EXTENT AND COMPOSITION OF THE NEW CRUST BENEATH RIFT BASINS IN THE NORTHERN GULF OF CALIFORNIA

The rupture of continental lithosphere in the northern Gulf of California occurs across the Wagner, Upper Delfin and Lower Delfin basins. Structural mapping on a 5-20 km grid of seismic reflection lines obtained by PEMEX constrains the maximum length of new crust in the Upper Delfin basin to <40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath the >5 km thick, wedge-shaped upper sequence. Numerous magmatic intrusives, volcanic edifices and their pyroclastic deposits are imaged within the Upper and Lower Delfin basins in both the PEMEX (6 s of TWTT) and the Ulloa 99-5 (2 s of TWTT) seismic lines. Samples of submarine and subaerial Quaternary volcanoes are rhyolitic to andesitic with geochemical characteristics and Sr-Nd isotope ratios that suggest a MORB-type parent magma. We infer that basaltic melts intrude the base of Late Miocene sedimentary deposits, whereas differentiated magma rise to shallower levels within the sediments and produce submarine volcanic eruptions. In order to investigate the effects of the thick sedimentary lid in the ascent of magma we constructed a simple hydraulic model to calculate the neutral buoyancy level of basaltic melts (2.68 g/cc), andesite (2.45), and rhyolite (2.25). The structure of the lithosphere is constrained with the wide-angle seismic refraction profile of González-Fernández et al. (2005). Density of sediments is constrained with well density logs, and magma pressure is controlled by density contrast and tensile strength of rocks (9 to 1 MPa). The model predicts that basaltic magma may reach a maximum of 1-1.3 km beneath the seafloor within the axial trough in the Upper Delfin basin. Only andesite to rhyolite magma reaches shallower levels, where they exsolve volatiles and produce submarine eruptions. Our results indicate that thick sedimentary deposits control magma ascent and differentiation in rift basins and contribute to produce a hybrid type of crust composed of basaltic and differentiated intrusive-extrusive rocks and meta- to unmetamorphosed sedimentary rocks.