spatially constrained mobile belts that are characterized by basinal and trough-like structures and are associated with A-type granites and bimodal volcanism. Spatial distribution of enriched initial Sr isotopic data (>0.706) confirms the sialic character of the crust associated with the gravity minimum at least in western Chihuahua. The mobile belts surrounding the WCMB contain geologic records of the events since 1.8–0.9Ga. These ages from within and just adjacent to the block correlate well with the Rodinia Neoproterozoic supercontinent that was assembled 1.3–0.9Ga. We propose that the northwest-southeast directed WCMB is a cratonic region underlain by Proterozoic crust. The WCMB is most likely separated from the deformed and active southwestern part of the North American Proterozoic craton by repeated thermotectonic activities within the surrounding mobile belts. These events have helped to dislocate and transport the block to its present location through processes of repeated compression, extension, and transform faulting.