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1. Tectonism and Its Relation to Magmatism Around Santorini Volcano From Upper Crustal P Wave Velocity

   https://doi.org/10.1029/2019JB017699  

   Heath, B. A.; Hooft, E. E. E.; Toomey, D. R.; Papazachos, C. B.; Nomikou, P.; Paulatto, M.; Morgan, J. V.; Warner, M. R. (October 2019, Journal of Geophysical Research: Solid Earth)

   Abstract At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short‐period seismometers, covering an area 120 km by 45 km. Here aPwave velocity model of the shallow, upper‐crustal structure (<3‐km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono‐magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E‐W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE‐SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE‐SW trending basin‐like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE‐SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation. 

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2. The Impact of Complex Volcanic Plumbing on the Nature of Seismicity in the Developing Magmatic Natron Rift, Tanzania

   https://doi.org/10.3389/feart.2020.609805  

   Reiss, Miriam Christina; Muirhead, James D.; Laizer, Amani S.; Link, Frederik; Kazimoto, Emmanuel O.; Ebinger, Cynthia J.; Rümpker, Georg (February 2021, Frontiers in Earth Science)

   null (Ed.)

   Constraining the architecture of complex 3D volcanic plumbing systems within active rifts, and their impact on rift processes, is critical for examining the interplay between faulting, magmatism and magmatic fluids in developing rift segments. The Natron basin of the East African Rift System provides an ideal location to study these processes, owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. Here, we report seismicity and fault plane solutions from a 10 month-long temporary seismic network spanning Oldoinyo Lengai, Naibor Soito volcanic field and Gelai volcano. We locate 6,827 earthquakes with M L −0.85 to 3.6, which are related to previous and ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ∼17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3–10 km) beneath Gelai, including two swarms. The deepest seismicity (∼down to 20 km) occurs above a previously imaged magma body below Naibor Soito. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills. Focal mechanisms vary spatially. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These data support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment. 

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3. How a strong low-angle normal fault formed: The Whipple detachment, southeastern California

   https://doi.org/10.1130/B35386.1  

   Axen, Gary J. (December 2019, GSA Bulletin)

   Abstract Many low-angle normal faults (dip ≤30°) accommodate tens of kilometers of crustal extension, but their mechanics remain contentious. Most models for low-angle normal fault slip assume vertical maximum principal stress σ1, leading many authors to conclude that low-angle normal faults are poorly oriented in the stress field (≥60° from σ1) and weak (low friction). In contrast, models for low-angle normal fault formation in isotropic rocks typically assume Coulomb failure and require inclined σ1 (no misorientation). Here, a data-based, mechanical-tectonic model is presented for formation of the Whipple detachment fault, southeastern California. The model honors local and regional geologic and tectonic history and laboratory friction measurements. The Whipple detachment fault formed progressively in the brittle-plastic transition by linking of “minidetachments,” which are small-scale analogs (meters to kilometers in length) in the upper footwall. Minidetachments followed mylonitic anisotropy along planes of maximum shear stress (45° from the maximum principal stress), not Coulomb fractures. They evolved from mylonitic flow to cataclasis and frictional slip at 300–400 °C and ∼9.5 km depth, while fluid pressure fell from lithostatic to hydrostatic levels. Minidetachment friction was presumably high (0.6–0.85), based upon formation of quartzofeldspathic cataclasite and pseudotachylyte. Similar mechanics are inferred for both the minidetachments and the Whipple detachment fault, driven by high differential stress (∼150–160 MPa). A Mohr construction is presented with the fault dip as the main free parameter. Using “Byerlee friction” (0.6–0.85) on the minidetachments and the Whipple detachment fault, and internal friction (1.0–1.7) on newly formed Reidel shears, the initial fault dips are calculated at 16°–26°, with σ1 plunging ∼61°–71° northeast. Linked minidetachments probably were not well aligned, and slip on the evolving Whipple detachment fault probably contributed to fault smoothing, by off-fault fracturing and cataclasis, and to formation of the fault core and fractured damage zone. Stress rotation may have occurred only within the mylonitic shear zone, but asymmetric tectonic forces applied to the brittle crust probably caused gradual rotation of σ1 above it as a result of: (1) the upward force applied to the base of marginal North America by buoyant asthenosphere upwelling into an opening slab-free window and/or (2) basal, top-to-the-NE shear traction due to midcrustal mylonitic flow during tectonic exhumation of the Orocopia Schist. The mechanical-tectonic model probably applies directly to low-angle normal faults of the lower Colorado River extensional corridor, and aspects of the model (e.g., significance of anisotropy, stress rotation) likely apply to formation of other strong low-angle normal faults. 

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4. Normal fault-tip damage zone structures and geometries from the Sevier fault, Utah

   https://doi.org/10.18277/AKRSG.2023.35.04  

   Nishimoto, Michelle; Surpless, Benjamin (July 2023, Keck Geology Consortium)

   Davidson, Cam; Wirth, Karl (Ed.)

   Fault-tip damage zones develop in response to fault propagation and displacement and are caused by the local amplification of stresses at the fault tip. Understanding the geometry and intensity of damage zones is crucial for evaluating earthquake hazards and assessing the potentials of oil and gas production, geothermal energy, and groundwater resources. Fractures initiate as a result of stresses exceeding rock strength and propagate based on the stress field at the fault tip. We investigate the damage zone of a fault segment within the Sevier normal fault zone near Orderville, Utah, focusing on fractures that developed within the Jurassic Navajo Sandstone, the Temple Cap Formation, and the oldest beds of the Carmel Formation. Because normal faults grow laterally as slip and displacement increase, we focus on the tip zone of a fault segment where fracturing is well-exposed. We executed a series of unmanned aerial vehicle (UAV) flights to capture high-resolution imagery of inaccessible rock exposures. We use these images to construct structure-from-motion (SfM) virtual outcrop models (VOMs) that we georeference and analyze using Agisoft Metashape. We collected and analyzed fracture orientation and intensity data in the field and with VOMs. Both types of data reveal a distribution of fracture intensity that is consistent with inner and outer damage zones similar to previous studies of other fault systems. Adjacent to the tip, the inner damage zone has a higher fracture intensity on the hanging wall compared to the footwall. This high fracture intensity on the hanging wall ends 30 meters over from the fault core where the intensity of the outer damage zone of the hanging wall becomes similar to that within the inner damage zone of the footwall. Laterally, along strike of the fault tip, intense fracturing ends 60 meters to the south and all fracturing ends 350 meters from the fault tip. Our results have implications for the spatial distribution of fracturing and related permeability in similar normal fault systems. 

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5. Magmatism controls global oceanic transform fault topography

   https://doi.org/10.1038/s41467-024-46197-9  

   Tian, Xiaochuan; Behn, Mark D; Ito, Garrett; Schierjott, Jana C; Kaus, Boris_J P; Popov, Anton A (December 2024, Nature Communications)

   Abstract Oceanic transform faults play an essential role in plate tectonics. Yet to date, there is no unifying explanation for the global trend in broad-scale transform fault topography, ranging from deep valleys to shallow topographic highs. Using three-dimensional numerical models, we find that spreading-rate dependent magmatism within the transform domain exerts a first-order control on the observed spectrum of transform fault depths. Low-rate magmatism results in deep transform valleys caused by transform-parallel tectonic stretching; intermediate-rate magmatism fully accommodates far-field stretching, but strike-slip motion induces across-transform tension, producing transform strength dependent shallow valleys; high-rate magmatism produces elevated transform zones due to local compression. Our models also address the observation that fracture zones are consistently shallower than their adjacent transform fault zones. These results suggest that plate motion change is not a necessary condition for reproducing oceanic transform topography and that oceanic transform faults are not simple conservative strike-slip plate boundaries. 

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