USING 40AR/39AR THERMOCHRONOLOGY TO ASSESS THE TILTING AND EXHUMATION OF CONTINENTAL CRUSTAL SECTIONS: AN EXAMPLE FROM THE SIERRA MAZATAN CORE COMPLEX, SONORA, MEXICO

The degree of tilting of a crustal section is a critical parameter in assessing crustal exposure depths.  However, many crustal sections lack structural markers that unambiguously document the magnitude of tilt.  The Sierra Mazatan core complex was tectonically unroofed by large-magnitude WSW slip on a major low-angle normal fault and exposes a crustal section in its footwall composed of Paleocene plutons and associated wall rock.  We demonstrate the utility of ⁴⁰Ar/³⁹Ar thermochronology in constraining the tilt of this footwall crustal section and compare these results with geologic constraints.  Data from this crustal section also yield insights on the tectonic exhumation of this core complex.

⁴⁰Ar/³⁹Ar ages young towards the west for all mineral systems, indicating that the western footwall exposes the deepest structural levels. The thermochronologic data indicate that a ~300 °C temperature difference existed across 25 km of the footwall in the slip direction prior to major slip, implying 25-50° of NE footwall tilt depending on the assumed paleogeothermal gradient (15-30 °C/km).  The higher tilt estimates are supported by geologic data including ~45° SW dipping footwall dikes and ~45° NE dipping footwall sediments.  These results demonstrate that the Sierra Mazatan footwall exposes a relatively continuous crustal section down to depths of ~15-20 km and show that thermochronologic data alone can provide reliable tilt estimates.

Thermochronologic data from this crustal section also provide insights on the tectonic unroofing of the core complex. Accounting for significant footwall tilt would restore the presently low-angle normal fault to a steep initial dip of 45-60° and implies relatively low geothermal gradients prior to extension (~20 °C/km).  The thermochronologic data reveal two distinct pulses of rapid footwall cooling indicating a poly-phase slip history with early slip from 25-23 Ma followed by a major slip event from 21-16 Ma.  Total footwall cooling of >300 °C requires 15-35 km of slip with geologic data constraining slip to <20 km.  Taken together, these data imply fault slip rates of 3-4 mm/yr during major slip.  Thermochronology from this tilted crustal section demonstrate that the core complex formed by moderate amounts of slip on an initially steep normal fault that experienced a multi-phase slip history.