The PIs propose to measure and interpret U-Pb crystallization ages of a varied suite of volcanic and plutonic rocks recovered from water depths of 300-3500m at several hundred sites on the submerged and previously unsampled rifted continental crust that underlies most of the southern Gulf of California. Many samples were collected along traverses up fault scarps and across volcanic features by a remotely operated vehicle; 50 dredge hauls also recovered igneous rock. All relevant samples have been petrographically described and geochemically analyzed. These samples record a long and complex magmatic history in this region, which in the past 20 Myr has changed from a volcanic arc to a subaerial intra-continental rift, been flooded by a marine incursion, and developed a chain of axial basins growing by seafloor spreading. Radiometric dates are needed to define this history, and to test hypotheses that up till now have been solely derived from still-subaerial outcrops at the margins of the rift. Existing dating show striking discordance between the age estimates, hinting at interesting cooling histories with likely tectonic implications. Targeted zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronologic analyses and modeling will constrain thermal histories, thereby contributing to understanding rift tectonics including continental uplift and subsidence in the gulf rift.
The work will very likely deepen our understanding of rifting history of the southern Gulf of California. Funding will also provide research opportunities for graduate students, foster collaboration with Mexican scientists and the results will be used in ongoing education and outreach programs, such as SDSU's K-8 project.
This award was one component of a 2-year collaborative project involving geological oceanographer Lonsdale, who has conducted research in the Gulf of California for 4 decades, and Californian continental geologists Dave Kimbrough (San Diego State U.) and Marty Grove (Stanford U.), who have much experience interpreting the onshore geology of the adjacent Baja California peninsula and several of its offshore islands, in part by radiometric dating of the time of emplacement of the igneous rocks and of their subsequent cooling history. A principal purpose was to apply these same techniques to igneous rocks that Lonsdale had located by sonar and seismic reflection surveys of submerged continental crust in the generally muddy floor of the southern Gulf (Fig.1), and selectively sampled from well imaged outcrops on steep fault scarps (Fig.2) with the mechanical arms of a video-streaming Remotely Operated Vehicle (ROV). Our collaboration had begun informally with State of California support as early as 2005, obtaining radiometric dates from scarp-foot rubble collected by blind rock-dredging, and several of the ROV dive sites were chosen because this reconnaissance dredging and dating had discovered radiometrically interesting stones (e.g., young Miocene granites and dike rocks within the prevailing Cretaceous basement, and rocks with dramatically different emplacement ages and cooling ages) that had presumably fallen from shallower outcrops. Dating precisely located in situ ROV samples from such outcrops proved essential for meeting our scientific objectives: (1) To define how the types, patterns and causes of magmatic intrusion and volcanism changed in the continental crust east of Baja California during the past 25 million years, as the region transitioned from a volcanic arc overlying subducting oceanic lithosphere to a deepening, widening intra-continental rift that was eventually flooded by the Pacific Ocean to create the Gulf of California. (2) To learn from the cooling histories of intrusions the timing of tectonic extension and uplift of parts of the Gulf floor; and (3) To incorporate this new information into new multi-authored syntheses of the origin and geologic history of the region. In all these objectives we were greatly helped by involvement of senior Mexican land-geologists and their students, who had assisted in the at-sea ROV fieldwork (as did Kimbrough and his post-doctoral scholar), were eager to supplement our radiometric analyses with similar analyses in their well equipped labs, and offered to apply their expert knowledge of the onshore magmatic and tectonic history on the mainland side of the Gulf to help interpret the geology of its floor. In the narrow context of Lonsdale’s 2-year award, where his main tasks were (1) recommending which of the samples should be prioritized for radiometric studies, and (2) contributing to syntheses of geologic history that incorporated the new dating results, the main outcomes relevant to both tasks were providing collaborators with (i) a large number of geochemically analyzed seafloor samples and a characterization of each sampling site, from interpretation of geophysical or video data; (ii) an appreciation, especially benefiting land-geologists and students unfamiliar with marine-geology techniques, of the limitations of blind rock-dredging of rubble piles as a method of obtaining samples for anything except reconnaissance geochronology studies; (iii) an emphasis of the much greater value of scientist-selected samples collected from outcrop by an ROV, an activity that can closely resemble normal onshore sampling for geochronology studies (Fig.3), especially if geologic features crucial to correct geochronologic interpretations, such as fault zones and dikes crossing older bedrock, are recognized during the sampling operation (as they were at several sites). Publications that have resulted from this project, and others still in preparation, reflect these contributions, and demonstrate the effectiveness of a project that was both "amphibious" (studying submerged continental rocks and structures, by both oceanographers and land-geologists) and binational (U.S. and Mexican researchers and students combining their different skills and experience).
