Geophysics & Tectonics

Our research group uses geophysical data to image the in situ structural architecture and properties of the subsurface. Interests span spatially from the Earth’s surface to the base of the lithosphere across all tectonic settings, and temporally from the creation of new lithosphere to its destruction at subduction zones. We synthesize various techniques to evaluate the evolution of the lithosphere over geologic timescales and implications for tectonic hazards at plate boundaries.

Subduction zones drive plate tectonics and post the greatest tectonic hazards, yet how they initiate remains unsolved...

Subduction Zone Initiation

The Puysegur Trench, a young subduction zone south of New Zealand, offers a rare opportunity to study subduction initiation in situ. Over the last 45 million years, the Puysegur margin has transitioned through a series of tectonic phases — rifting, seafloor spreading, strike-slip, and now early-stage subduction.

Analyzing a new 2018 seismic dataset, we mapped lithospheric structures to constrain their tectonic affinity and the margin’s evolution. These images reveal that the overriding Pacific Plate consists of stretched continental crust formed during Eocene-Oligocene rifting. Although rifting was more advanced to the south, it never fully progressed to seafloor spreading. Instead, subsequent strike-slip motion along the plate boundary juxtaposed dense Australian oceanic lithosphere from farther south with buoyant continental lithosphere at a collision zone.

Subduction initiation at Puysegur was facilitated by lithospheric buoyancy contrasts and pre-existing tectonic structures. Our findings also show a shift in stress patterns during subduction initiation: horizontal compression and uplift dominated initially, followed by extension and subsidence as subduction self-sustained and propagated along-strike. This stress evolution accelerated over time as the plate interface progressively weakened along the propagating trench tip, driving faster and easier subduction initiation along the margin.

The Puysegur Trench highlights the interplay of inherited lithospheric structures, dynamic forces, and the 4D nature of subduction initiation, which consists of localized nucleation followed by along-strike propagation phases.

Publications: Shuck et al. (2021), Tectonics; Shuck et al. (2022), Nature Geoscience; Gurnis et al. (2018), EPSL; Patel et al. (2020), Basin Research

Subduction Zone Termination

Similarly, the termination of subduction zones and how they ‘turn off’ remains enigmatic…

Like Puysegur, we are investigating subduction termination by imaging the northern Cascadia Subduction Zone, where cessation of subduction is imminent. This complex region hosts several tectonic triple-junctions and the Nootka Fault Zone, an active left-lateral transform, that segments the incoming oceanic plate as it dives beneath North America.

Using the CASIE21 dataset, we have new structural images across the ridge-trench-fault triple-junction, the NFZ, and the Explorer and Juan de Fuca slabs. We integrate our interpretations with recent earthquake hypocenter and focal mechanism catalogs to piece together the evolution of the NFZ and its role in slowing subduction.

Our images reveal the NFZ began at ~4 Ma as a broad shear zone within young oceanic lithosphere and progressively localized into a mature transform boundary, forming the Explorer microplate and facilitating its diminished subduction relative to the adjacent Juan de Fuca plate. Past the trench, we image trench-parallel tears in the Explorer and Juan de Fuca slabs offset by the NFZ transform. Our findings suggest a single tear propagated laterally across the paleo-NFZ wake but was intersected by the NFZ, enhancing Explorer slab tearing yet allowing continued Juan de Fuca subduction. We are proposing a 4D subduction termination model where transform boundaries enable laterally diachronous fragmentation of oceanic lithosphere, segmented slab breakoff and microplate capture, and discrete jumps of triple junctions.

Publications: Shuck et al. (2021), in prep;

Subduction Zone Geodynamics & Tectonic Hazards

Why some subduction zones produce devastating earthquakes and tsunamis and others do not, remains unclear…

Mapping the Cascadia megathrust

The Cascadia subduction zone offshore Pacific Northwest, USA, poses the greatest domestic natural hazard threat. Paleoseismic data indicate the plate boundary has ruptured in massive earthquakes resulting in tsunamis and landslides in the past. Although at present-day this system is seismically ‘quiet’, we know that it is locked and loaded for another major event in the future.

The CASIE21 research team, led by Suzanne Carbotte (LDEO) and many others, has collected a new regional seismic dataset along the margin. These data constrain a new 3D subduction interface grid which has significantly more detail than previous smooth models. The results show that the young and weak Juan de Fuca plate has multiple tears and is segmented into four primary domains. These segment boundaries are well aligned with earthquake rupture boundaries inferred by the extensive paleoseismic record.

We are working with many scientists to understand the implications of a segmented plate interface, and the associated hazards from different earthquake scenarios, including full-margin vs partial-margin ruptures.

Southern Cascadia Hazards

Although the Cascadia Subduction Zone extends from Northern California to Vancouver Island, the CASIE21 experiment was not able to collect new data offshore California. This data gap leaves a detailed full-margin grid of the slab incomplete and hinders earthquake rupture and tsunami hazard modeling efforts.

Funded by the USGS National Earthquake Hazards Reduction Program, we are reprocessing legacy seismic data in southern Cascadia with modern techniques to improve imaging in this region. With these results, we will map the megathrust and other faults in the overriding plate with greater detail. These will be synthesized with the CASIE21 footprint to complete a 3D seismic velocity model and subducting plate interface for the entire margin, which will enhance subsequent hazard assessments.

Subducting Sediments, Water, and Fault Zone Properties

mapping the incoming plate stratigraphy

Sediment properties along the incoming plate can influence where and how the décollement forms, and hence control variations in sediment subduction versus accretion, the frictional properties of the megathrust, and geochemical cycling. Sediments in Cascadia are routed by several submarine canyon-fan systems that deposited terrigeneous coarse sediment throughout Pleistocene glaciation cycles, interbedded with hemipelagic mud during sea level high-stands. We are tying ocean drilling lithostratigraphic and biostratigraphic data to the CASIE21 seismic profiles to determine the nature of sediments, depositional rates, and alteration features to determine the material properties that form the plate interface and control earthquake nucleation.

Hydrous fluids can be entrained within oceanic plates during hydrothermal circulation near the ridge axis and during plate-bend faulting at the outer rise. Downdip, fluids released by metamorphic dehydration reactions may accumulate along the megathrust, leading to high pore-fluid pressure and influencing seismogenesis.

We are working with novel techniques, such as Downward Continuation of marine seismic data, followed by high-resolution streamer tomography with millions of picks to constrain the velocity structure of oceanic plates. From these results, we seek to infer the distribution and volume of fluids carried into the Cascadia subduction zone.

We are also examining the thermal evolution of oceanic crust and metamorphic alteration of overlying sediments. Basal sediments at the deformation front have already undergone the smectite to illite transition, promoting a frictionally locked megathrust with seismogenic behavior.

Publications: Carbotte et al. (2024), Science Advances

fluid-rock interactions and rheology

Continental Rifting and Passive Margin Architecture

The primary controls on continental breakup leading to new ocean basins is debated…

The Eastern North American Margin contains the geologic record of the formation and breakup of the Pangea supercontinent. Rifting began in the Late Triassic and continued into the Early Jurassic. The rift sequence has classically been described as a ‘magma-rich’ margin, that swiftly underwent breakup and into seafloor spreading.

The 2014 ENAM Community Seismic Experiment provided new seismic data to image the crust and mantle properties along the continent-ocean transition to evaluate melt-rock interactions and the timing of lithospheric rupture.

Inverting ocean-bottom seismometer data and reflection images, we found an anomalous 200-km-wide zone of thin, rough, and high-velocity ‘proto-oceanic’ crust with an abrupt transition to thick and smooth crust further seaward. We conducted a suite of petrological models of mantle melting and crystallization, which show that the proto-oceanic crust formed by diffuse melt percolation through a thermally eroding lithospheric lid, whereas the transition to smooth and thicker crust represents full lithospheric rupture and the onset of seafloor spreading in the Atlantic Ocean, ~25 Myr later than previously thought.

Oceanic Core Complexes and Mantle Serpentinization

How fluids alter the mantle rocks and their physical properties has rarely been studied in situ…

Structural deformation and chemical reactions operating within the Earth’s mantle affect a vast range of tectonic processes, such as mantle convection, rheological and seismogenic behavior, and geochemical exchanges. However, the nature of these processes and mechanical behavior of the mantle is poorly understood due to the lack of direct sampling of in situ mantle rocks.

We are using samples collected by IODP Expedition 402 to re-evaluate relationships between geological and geophysical properties of mantle peridotites: (1) alteration and hydration of mantle peridotites via serpentinization, and (2) structural and mineral fabric development via mantle flow and detachment fault zone deformation.

We are collaborating with an international team of scientists to perform microCT scanning of peridotite phases and crack networks, measure their P-wave and S-wave velocities and anisotropy under different temperature and pressure conditions, and corresponding thin section petrography and EBSD to determine mineral phases and their crystallographic properties. This project will enhance relationships to assess the distribution and volume of fluids within oceanic lithosphere across tectonic settings.

Mantle Dynamics during Incipient Seafloor Spreading

How anomalous mantle conditions from prior tectonic events persist beneath mid-ocean ridges is unknown…

COMING FALL 2025

This project is funded by the National Science Foundation Marine Geology & Geophysics program to acquire a new marine active-source seismic dataset in the western Atlantic Ocean. The 43-day field program is slated for Sep 27 – Nov 10, 2025, aboard the R/V Marcus G. Langseth, and will acquire multi-channel seismic reflection and wide-angle seismic data with 52 ocean-bottom seismometers.

The scientific objective of the experiment is to investigate inherited mantle properties and their controls on the formation of oceanic lithosphere during incipient seafloor spreading following the breakup of Pangea. Specifically, we seek to test how anomalous mantle temperature, composition, and flow patterns from prior subduction & supercontinent cycle influence subsequent mid-ocean ridge processes. The new dataset spans the first ~50 Myr of seafloor spreading at the nascent Mid Atlantic Ridge and will constrain the thickness, seismic velocity, and roughness of oceanic crust and frozen anisotropic fabrics preserved in the mantle lithosphere, which will be synthesized with petrological and geodynamic models to backout how mantle temperature, composition, and flow patterns evolved during embryonic seafloor spreading.