Denudation of metamorphic core complexes and the reconstruction of the transition zone, West central Arizona: Constraints from apatite fission track thermochronology

Abstract

Apatite fission track thermochronologic results from transects across the Basin and Range and Transition Zone provinces in west central Arizona provide constraints on the denudational history and structural framework of the region. Apatite fission track ages decrease from ∼21 to 14 Ma in the Harcuvar Mountains and from ∼16 to 13 Ma in the Buckskin‐Rawhide Mountains in the slip direction (SW ‐ NE) of detachment faults in the lower plates of the metamorphic core complexes. Mean lengths of confined fission tracks from the core complexes are all >14 μm, indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone (<110°C) to near‐surface conditions. These data give the time at which progressively deeper parts of the lower plates were drawn up through the apatite annealing zone and suggest that the slip rate on detachment faults in the Whipple tilt domain was ∼7–8 mm/yr. The apatite fission track results also indicate that detachment faulting ended at ∼13–14 Ma in this area and, when they are combined with other thermochronologic data, yield cooling rates of >40°C/m.y. for lower plate rocks. Apparent apatite ages in the Transition Zone province generally increase from ∼25 Ma to ∼100 Ma away from the Basin and Range province. This trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the Weaver Mountains and the Colorado Plateau. Gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the Phillips‐Kirkland drill hole reveal parts of denuded Mesozoic and Cenozoic apatite partial annealing zones. These paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks. The reconstructions indicate that the major faults in the Transition Zone province have relative displacements of >1 km and that offset on them occurred mostly after 25 Ma. These data also indicate that most of the rocks now exposed in the Transition Zone of west central Arizona were not exposed until Miocene time.