Landscape evolution at a young rifted margin : the Loreto region of Baja California Sur, Mexico

摘要

Continental rifts are commonly flanked by zones of high elevation. Proposed uplift mechanisms include active and induced asthenospheric upwelling, and isostatically driven lithospheric flexure. Although these hypotheses make testable and distinct predictions of the relative timing of crustal extension and rift flank uplift, the difficulty of closely constraining these processes in modern or ancient rift zones means that the issue remains controversial. This study focuses on the Loreto rift segment of the Baja California peninsula, which forms the western margin of the Late Neogene Gulf of California rift. The Loreto region is characterised by a prominent east-facing rift escarpment which separates a low-elevation coastal plain, which hosts rift-bounding faults, from a west-tilted, topographically asymmetric rift flank, incised by west-draining canyons. On the coastal plain, slip on the rift-bounding Loreto fault has driven westward retreat of the escarpment. Footwall exhumation due to escarpment retreat is reconstructed using the apatite fission track and apatite (U-Th)/He low-temperature thermochronometers to constrain the minimum age of escarpment retreat and thus also Loreto fault slip. On the rift flank west of the escarpment, canyon incision depths are obtained by analysis of digital elevation models and used as a proxy for minimum uplift magnitude. The timing and rate of rift flank canyon incision, a proxy for the timing and magnitude of rift flank surface uplift, is constrained using 40Ar/39Ar dating of lavas which display cut and fill relations with the rift flank canyons. These lavas also provide a resistant cap atop canyon interfluve mesas, and the extent of this resistant cap likely controls the extent of rift flank catchment denudation in response to uplift. The principal finding of this thesis is that rift flank surface uplift was coeval with crustal extension at Loreto, consistent with predictions made by models of rift flank uplift driven by the flexurally-distributed isostatic response to the lithospheric unloading associated with crustal extension.