Abstract
The modern topography of the Atlanta lobe of the Idaho batholith in the North American Cordillera has been identified as a dissected plateau. The plateau was suggested to exist since
Late Cretaceous, while the dissection was advocated to take place since Late Miocene,
associated with unusual regional topographic rejuvenation in a post-orogenic extensional
tectonic setting far away from the convergent plate boundary. However, the previous evidence
supporting the timing and mechanism of the dissection is limited. Spatial variability of multiple
aspects exists within the Atlanta lobe. Previous studies identified a Miocene cooling and
exhumation signal in the southern Atlanta lobe, but the spatial extent of this signal is unknown.
To further reconstruct the exhumation history of the region and investigate the mechanisms
that cause the internal variability of the Atlanta lobe, we investigate the area using low
temperature thermochronology. We sampled Late Cretaceous and Miocene aged granitoid
bedrock along the South Fork Payette River (SFPR) and the Middle Fork Boise River (MFBR)
in the Boise Mountains, resulting in three vertical and two horizontal sample transects, a total
of 21 new apatite (U-Th)/He (AHe) sample dates and 2 new apatite fission track (AFT) dates
from previous samples. The AHe dates range from 29.9 ± 3.1 Ma to 3.9 ± 0.8 Ma, and most
cooling dates are Miocene to Pliocene. The two AFT dates are 37.89 ± 4.6 Ma and 40.7 ± 4.72
Ma. The thermal history inverse models from the vertical transects show a two-stage cooling
during Eocene-Oligocene and Late Miocene–Pliocene, supporting the Late Miocene incision.
The pattern of cooling dates along the river profiles from the horizontal transects suggests that
the incision of modern SFPR and MFBR occurred after the major Late Miocene cooling period,
and has been disrupted by faults. Combining our thermochronology data with other geological
evidence, we propose a tectonic model to explain the exhumation history and the variability
inside the Atlanta lobe. We suggest the Eocene exhumation was due to extension from overthickened crust, thermal-dynamic uplift, and potentially delamination under the Atlanta lobe
due to the influx of hot asthenosphere. The Miocene exhumation could have been driven by
thermal-dynamic uplift and/or delamination under the Boise Mountains triggered by the nearby
Yellowstone Hotspot. The incision of the SFPR and MFBR was likely enhanced by base level
drop due to the draining of the Paleo-Lake Idaho, and disrupted by activity on local Basin and
Range faults.