The Sills of Guaymas Basin, Gulf of California: Their Distribution and Seismic Estimates of Their Thickness and Alteration of Intruded Sediments

Abstract:

A variety of observations, including seismic images and seafloor backscatter and photography, indicate that active igneous sill intrusion into sediments occurs over a broad region, up to 40 km from the plate boundary, in Guaymas Basin in the Gulf of California. The two, young (<6 m.y.) spreading segments within Guaymas Basin are creating new igneous crust that is essentially oceanic; however, as a consequence of high sedimentation rates (1-2 km/m.y.), this crust does not have a typical oceanic Layer 2, but instead has an upper layer of intercalated sediments and igneous intrusions. The active addition to this layer over a broad region has three broad implications: it reveals something about the focusing mechanisms of melt transport from the mantle to the crust; it suggests that the role of this and other sedimented spreading centers in global carbon budgets should be assessed due to the large quantity of CO₂ and CH₄ released during intrusion-related sediment alteration; and, similarly, this setting provides an ideal active analogue for past igneous events for which sill intrusion has been implicated in large, rapid excursions in oceanic and atmospheric chemistry. Understanding the processes and consequences of igneous sill intrusion into sediments requires a means of quantifying both the alteration of sediments following intrusion and the thickness of intruded sills, which scales with the sill’s thermal energy. We present results of seismic analysis techniques that enable us to quantify both of these things. These types of measurements, with ground truthing from drilling results, can provide strong constraints for models of sill/sediment hydro-thermal-chemical dynamics that span a range of environments and age. We also present new seismic images of sill intrusions and subbottom images of features related to gas expulsion that demonstrate some of this range present in Guaymas Basin.