Magmatic origin of hydrothermal response to earthquake swarms: Constraints from heat flow and geochemical data

摘要

The hydrothermal responses to seismicity on the East Pacific Rise (EPR) near 9°50'N and elsewhere provide insight into the links among magmatic, tectonic, and hydrothermal processes at mid-ocean ridges. We argue that March 1995 seismic activity at 9°50'N could have resulted from a noneruptive diking event. This scenario is consistent with the thin,lens-like shape of the EPR magma chamber, the seismic pattern displaced to its west margin, and observations suggesting magma replenishment between major eruption episodes in 1991/1992 and 2005/2006. We model dike propagation numerically and show that the dike is likely to propagate subvertically from the lens margins or from edges of a discontinuity in the lens. We use scale analysis to estimate mass transport in the hydrothermal system directly from the available heat flow and geochemical data. Our results show that the increase of the vent temperature, observed at the seafloor only a few days after the seismic swarm, does not necessarily reflect heat transported directly from the magma chamber or constrain the residence time of fluid in the entire discharge zone. We suggest that the increase in vent temperature may have been caused by deep permeability changes associated with the same diking event. Such permeability changes lead to a thermal perturbation in the upper —100 m of crust where high-temperature fluid mixes with cool seawater and hydrothermal heat flux is partitioned into focused and diffuse flow components. The analysis developed in this paper is not restricted to the March 1995 earthquake swarm at EPR 9°50'N but may also be applicable to other locales in which coincident seismicity and thermal responses have been observed.