Geophysical and geochemical constraints on the evolution of oceanic lithosphere from formation to subduction

Abstract

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. Chapter 1: IntroductionThe total water content of the oceanic lithosphere is the sum of the free water in pore spaces in the sedimentary cover and highly porous upper crust, chemically bound water in sediment clay minerals, chemically bound water in the upper and lower crust resulting from the precipitation of secondary hydrous alteration products, free water in fault zones, and bound water of hydrous minerals in the uppermost mantle resulting from serpentinization of peridotite. This thesis is focused on characterizing the location, extent, and timing of the processes that result in this alteration of the lithosphere.In Chapter 2 I focus on the alteration that occurs in the near axis region at an oceanic core complex located on the slow spreading Mid-Atlantic Ridge. At the Rainbow massif long-lived high-T hydrothermal venting h...

Three‐Dimensional Seismic Structure of the Mid‐Atlantic Ridge: An Investigation of Tectonic, Magmatic, and Hydrothermal Processes in the Rainbow Area

“…In this complex setting, separating the effects of these factors requires additional contextual information. We therefore draw on a variety of data sets from this area in the interpretation of the seismic images, including bathymetry and gravity (Paulatto et al, 2015), sonar backscatter (Eason et al, 2016), microseismicity (Horning, 2017), seismic reflection images (Canales et al, 2017), and seafloor sample compositions (Sigurdsson, 1981;Stakes et al, 1984;Fouquet et al, 1997;Gale et al, 2013;Andreani et al, 2014).…”

“…The high‐velocity wedge itself has values greater than that of pure gabbro (e.g., Miller & Christensen, ), but less than that of hot uplifted mantle, and can be explained by a mix of unaltered or lightly altered mantle plus scattered gabbroic intrusions. The 9 month microearthquake study of this area (Horning, ) detected over 65,000 events, the majority of which are confined to the high‐velocity interior of the massif. These interior events do not line up along specific fault‐like structures but instead establish a diffuse zone of deformation in a region where seismic velocities indicate largely ultramafic host rock above seismic reflectors interpreted to be magmatic sills driving hydrothermal convection in the ultramafic massif.…”

“…Instead, the massif is crosscut by two sets of secondary normal faults oriented north‐south and southwest‐northeast. The detachment fault itself is inferred to be inactive due to the widespread sediment cover, lack of recent detachment fault exposure, presence of the secondary faults, and absence of seismicity along any detachment fault plane (Andreani et al, ; Horning, ).…”

“…The experiments included sonar mapping, gravity and magnetics, and multiple seismic surveys. Results of the shipboard sonar and geopotential survey from this cruise were published in Paulatto et al () and Eason et al (); results from a multichannel seismic (MCS) reflection survey, using a towed streamer system, were published in Canales et al (); and results from a microseismicity study of the area are reported in Horning (). In this paper, we present three‐dimensional seismic images produced by seismic refraction tomography.…”