EXAMINING CENOZOIC EXTENSION ACROSS THE CORDILLERA THROUGH LOW-TEMPERATURE THERMOCHRONOLOGY

Much of the North American Cordillera has been in a state of extension since the Eocene. This is most evident from the metamorphic core complexes found across Mexico, the USA and Canada. However, the duration of these major extensional phases and the extent of crustal thinning across the Cordillera throughout the Neogene and Quaternary remains elusive due to an incomplete sedimentary record and difficult identifying fault activity. With limited physical evidence for extension, low-temperature thermochronology becomes our greatest tool in studying extension. This method helps to infer the timing and duration of tectonic phases through the generation of thermal histories and has been used to study extension across Mexico, Arizona, Nevada, and British Columbia (BC). Collectively, this work depicts a complex pattern of extension stretching from multiphase episodes of fault activity in the north to significant crustal stretching in the south.

We present new apatite (U-Th)/He and fission-track data from BC revealing several phases of extension and exhumation through out the Cenozoic. Results show the Monashee Complex exhumed up to 2 km following Miocene activity along the Columbia River Fault. The Malton Gneiss Complex exhumed in the Miocene due to activity on both the North Thompson Albreda Fault and the Rocky Mountain Trench. Moreover, the Bugaboo Batholith shows evidence of up to 2 km of exhumation during the late Miocene and Quaternary. When integrated with published data a pattern of extension related exhumation is evident throughout the Paleogene and Neogene in BC Columbia, quite different to the pattern evident in the Southern Cordillera. These results suggest late Cenozoic deformation of the Cordillera was driven by the high gravitational potential and the tectonic reorganization along the Pacific–North American margin resulting in reactivation of major faults as splay faults facilitated by dextral motion of the Rocky Mountain Trench.

This work highlights the insights low-temperature thermochronology can provide into the extension of the Cordillera. Continued work across Washington and Oregon would link the Southern Cordillera with the evolution in BC and reveal processes that explain the diversity of tectonics throughout the Cenozoic.