Mid-Miocene low-angle normal faulting is an integral part of the regional tectonic strain of the southern Basin and Range. The distinguishing characteristic is the presence of “detachments” — allochthons of moderately to steeply dipping hanging-wall rocks resting in very low angle fault contact on structurally deeper, generally older, footwall rocks. Rotation of detachment strata was accomplished by imbricate listric(?) normal faulting and coordinated tear faulting. Regional structural domains, each thousands of kilometres in area, contain detachments marked by uniform sense of detachment-strata rotation.
The Eagle Pass detachment, 70 km northeast of Tucson, Arizona, reveals informative structural details of parts of the system. Steeply southwest-dipping mid-Miocene volcanic and sedimentary rocks, several thousands of metres in thickness, rest in very low angle (10°±) fault contact on Precambrian granitic basement. Detachment strata were faulted into position by NE-directed translation, accompanied by rotation of strata to steep SW dips. The detachment fault which underlies the 25 km2 detachment has the form of a large northeast-trending mullion trough with relief of at least 500 m. Along the keel of the detachment, Precambrian quartz monzonite was transformed into a hydrothermally altered selvage of microbreccia. The steep (± 80°) northwest margin of the mullion trough served as a tear fault along which quartz monzonite was crushed, volcanics were dragged, and fanglomerate was locally sheared into flattened-pebble conglomerate.
Detachments like that at Eagle Pass resemble slump blocks on a grand scale. They are elements of a regional system of extensional deformation which coincides with the belt of Cordilleran metamorphic core complexes. Mid-Miocene detachment faulting post-dates the formation of exposed mylonitic tectonites of core complex affinity, but the direction of translation of the detachments is parallel to penetrative lineation in nearby and/or underlying tectonite. Moreover, in southeastern Arizona examples, the sense of translation of detachments matches the sense of simple-shear movements disclosed by tectonite fabrics. We speculate that large crustal displacements achieved during extension-induced formation of tectonite produced a geometric and/or dynamic condition which strongly influenced subsequent detachment faulting. Consequently, with renewed(?) or continued(?) extension in mid-Miocene time, deep upper-crustal slumping was triggered and detachment rocks were translated along listric(?) normal faults in the direction and sense of the earlier displacements which had accommodated the formation of tectonite.