TIMING, MAGNITUDE, AND RATE OF EXTENSIONAL UNROOFING AT THE SIERRA MAZATAN METAMORPHIC CORE COMPLEX, SONORA, MEXICO: RESULTS FROM 40AR/39AR THERMOCHRONOLOGY

Sierra Mazatan in Sonora, Mexico, is the southernmost core complex in the Cordillera. It is included within both the southern Basin and Range and the Gulf Extensional Province, but it is unclear whether Sonoran core complexes are mainly the result of earlier (syn-subduction) extension or later “proto-Gulf” (post-12 Ma) extension. We present new ⁴⁰Ar/³⁹Ar thermochronologic data that constrain the timing, magnitude, and rate of extensional unroofing at Sierra Mazatan. Samples were collected from footwall rocks in a transect parallel to the slip direction (top to the WSW). Kspar samples from the eastern part of the range yield age spectra that are flat at ~20 Ma at low T and climb to ~26 Ma at high T. Diffusion modeling indicates that the eastern part of the range resided at ~300°C prior to 20 Ma and then cooled rapidly after 20 Ma. Kspar samples from progressively westward yield flatter age spectra that young in the direction of upper plate slip. The westernmost Kspar sample yields a nearly flat spectrum that climbs from 16 to 18 Ma.

These data indicate that footwall rocks cooled rapidly more than 200°C (~70°C/m.y.) from 20-16 Ma. Cooling of this rate and magnitude by tectonic exhumation indicates that rapid extension initiated at ~20 Ma and continued until ~16-15 Ma. A 12.5 Ma ignimbrite unconformably overlies eastern footwall rocks, demonstrating that major extensional slip was completed by that time. The western (structurally deepest) mylonitic part of the footwall was well above Kspar closure temperature (>350°C) at the start of extension and records rapid extension-related cooling to <200°C by 16 Ma. Depending on the paleogeothermal gradient and initial fault dip, this data implies 12-30 km of slip on the detachment fault with a slip rate of 3-7.5 km/m.y. The spatial array of thermochronologic data indicate that the western part of the range was 100-150°C hotter than the eastern part prior to extension, suggesting that Sierra Mazatan has been tilted 30-50° eastward since 20 Ma. This would restore the detachment fault to an initial dip of 40-60° W. Thus, Sierra Mazatan was rapidly unroofed during the early-mid Miocene (20-15 Ma) by large-scale slip on a rotational detachment fault, and this large-magnitude extension is distinctly older than the post-12 Ma “proto-Gulf” extension related to the change in plate boundary configuration.