Sill stacking in subseafloor unconsolidated sediments and control on
sustained hydrothermal systems: evidence from IODP drilling in the
Guaymas Basin, Gulf of California
Abstract
Magma emplacement in the top unconsolidated sediments of rift basins is
poorly constrained in terms of mechanics and associated hydrothermal
activity. Our study compares two shallow sills from the Guaymas Basin
(Gulf of California) using core data and analyses from IODP Expedition
385, and high-resolution 2D seismic data. We show that magma stalling in
the top uncemented sediment layer is controlled by the transition from
siliceous claystone to uncemented silica-rich sediment, promoting flat
sill formation. Space is created through a combination of viscous
indentation, magma-sediment mingling and fluidization processes. In low
magma input regions, sills form above the opal-A/CT diagenetic barrier,
while high magma input leads to upward stacking of sills, forming
funnel-shaped intrusions near the seafloor. Our petrophysical,
petrographic, and textural analyses show that magma-sediment mingling
creates significant porosity (up to 20%) through thermal cracking of
the assimilated sediment. Stable isotope data of carbonate precipitates
indicate formation temperatures of 70−90°C, consistent with the current
background geothermal gradient at 250−325 m depth. The unconsolidated,
water-rich host sediments produce little thermogenic gas through contact
metamorphism, but deep diagenetically formed gas bypasses the
low-permeability top sediments via hydrothermal fluids flowing through
the magma plumbing system. This hydrothermal system provides a steady
supply of hydrocarbons at temperatures amendable for microbial life,
acting as a major microbial incubator. Similar hydrothermal systems are
expected to be abundant in magma-rich young rift basins and play a key
role in sustaining subseafloor ecosystems.