Abstract
The evolving landscape of the Baja California (BC) peninsula governs its floral and faunal diversity. The positions of Pacific and Gulf of California (GOC) shorelines and related terrestrial habitats along the Baja California peninsula have been controlled by volcanic topography and vertical crustal motions driven by subduction, rifting, faulting, and magmatism since the Miocene. New geologic mapping has begun to reveal the spatio-temporal patterns of landscape evolution and the tectonic drivers of genetic divergence in central BC. A chain of extinct, early to middle Miocene stratovolcanoes (Comondu Group) are spaced evenly along the NW-trending topographic crest of the BC peninsula. At the latitude of San Ignacio-Santa Rosalia (27.4 degrees N), this Pacific-GOC drainage divide is a low (400-500 m asl), broad (20-30 km-wide) pass between two stratovolcanoes. Here, Comondu Group is overlain by a sequence of mafic lavas, fluvial conglomerate, eolian sandstone, and the approximately 10-Ma basalt of Esperanza. Within 5-10 km west of the divide, we map newly discovered approximately 3-4-Ma tidal sediments that overlie subaerial flows of the basalt of Esperanza up to approximately 320 m asl, similar to the highest elevations of Pliocene marine deposits east of the divide in the Santa Rosalia basin. The Pacific-GOC divide is controlled by the NW-striking Campamento fault, which is mapped at the base of the 100-200 m-high, NE-facing rift escarpment. The densely-welded approximately 1.4-Ma La Reforma tuff caps a >300 m-high bench of marine deposits east of the divide and is plastered (inclined 10-21 degrees ) to the base of this escarpment, indicating it formed near sea level by the early Quaternary. These observations document late Miocene to Pliocene subsidence followed by >300 m of Quaternary uplift, the latter likely driven by transtensional faulting and magmatic inflation. Ongoing mapping will constrain the timing of vertical crustal motions and whether or not the Pacific tidal embayment connected as a seaway across the BC peninsula to the Pliocene GOC. Ascertaining whether a transpeninsular seaway or a narrow, low-relief isthmus existed at the modern drainage divide has significant implications for how tectonics influenced habitat distribution and possibly restricted north-south gene flow that drove the genetic divergence documented within multiple species along the BC peninsula.