177-3 Timing and Magnitude of Cenozoic Exhumation in southern Yukon and northwestern British Columbia

Session: Chronology of Orogenesis: Unlocking the Timelines of Mountain Building

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Abstract:

The Northern Canadian Cordillera is an accretionary orogen that experienced large-scale Cenozoic dextral displacement distributed across a wide network of orogen-parallel faults that transect previously-accreted terranes. We aim to constrain the timing and magnitude of exhumation in the northern Canadian Cordillera via low temperature thermochronology to understand the Cenozoic evolution of the orogen. 

We present new apatite and zircon (U-Th)/He (AHe & ZHe) and apatite fission-track (AFT) data from 29 samples collected in the Cassiar Mountains of southernmost Yukon and northwestern British Columbia. This area is bound by two large dextral faults: the purportedly Cretaceous Teslin fault to the west; and the lithospheric-scale Tintina fault to the east, which accommodated ~430 km of dextral displacement in the early Cenozoic. To constrain along-strike and across-strike cooling trends, we integrate our new data from the Cassiar Mountains with published data from the early Mesozoic Whitehorse Trough to the west of the Teslin fault and from mid-Cretaceous plutons of the Upper Hyland River region east of the Tintina fault.  

AHe data from 25 samples yielded ages that range 77–5 Ma, AFT data from 21 samples range 78–22 Ma, and ZHe data from 29 samples range 142–31 Ma.  

In the Cassiar Mountains, older ZHe ages (ca. 100–80 Ma) are found near the center of the range. ZHe ages decrease to ca. 50 Ma near the Tintina and Teslin faults. AHe ages increase from ca. 30–20 Ma to ca. 50 Ma with increasing distance to the east of the Teslin fault. 

AHe and AFT are generally younger (ca. 50–40 Ma) west of the Tintina fault in the Cassiar Mountains and Whitehorse Trough, compared to ca. 60 Ma ages east of the fault. ZHe ages in the Cassiar Mountains are noticeably younger (ca. 50 Ma) than in the adjacent Whitehorse Trough (ca. 90 Ma and older) to the west and Upper Hyland River Valley (ca. 90–80 Ma) to the east. This suggests that the distribution of Cenozoic cooling and exhumation was influenced by older structures, such as the Teslin fault, that likely reactivated during the Paleocene.   

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9331

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