EVIDENCE FOR NORTHWESTERLY EXTENSION IN THE TUCSON BASIN, ARIZONA, USA, FROM PSEUDOTACHYLITE BRECCIA ALONG THE MAIN DETACHMENT FAULT OF THE SANTA-CATALINA RINCON MOUNTAINS CORE COMPLEX

Pseudotachylite along the Santa-Catalina detachment fault at Tanque Verde Ridge in the Rincon Mountains near Tucson, Arizona occurs as a mappable (1:24,000 scale) unit up to 30 m-thick that consists of as many as three distinct divisions. A lower division is composed of slabs of semi-intact mylonitic footwall granite mixed with large (>50 cm) suspended clasts of the same granite floating in a matrix of pseudotachylite. Mylonitic stretching lineation in the semi-intact blocks are generally parallel to lineations in the core of the core complex (~060) as are some of the lineations in the suspended blocks. The lower division grades into a middle division in which angular to sub-angular, cobble-pebble clasts of mylonitic granite are suspended in pseudotachylite. The clasts are highly concentrated, with the texture varying from clast- to matrix-supported. Many of the clasts are elongate, generally parallel to a stretching lineation they inherited (presumably) from the footwall core prior to formation of the pseudotachylite breccia. The clasts record a consistent elongation direction of 312 (n=15) along a 5 km-long exposure of the fault. An upper division of the mappable unit, consisting of matrix-supported pseudotachylite breccia with 0-5% <2cm clasts is locally present above a sharp, yet irregular contact with the middle division.

The elongation lineation of 312, oriented at right-angle to the average stretching lineation in the core of the Santa Catalina Rincon Mountains core complex is consistent with the interpretation that the Santa-Catalina detachment underwent a phase of intense, northwesterly-southeasterly shear after the core’s northeasterly stretching lineation was developed. An important phase of northwest-southeasterly extension is well-documented in the immediate hangingwall where an array of early to mid-Miocene normal faults accommodate at least 100% extension. The hangingwall also includes a large (>20 km diameter) west-facing silicic caldera fragment (Mt Fagan) that is the same age, has the same phenocryst mineralogy, and the same paleomagnetic polarity as the upper Cretaceous (~73Ma) Cat Mountain Rhyolite whose caldera lies in the Tucson Mountains 35 km west-northwest of Tanque Verde Ridge. An extension vector of 315 would restore the two caldera fragments.