The Basin and Range Province is a major region of Cenozoic crustal extension. Our work has concentrated on the northern Basin and Range in Nevada, Utah, and adjacent states, integrating mapping, petrologic, geochronologic, thermochronologic, sedimentary, volcanologic, seismic, and other approaches.
Thermochronology of Extensional Fault Blocks
Major extension in the brittle upper crust is generally accommodated by large, high-angle normal faults. This highly simplified cartoon shows large, domino-style tilt blocks characteristic of many parts of the Basin and Range. During extension, rocks in the footwalls of the normal faults are uplifted and exhumed, forming mountain ranges, whereas rocks in the hanging walls subside, forming the intervening valleys. The angles of faults become shallower as the blocks rotate. The exhumed rocks in the footwalls are key targets for thermochronologic sampling.
Expected FT and He Data Patterns
Before the crust begins to extend, FT and He parameters define a subhorizontal "thermostratigraphy" within the upper crust. For example, apatite FT ages deeper than about 5 km will be reset to zero age by subsurface temperatures hotter than about 120°C. FT ages at depths shallower than about 5 km will become progressively younger at shallower depths.
As the crust extends, samples from various depths will be exhumed toward the Earth's surface. Samples formerly deeper than 5 km will start retaining fission tracks and will yield FT ages dating their times of cooling below about 110°C. Because the cooling was directly associated with major fault slip, the FT ages directly date the time of fault slip and range formation.
