2024 Theses Doctoral
Characterizing Incoming Plate Hydration and Overriding Plate Structure at Subduction Zones: Implications for Plate Boundary Slip Behavior
Subduction zones, where one tectonic plate descends beneath another, are the most seismically active regions on Earth and have produced the largest earthquakes and some of the most destructive tsunamis ever recorded. Significant questions remain regarding the roles both the downgoing and overriding plates play in contributing to varying styles of rupture along the main seismogenic contact between the two plates, or megathrust, where such great (Mw > 8) earthquakes are generated. In the last few decades, the scientific community has recognized how different structural and compositional properties of both plates, and in particular the hydration state of the incoming plate can contribute to variations in megathrust slip behaviors.
In this thesis, I show how marine multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data can be used to investigate structural controls on megathrust slip behavior including the different styles of great earthquakes and/or the generation of slow slip events. Offshore Alaska and Sumatra, we used long-streamer multichannel seismic data to create a high-resolution P-wave velocity (Vp) model of the upper oceanic crust prior to subduction. Using a differential effective medium theory, we place the first constraints on the amounts pore (free) water contained therein.
Our results indicate that the uppermost oceanic crust of the incoming plates in both regions is significantly hydrated. Offshore Alaska, we show that pervasive faulting in the bending area allows seawater to penetrate into the uppermost crust. We propose that high water content in uppermost crust might contribute to observations of low coupling along the shallow plate interface in this area through the expulsion of pore fluids. Geochemical analyses of arc lavas in this segment of the Alaska subduction zone suggests significant fluid release from the downgoing crust compared to adjacent segments. Thus, we propose that during subduction, additional bending and high-temperature circulation of remaining pore fluids could further alter the upper oceanic crust that dehydrates around sub-arc depths. Offshore Sumatra, few bending-related faults are observed; however, evidence for significant and homogeneous hydration within the the uppermost crustal layer 2A (extrusives) suggests that plate bending plays a role in the shallow reopening cracks, facilitating the shallow penetration of seawater. In layer 2B (sheeted dikes) just below, our results suggest heterogeneous, yet significant, hydration that we attribute to the slow and diffuse deformation taking place in the Wharton Basin. We speculate that the large amounts of upper-crustal water carried into the Sumatra subduction zone can influence shallow slip behavior, as evidenced by recent records of a long-lasting slow slip event in the area.
To further explore potential structural and compositional controls on spatial varia- tions in megathrust slip behavior in Alaska, we use OBS data to create a 3D Vp model of the Alaska Peninsula Subduction zone within a 500-by-400 km wide area with good resolution down to 20-25 km depths in both the incoming and overriding plates. Our model samples two subduction zone segments that exhibit differences in history and style of megathrust rupture. We interpret reductions in seismic velocities within the incoming plate as evidence for modest hydration of the Pacific oceanic plate resulting from a series of fracture zones and the formation of large seamounts and an associated basement swell, or platform. The bathymetry of the seamounts and platform in part modulates the distribution and lithology of subducting sediments across the margin that we propose might influence shallow slip behavior. Within the overriding North American plate, we see evidence for contrasting styles of deformation and variations in composition (i.e., rigidity) that agrees well with observed changes in plate coupling and great earthquake history. These results emphasize the importance of considering not only one, but several factors related to both the incoming and overriding plates which collectively contribute to along-strike and downdip variations in megathrust slip behavior between segments.
Our final study looks at the incoming Cocos plate just before it subducts offshore Mexico beneath the North American plate. Here we jointly inverted 2D OBS and long-offset MCS data acquired parallel to the trench to derive a 270 km-long, high-resolution Vp model of the entire oceanic crust and uppermost mantle. We provide the first constraints on the quantities of both free and structural (i.e., mineral-bound) water contained within the Cocos plate outboard of the Guerrero Gap and adjacent segments of the Mexican subduction zone. The Guerrero gap hosts large slow slip events that are commonly explained through the release of water through the dehydration of altered sediments and upper oceanic crust downdip. Strikingly, our results show that while the Cocos plate is hydrated offshore Mexico, nearly all of the water is contained within the upper oceanic crust.
Moreover, we see that most of the water by weight is present as free fluids in the pores and that the upper oceanic crust is only moderately altered (0.3-1.3 wt.%) compared to global averages (> 1.5-3 wt.%). While the upper crust appears hydrated everywhere across our profile, we find that ∼30% more water is subducting outboard the Guerrero seismic gap where large seamounts contribute to a thicker extrusive layer and more alteration. This, along with evidence for the subduction of seamounts in Guerrero might help explain observations of weak shallow plate coupling and a greater propensity for slow slip at greater seismogenic depths compared to adjacent segments. These results provide important new constraints on how much pore and structural water is carried in the Cocos plate offshore Mexico. We propose that global estimates of incoming structural water content are not applicable everywhere, as is commonly assumed by petrologic and thermal models. Much less structural water may be needed within the upper oceanic crust just before subduction to explain the occurrence of slow-slip events downdip in some subduction zones.
Geographic Areas
Files
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Acquisto_columbia_0054D_18850.pdf application/pdf 14.9 MB Download File
More About This Work
- Academic Units
- Earth and Environmental Sciences
- Thesis Advisors
- Becel, Anne
- Degree
- Ph.D., Columbia University
- Published Here
- October 16, 2024