Paleomagnetism of the Guaymas Basin, Gulf of California: New Results from IODP Expedition 385

Abstract

<p>Sill-driven hydrothermal circulation results in geochemical exchanges between ocean crust, sediments, and seawater. IODP Expedition 385 drilled sill/sediment sequences at eight sites in the Guaymas Basin to better understand the interactions between magmatic, hydrothermal, and sedimentary systems in a highly active marine environment.</p><p>We focus on the magnetic data of two adjacent sites with similar sedimentation history, but different alteration degree due to an intruding sill. Site U1546 recovered a 540m sedimentary sequence disrupted by an 80m wide sill between 350/430 meters below seafloor (mbsf). The sill terminates towards Site U1545 which recovered 507m of an undisrupted sedimentary sequence. These sites allow us to directly compare paleomagnetic measurements and changes related to the different heat system in both sites.</p><p>Detailed paleomagnetic measurements were conducted, including anisotropy of magnetic susceptibility (AMS), thermal demagnetization, and alternating field demagnetization on both discrete samples and section halves. Both sites feature a decrease of magnetic intensity (NRM) and magnetic susceptibility between 50-80 mbsf, associated with a change in coercivity (averaging from 24 to 10 mT from MDF). This correlates with the sulphate-methane transition zone (SMTZ) where magnetic mineral assemblages change because of biogeochemical processes. A stable magnetic component was identified in most samples after removal of a low coercivity magnetic drilling overprint (generally removed after AF 5-10 mT), resulting in a mean inclination of ~46°, close to the geocentric axial dipole (GAD) inclination of 46.3°. Cores were assigned to the C1n Brunhes chron.  In sill-bearing Site U1546, magnetic intensity is found to gradually increase toward the sill and to decrease below the mafic sill (basaltic, doleritic, and gabbroic textures), suggesting authigenesis of magnetic minerals in the contact zone, likely triggered by the heat of the igneous intrusion in combination with hydrothermal fluid circulation.</p><p>Here we will present an overview of the paleomagnetic and rock magnetic data used to identify the magnetic mineralogy through the oceanic crustal sections of the Guaymas Basin, and to compare these two adjacent sites to assess the effect of heat and hydrothermal circulation on the paleomagnetic record in the Guaymas Basin.</p>