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To investigate the mechanical and microstructural properties of mafic blueschists, we conducted deformation experiments on powdered natural blueschist aggregates using the general shear geometry in the Griggs apparatus. Experiments were performed at ∼1.0 GPa and temperatures ranging from 650 to 700°C. The blueschist starting material consists primarily of sodic amphibole and epidote, with minor amounts of quartz, titanite, albite, and white mica. Strain rate stepping experiments provided mechanical data with stress exponents ranging from 1.8 to 2.2. Microstructural analysis of the deformed samples show that the blueschist aggregates were deforming by microboudinage of the sodic amphibole, with a chemically new sodic‐calcic amphibole diffused into the boudin neck. Based on these results, we interpret the samples to have deformed by diffusion creep of the sodic‐calcic amphibole, which was rate‐limited by diffusion into the boudin neck. We developed a microboudinage diffusion creep flow law using a least square regression, with parameters of A= 2.43e11 MPa−nμm s−1, n= 2.0 ± 0.3, m= 1.0, and Q= 384 ± 15 kJ/mol. Extrapolation of the flow law to the blueschist stability field suggests viscosities that are higher than metasedimentary rocks (quartz dislocation creep flow law) and lower than eclogitic rocks (omphacite dislocation creep flow law) consistent with field observations. We also show that this type of deformation mechanism matches observations of natural rocks in paleosubduction zone environments, supporting the application of this flow law to estimate amphibole rheology in modern subduction zones. We conducted deformation experiments on samples of metamorphosed oceanic crust called blueschist, which is a rock abundant in sodium‐rich amphibole and epidote. These rocks and minerals occur at high pressures (∼1 GPa) and low temperatures (∼400°C) within the earth, primarily along the subduction zone interface. By conducting deformation experiments we can characterize and quantify the microstructural and mechanical properties of our samples. We find sodic amphibole, the most abundant mineral in our samples, deforms by a process called microboudinage, where the sodic amphibole fractures and new material simultaneously diffuses into the fracture, allowing the sample to accommodate strain. Using existing theory on microboudinage and the mechanical data from our deformation experiments, we develop a mechanical relationship (e.g., flow law) that can be extrapolated to geologic conditions to provide constraints on blueschist deformation along the subduction zone interface. To support the extrapolation of our flow law, we show that microboudinage has been widely observed in naturally deformed amphiboles from paleosubduction zone environments. Experimentally deformed sodic amphibole deforms by microboudinage with sodic‐calcic amphibole diffusing into the boudin neckWe developed a microboudinage flow law with a stress exponent of 2.0Extrapolation of the flow law provides viscosity estimates consistent with existing flow laws representative of the subduction interface Experimentally deformed sodic amphibole deforms by microboudinage with sodic‐calcic amphibole diffusing into the boudin neck We developed a microboudinage flow law with a stress exponent of 2.0 Extrapolation of the flow law provides viscosity estimates consistent with existing flow laws representative of the subduction interface

We use xenoliths from young cinder cones in the eastern Mojave region of Southern California to investigate deformation fabrics and their implications for strain localization, lithospheric viscosity, and controls on mineral lattice‐preferred orientations (LPOs) and seismic anisotropy at Moho depths. Lower crustal gabbros and upper mantle peridotites were collected from two areas separated by ∼80 km—the Cima and Deadman Lake Volcanic Fields. At both localities, mantle peridotites exhibit solid‐state deformation fabrics that show strong LPOs and other evidence for dislocation creep as the dominant deformation mechanism. The preservation of microstructures ranging from annealed, granular, to mylonitic indicates that there is substantial strain localization in the Mojave mantle near the Moho. Paleopiezometry conducted on subgrains in olivine indicates that stress magnitudes during deformation were 12–26 MPa. Calculations using olivine flow laws yield strain rates on the order of ∼10−12/s and an associated viscosity of ∼1019Pa s, consistent with estimates of mantle lid viscosity from postseismic relaxation studies. Two types of olivine LPOs were observed in the peridotites: A‐type and E‐type fabrics, both of which predict seismic fast axes that align parallel to the lineation within the foliation plane. The occurrences of the two fabric types appear to correlate with strain magnitude but not with water contents measured using Fourier transform infrared spectroscopy. Lower crustal fabrics are dominated by magmatic foliations and LPOs in plagioclase, which produce seismic fast axes oriented perpendicular to the foliation plane. This suggests that even where mantle and lower crustal fabrics are kinematically linked, their seismic fast axes may appear anticorrelated across the Moho. A wide range of xenolith textures indicates significant strain localization at and below Moho depths in the MojaveA‐ and E‐type olivine LPOs in the mantle may result in fast axes oriented orthogonally to plagioclase fast axes aligned in the lower crustXenolith‐constrained flow laws suggest fast mantle strain rates ∼10−12/s and viscosity of ∼1019Pa s in the Mojave upper mantle

Northwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25 ± 5 m from its source by the Banning strand to between 5.1 ± 0.4 ka (95% confidence interval (CI)) and 6.4 + 3.7/−2.1 ka (95% CI) using U‐series dating of pedogenic carbonate clast coatings and 10Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9 + 2.3/−1.6 mm/yr (median, 95% CI) to 4.9 + 1.0/−0.9 mm/yr (median, 95% CI). This rate represents only 25–35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U‐series and 10Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms. The Holocene slip rate for the Banning strand of the southern San Andreas fault is 4–5 mm/yrThis new geologic slip rate is only 25–35% of total southern San Andreas slip, lower than expectedSlip is not concentrated on the Banning strand; regional earthquake hazard models need expansion

A global compilation of shear stress magnitude from mylonites developed along major fault zones suggests that maximum shear stresses between 80 and 120 MPa are reached at temperatures between 300 and 350°C on normal, thrust, and strike‐slip faults. These shear stresses are consistent with estimates of brittle rock strengths based on sliding friction (e.g., Byerlee's law), and with in situ measurements of crustal stress measured in boreholes. This confirms previous suggestions that in some areas at least, the continental crust is stressed close to failure down to the brittle‐ductile transition. Many major active faults in all tectonic regimes are considered to be relatively weak, however; peak static shear stresses for brittle faults estimated by a variety of techniques lie in the range of 1–50 MPa. The sharp contrast between static shear stresses estimated on the seismogenic parts of major faults and those estimated from steady‐state microstructures in ductile rocks immediately below the seismogenic zone suggests that there is an abrupt downward termination, probably controlled by temperature, of the weakening processes that govern fault behavior in the upper crust. These data also imply that seismogenic parts of major fault zones contribute little to lithospheric strength, and are unlikely to have much influence on either the slip rate or the location of the faults. Conversely, ductile middle crust immediately below the brittle‐ductile transition deforms at high stresses, and forms a significant load‐bearing element within the lithosphere. Stresses on major brittle faults are low but are high in the middle crustSeismogenic parts of major fault zones do not influence lithospheric strengthMiddle crust below the BDT forms a load‐bearing element within the lithosphere

Exhumed high‐pressure/low‐temperature (HP/LT) metamorphic rocks provide insights into deep (∼20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist‐to‐eclogite facies oceanic‐ and continental‐affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P‐T‐D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn‐subduction return flow along similar P‐T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top‐to‐the‐S‐SW shear (prograde‐to‐peak subduction), top‐to‐the‐NE shear (blueschist facies exhumation), and then E‐W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi‐mineral Rb‐Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ∼50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ∼50–40 and ∼43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene‐Oligocene subduction channel proceeded at rates of ∼1.5–5 mm/yr and accommodated ∼80% of the total exhumation of this HP/LT complex. Syros is a tectonic stack composed of three slices constructed by subduction and underplating; peak subduction ages young with structural depthThe subduction‐to‐exhumation transition is marked by kinematic rotation and cooling during decompressionMetamorphic geochronology indicates syn‐subduction exhumation occurred continuously in an Eocene‐Oligocene subduction channel Syros is a tectonic stack composed of three slices constructed by subduction and underplating; peak subduction ages young with structural depth The subduction‐to‐exhumation transition is marked by kinematic rotation and cooling during decompression Metamorphic geochronology indicates syn‐subduction exhumation occurred continuously in an Eocene‐Oligocene subduction channel

The recently developed titanium-in-quartz (TitaniQ) geothermobarometer of Wark and Watson (2006) and Thomas et al. (2010) has the potential to be applied to a wide range of igneous and metamorphic rocks. For Ti concentrations > -10 ppm, the concentrations can be measured using an electron microprobe, but lower concentrations are below detection limits and require techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). SIMS is ideal for this purpose as it maximizes lateral and depth resolution. We used SIMS to analyze synthetic quartz crystals characterized for Ti concentration by electron probe (18-813 ppm Ti) and compare this calibration to the commonly available NIST 61X glasses using both high mass resolution (HMR) and conventional energy filtering (CEF) techniques. We used a primary beam of 16O-ions and detected positive secondary ions. During HMR sessions, the mass spectrometer was operated at a mass resolving power (M/ΔM) of ~2000 to separate molecular ions from elemental Ti peaks. For CEF analyses, the instrument detected secondary ions sputtered from the sample with excess kinetic energies of 75 ± 20 eV. Titanian quartz measurements reveal general homogeneity and a linear increase in Ti+/Si+ion ratios with increasing Ti concentrations. Background signals represent 0-1 ppm. The slopes of the calibration curves for the titanian quartz crystals are -70% of the curves constructed using NIST glasses, indicating a much higher ion yield for Ti from the glasses compared to the simple oxide of silicon. We demonstrate, however, that a simple correction factor allows NIST glasses to be used to quantitatively determine the Ti concentrations of quartz (with 3.8% error using HMR, and 8.2% error using CEF) until homogenous, well-characterized samples of SiO2become generally available.

Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, particularly in the shallow megathrust region of the seismogenic zone. We describe subduction‐related deformation structures and characterize deformation conditions from an exhumed, basalt‐hosted megathrust in the Chugach accretionary complex of south‐central Alaska. Rocks in the Chugach preserve a record of seafloor mineralogical changes from pre‐subduction, hydrothermal circulation that produced sheet silicates with a lower frictional strength than intact basalt. Pre‐subduction alteration also served to introduce hydrous phases that can expel water during deformation and raise the pore fluid pressure. Once strain localized within basalts onto a megathrust fault plane at lithostatic pore fluid pressures, the basalt weakened further through a combination of cataclasis, dilatational shear fracturing, and slip on chlorite‐rich shear bands. This process occurred in a narrower fault zone, and at higher maximum differential stress and greater pore fluid pressure fluctuations than recorded in some sediment‐hosted megathrusts at similar pressure and temperature conditions. Our data indicate that when the lower plate contains basalt bathymetric features, basalt dismembers during subduction into a chlorite‐rich fault gouge that surrounds lenses or slices of intact, less‐altered basalt. When one tectonic plate slides beneath another (the process of subduction), the surface roughness and the composition of the lower plate can change how easily and how rapidly this convergence occurs. Typically, the subduction process is neither smooth nor steady, and this results in seismic activity that varies in magnitude—small to large—and character—from fast, infrequent earthquakes to slow and frequently recurring earthquakes. It is critical to determine the properties of the rocks that make up the lower plate, so that we can better understand which features lead to various types of earthquake activity. We describe the characteristics of volcanic rocks that once occupied the lower plate and that are now exposed in south‐central Alaska. These rocks record a pattern of breakdown that is dependent on compositional changes from chemical alteration on the seafloor prior to subduction and high fluid pressures during subduction. Our data indicate that volcanic topographic features on the seafloor do break apart and undergo significant compositional and mechanical changes, and the strength of the resulting rock is controlled primarily by the strength of the mineral chlorite. Seafloor alteration produces hydrous phases and phyllosilicates that impact Pfand frictional strength of basaltBasalt weakens through a consistent pattern of deformation during subduction that depends on Pfand phyllosilicate contentSubducted basalt hosts larger shear stresses than sediment megathrusts and deforms at a narrower width, suggesting faster strain rates Seafloor alteration produces hydrous phases and phyllosilicates that impact Pfand frictional strength of basalt Basalt weakens through a consistent pattern of deformation during subduction that depends on Pfand phyllosilicate content Subducted basalt hosts larger shear stresses than sediment megathrusts and deforms at a narrower width, suggesting faster strain rates