THERMOPHYSICAL PROPERTIES AND MECHANISMS OF SILL EMPLACEMENT AT THE RINGVENT SITE, GULF OF CALIFORNIA

The Guaymas Basin, the largest enclosed basin in the Gulf of California, is a dynamic young seafloor spreading center where new oceanic crust forms amidst rapid sedimentation and frequent sill intrusions. The Ringvent site, characterized by a young saucer-shaped sill intrusion, features a circular array of active hydrothermal vents with an 800m diameter, situated 28 km northwest of the northern trough.

This seafloor structure, positioned atop a 150m deep saucer-shaped sill, exhibits high thermal gradient anomalies, localized fluid flow, methane gas seepage, and the development of diverse microbial communities. It is believed to be transitioning from sill-driven hydrothermalism to cooling dominated by fluid flow. The sill is embedded in a layered succession of diatom oozes deposited atop indurated claystone. During IODP Expedition 385, Ringvent was extensively cored and logged to investigate the physical properties at the sill-host rock contact zone. Properties measured include density, porosity, gamma ray emissions, thermal conductivity, shear strength, p-wave velocity, and magnetic susceptibility. The raw data sets were organized, filtered, and plotted to identify patterns in the sediment's behavior with depth and around the sill, aiming to elucidate the mechanisms involved in sill emplacement.

We identified common trends in physical properties regardless of sill intrusions. Our study reveals clear patterns of increasing shear strength, density, and thermal conductivity with depth influenced by compaction and diagenetic processes. These trends are interrupted by lithological changes from diatom oozes to indurated claystone and the transition to recrystallized opal (CT opal) around the sill border. Diatom oozes deform plastically under stress, while indurated claystone behaves as a brittle elastic material. Moreover, the yield stress of the oozes is abnormally high for hemipelagic sediments. By contrast, the indurated claystone behaves as a brittle elastic material that breaks sharply once it reaches its yielding point.

The sill flowed laterally at the transition from brittle elastic to elastic-plastic behavior, suggesting a rheological control on its emplacement. This was facilitated by the unusual strength and behavior of the diatom oozes, which allowed sediments to flow around the intrusion stress field, preventing further magma ascent.