Insights on Metamorphic Core Complex Formation from the Sierra Mazatán Case Study

Metamorphic core complexes are critical tectonic elements of lithospheric extension. However, the mechanisms of core complex formation remain controversial. One critical issue is whether low-angle normal faults associated with core complexes formed in their current, mechanically unfavorable orientation. The Sierra Mazatán core complex in NW Mexico was exhumed during the Oligo-Miocene by ~20 km of top-WSW slip on a presently 15°W dipping fault. This core complex preserves key relationships that document its tectonic evolution and provides an important case study for core complex formation. Here we focus on the initial dip of the bounding normal fault.

Multiple lines of evidence document major footwall rotation. Thermochronologic data shows that prior to exhumation, a ~250-300°C difference existed across ~20 km of the footwall in the slip direction, implying 20-50° of footwall tilting (assuming geothermal gradients of 15-40°C/km). Variations in deformation mechanisms in footwall mylonites match this thermal gradient. Geologic relationships confirm major footwall tilting. An Oligocene dike swarm dips 40-50°SW throughout the northern footwall, indicating 40-50° of footwall tilt (assuming initially vertical dikes). Preliminary paleomagnetic data from these dikes (Platzman et al., 2006) supports similar footwall tilt. And finally, we interpret a contact in the easternmost footwall as a Cenozoic unconformity, above which Oligocene sediments dip 40-50° NE. Together, these results strongly support 40-50° of NE footwall tilting since the fault formed, indicating an initial fault dip of 50-60°. The lack of appropriate footwall structures and the presence of normal faults east of the core complex cause us to favor domino-style fault rotation over a rolling hinge model. These results suggest that not all core complexes represent a unique mode of extension but rather can be formed by major slip on an otherwise typical steeply dipping normal fault. It remains unclear how many other Cordilleran core complexes have experienced similar tilting.