(U-Th)/He apatite constraints on the erosional history of the southwestern Colorado Plateau and implications for Early Tertiary uplift and carving of a "Proto-Grand Canyon"

Abstract

Resolving the timing and relationships between regional unroofing, canyon incision, and topographic uplift in continental plateaus is a challenging problem. The regional unroofing history of the Colorado Plateau, and its relationship to Grand Canyon incision (up to 1.5 km) and plateau uplift (~1.9 km) since the Late Cretaceous, is controversial. We used (U-Th)/He apatite thermochronometry (36 samples, 230 single-grain analyses) across the southwestern quadrant of the Colorado Plateau to address these issues. Our data document overall southwest to northeast unroofing from plateau margin to plateau interior, during denudation phases in the Late Cretaceous/Early Tertiary (80 to 55 Ma), mid Tertiary (28 to 16 Ma), and Late Tertiary (<6 Ma). Distributions of apatite dates modeled using the radiation damage trapping model [1,2] suggest that eastern Grand Canyon samples from the basement and the Kaibab surface nearby had similar Early to mid-Tertiary thermal histories, despite their ~1500 m of stratigraphic separation. If these models are correct, they indicate that a significant (≥ 1000 m deep) paleo-Grand Canyon was carved in post-Paleozoic sediments in this region during Early Tertiary time. Evidence for kilometer-scale topographic relief would require substantial uplift during Sevier/Laramide time, preceding regional unroofing of this portion of the plateau interior. Although the data do not preclude additional post-Laramide uplift, the subsequent regional unroofing phases could be explained by drainage reorganization associated with rift-related lowering of adjacent regions without additional elevation gain of the plateau.

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