More Like this

1. Metasomatism and Slow Slip: Talc Production Along the Flat Subduction Plate Interface Beneath Mexico (Guerrero)

   https://doi.org/10.1029/2023GC010981  

   Lindquist, P. C.; Condit, C. B.; Hoover, W. F.; Hernández‐Uribe, D.; Guevara, V. E. (August 2023, Geochemistry, Geophysics, Geosystems)

   Talc‐rich rocks are common in exhumed subduction zone terranes and may explain geophysical observations of the subduction zone interface, particularly beneath Guerrero, Mexico, where the Cocos plate subducts horizontally beneath North America and episodic tremor and slow slip (ETS) occurs. We present petrologic models exploring (a) the degree of silica metasomatism required to produce talc in serpentinized peridotites at the pressure‐temperature conditions of the plate interface beneath Guerrero and (b) the amount of silica‐bearing water produced by rocks from the subducting Cocos plate and the location of fluid pulses. We estimate the volumes of talc produced by the advection of silica‐rich fluids into serpentinized peridotites at the plate interface over the history of the flat‐slab system. In the ETS‐hosting region, serpentinites must achieve ∼43 wt. % SiO₂to stabilize talc, but minor additions of silica beyond this produce large volumes of talc. Our models of Cocos plate dehydration predict that water flux into the interface averages 3.9 × 10⁴ kg m⁻² Myr⁻¹but suggest that only where subducting basalts undergo major dehydration reactions will sufficient amounts of silica‐rich fluids be produced to drive significant metasomatism. We suggest that talc produced by advective transport of aqueous silica alone cannot account for geophysical interpretations of km‐thick zones of talc‐rich rocks beneath Guerrero, although silica‐bearing fluids that migrate along the plate interface may promote broader metasomatism. Regions of predicted talc production do, however, overlap with the spatial occurrence of ETS, consistent with models of slow slip based on the frictional deformation of metasomatic lithologies. 

   more » « less
2. Formation of talc in the subduction interface: Mg isotopes demonstrate Mg loss over Si gain

   https://doi.org/10.1130/G52538.1  

   Easthouse, Griffin; Hoover, William; Teng, Fang-Zhen; Condit, Cailey; Pike, Courteney; Wang, Ze-Zhou; Berg, Anna; Wynn, Peter; Woods, Lauren; Poulaki, Eirini (February 2025, Geology)

   Abstract Talc-rich metasomatic rocks in subduction interface shear zones profoundly influence seismicity and arc magmatism, but their petrogenesis remains controversial. Magnesium isotope compositions of exhumed subduction interface rocks from the Catalina Schist (California, USA) record Mg exchange from ultramafic to crustal rocks. Preferential loss of isotopically light Mg from serpentinite produces isotopically heavy talc-rich metasomatic rocks. Addition of this isotopically light Mg to adjacent metasedimentary and metamafic rocks from the slab produces actinolite- and chlorite-rich metasomatic rocks, respectively, with convergent δ26Mg values relative to their protoliths. The addition of Ca to ultramafic- and metasedimentary-derived metasomatic rocks reflects a separate contribution from infiltrating metabasalt-derived fluids. Talc-rich rocks are formed by passive enrichment of Si in serpentinite during Mg loss to adjacent Mg sinks. These results and a global compilation of exhumed paleosubduction terranes suggest that talc is a common component of the subduction interface and often forms independent of Si metasomatism. Talc is likely prevalent along the interface from mantle wedge corner to subarc wherever ultramafic material is in contact with a Mg sink and where it could influence slow slip events, subduction interface rheology, and arc magmatism in modern subduction zones. 

   more » « less
3. SCENTAR: A High-Density Nodal Array to Study the Structure and Seismogenic Behavior of the Southern Cascadia Forearc

   https://doi.org/10.1785/0220220251  

   Delph, Jonathan R.; Thomas, Amanda M.; Stanciu, A. Christian; Aslam, Khurram; Chatterjee, Avigyan; Sassard, Vincent (November 2022, Seismological Research Letters)

   Abstract Tectonic and seismogenic variations in subduction forearcs can be linked through various processes associated with subduction. Along the Cascadia forearc, significant variations between different geologic expressions of subduction appear to correlate, such as episodic tremor-and-slip (ETS) recurrence interval, intraslab seismicity, slab dip, uplift and exhumation rates, and topography, which allows for the systematic study of the plausible controlling mechanisms behind these variations. Even though the southern Cascadia forearc has the broadest topographic expression and shortest ETS recurrence intervals along the margin, it has been relatively underinstrumented with modern seismic equipment. Therefore, better seismic images are needed before robust comparisons with other portions of the forearc can be made. In March 2020, we deployed the Southern Cascadia Earthquake and Tectonics Array throughout the southern Cascadia forearc. This array consisted of 60 continuously recording three-component nodal seismometers with an average station spacing of ∼15 km, and stations recorded ∼38 days of data on average. We will analyze this newly collected nodal dataset to better image the structural characteristics and constrain the seismogenic behavior of the southern Cascadia forearc. The main goals of this project are to (1) constrain the precise location of the plate interface through seismic imaging and the analysis of seismicity, (2) characterize the lower crustal architecture of the overriding forearc crust to understand the role that this plays in enabling the high nonvolcanic tremor density and short episodic slow-slip recurrence intervals in the region, and (3) attempt to decouple the contributions of subduction versus San Andreas–related deformation to uplift along this particularly elevated portion of the Cascadia forearc. The results of this project will shed light on the controlling mechanisms behind heterogeneous ETS behavior and variable forearc surficial responses to subduction in Cascadia, with implications for other analogous subduction margins. 

   more » « less
4. Episodic creep events on the San Andreas Fault caused by pore pressure variations

   https://doi.org/10.1038/s41561-018-0160-2  

   Khoshmanesh, Mostafa; Shirzaei, Manoochehr (June 2018, Nature Geoscience)

   Recent seismic and geodetic observations indicate that interseismic creep rate varies in both time and space. The spatial extent of creep pinpoints locked asperities, while its temporary accelerations, known as slow-slip events, may trigger earthquakes. Although the conditions promoting fault creep are well-studied, the mechanisms for initiating episodic slow-slip events are enigmatic. Here we investigate surface deformation measured by radar interferometry along the central San Andreas Fault between 2003 and 2010 to constrain the temporal evolution of creep. We show that slow-slip events are ensembles of localized creep bursts that aseismically rupture isolated fault compartments. Using a rate-and-state friction model, we show that effective normal stress is temporally variable on the fault, and support this using seismic observations. We propose that compaction-driven elevated pore fluid pressure in the hydraulically isolated fault zone and subsequent frictional dilation cause the observed slow-slip episodes. We further suggest that the 2004 Mw 6 Parkfield earthquake might have been triggered by a slow-slip event, which increased the Coulomb failure stress by up to 0.45 bar per year. This implies that while creeping segments are suggested to act as seismic rupture barriers, slow-slip events on these zones might promote seismicity on adjacent locked segments. 

   more » « less
5. Metamorphic Dehydration, Fluid Pressure, and the Frictional‐Viscous Transition Along Subduction Megathrusts: Case Study in Cascadia and Implications for Slow Earthquakes

   https://doi.org/10.1029/2025AV002000  

   Ozawa, So; Dunham, Eric M; Condit, Cailey B (June 2026, AGU Advances)

   Abstract Relative plate motion in subduction zones transitions from frictional slip to viscous flow with increasing depth and temperature. The frictional‐viscous transition can control the depth extent of megathrust earthquakes and episodic tremor and slip (ETS). Pore fluid pressure is a critical control on the transition, but models for its depth dependence are lacking. Here, we present a steady‐state modeling framework to calculate the fluid pressure and shear stress profile along the subduction interface. We consider fluid production from dehydration reactions in the subducting oceanic lithosphere, calculated from thermodynamic equilibrium models. These fluids are channeled updip, though in some models we allow for fluid loss into the overriding plate. The fluid pressure is calculated from Darcy's law, with permeability depending on effective stress, temperature, and slip rate. We allow for both rate‐state frictional sliding and thermally activated linear viscous flow, and solve for the partitioning of deformation between them. We apply the modeling framework to the Cascadia subduction zone. Our results show nearly uniform effective stress in the seismogenic zone, below which it decreases with depth and fluid pressure approaches lithostatic pressure. The frictional‐viscous transition spans a wide range of depths, and mixed frictional‐viscous deformation is predicted at the ETS source depth. A model with fluid leak‐off into the overlying plate produces a more heterogeneous effective stress, with a local minimum near the major dehydration depth. Our results provide important insight into earthquake hazards and the mechanism of ETS in Cascadia, and the modeling framework is applicable to global subduction zones. 

   more » « less