(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2017-02)
Horning, Gregory W.; Canales, Juan Pablo; Carbotte, Suzanne
This thesis investigates the evolution of the oceanic lithosphere in a broad sense from
formation to subduction, in a focused case at the ridge, and in a focused case proximal to
subduction. In general, alteration of the oceanic lithosphere begins at the ridge through
focused and diffuse hydrothermal flow, continues off axis through low temperature
circulation, and may occur approaching subduction zones as bending related faulting
provides fluid pathways. In Chapter 2 I use a dataset of thousands of microearthquakes
recorded at the Rainbow massif on the Mid-Atlantic Ridge to characterize the processes
which are responsible for the long-term, high-temperature, hydrothermal discharge found
hosted in this oceanic core complex. I find that the detachment fault responsible for the
uplift of the massif is inactive and that the axial valleys show no evidence for faulting or
active magma intrusion. I conclude that the continuous, low-magnitude seismicity located
in diffuse pattern in a region with seismic velocities indicating ultramafic host rock
suggests that serpentinization may play a role in microearthquake generation but the
seismic network was not capable of providing robust focal mechanism solutions to
constrain the source characteristics. In Chapter 3 I find that the Juan de Fuca plate, which
represents the young/hot end-member of oceanic plates, is lightly hydrated at upper
crustal levels except in regions affected by propagator wakes where hydration of lower
crust and upper mantle is evident. I conclude that at the subduction zone the plate is
nearly dry at upper mantle levels with the majority of water contained in the crust.
Finally, in Chapter 4 I examine samples of cretaceous age serpentinite sampled just
before subduction at the Puerto Rico Trench. I show that these upper mantle rocks were
completely serpentinized under static conditions at the Mid-Atlantic Ridge. Further, they
subsequently underwent 100 Ma of seafloor weathering wherein the alteration products
of serpentinization themselves continue to be altered. I conclude that complete hydration
of the upper mantle is not the end point in the evolution of oceanic lithosphere as it
spreads from the axis to subduction.
