METAMORPHIC CORE COMPLEX EVOLUTION: INSIGHTS FROM THE SIERRA MAZATÁN CASE STUDY

Metamorphic core complexes continue to be enigmatic features of extensional tectonics. At the center of the controversy is whether core complexes are unique modes of extension that initiated at shallower angles and accumulated larger amounts of slip at faster rates than modern normal faults. Resolving these questions remains central to understanding large-magnitude crustal extension.

Data from the Sierra Mazatán core complex in northwestern Mexico sheds new light on core complex formation. The Sierra Mazatán was exhumed along a >50 km long normal fault that presently dips 10-15° WSW. Fault slip began as early as 25 Ma but mainly occurred from 21-15 Ma based on thermochronology and syn-tectonic basin fill deposits. Geologic constraints limit total slip to <20 km indicating average slip rates of ~3-4 mm/yr. Multiple lines of evidence, including tilted footwall dikes and sediments, and thermochronologic data, indicate the footwall has been tilted ~40-50°, requiring that the detachment fault initiated at a steep dip (50-60°) and rotated to a shallow angle during slip. Little evidence is present for a rolling hinge, suggesting the core complex formed as a simple tilted fault block.

Footwall tilt bears on the origin of fault corrugations found at many core complexes. Fault corrugations at Sierra Mazatán have wavelengths of 40-50 km and map-view amplitudes of 5-15 km, similar to many other core complexes. Restoring this fault to an initial 60° dip reduces fault trace amplitudes to <5 km, similar or less than those at many large, active normal faults. Thus, fault corrugations may have been an original fault feature rather than the result of folding. The characteristic wavelength of these corrugations may result from fault segmentation of ~25 km long segments.

In summary, the Sierra Mazatán core complex formed along an initially steep normal fault as the footwall of a titled fault block at slip rates similar to large active normal faults. Fault corrugations may be original fault features whose trace is exaggerated by tilting. At least some core complexes need not be unique modes of crustal extension but may be the end product of fairly large amounts of slip on a typical high-angle normal fault. An outstanding question is why slip is focused on a single structure at core complexes rather than distributed among many faults.