Search

We employ an automatic earthquake detection algorithm to seismic waveforms recorded between the years 2000 and 2020 in southwest British Columbia. 32,121 events which possess at least three paired P‐ and S‐wave arrival times are located, compared to 21,538 seismic events in the existing Geologic Survey of Canada catalog. This augmented catalog is employed for double‐difference seismic tomography across Vancouver Island, with particular focus on the Nootka Fault zone (NFZ). The NFZ is a transform boundary that separates the Juan de Fuca and Explorer plates in a zone of distributed left‐lateral strike‐slip faulting. Tomographic results indicate that a double seismic zone exists within the NFZ that parallels Vp/Vs structure typically observed in subduction zones. Specifically, a dipping high Vp/Vs layer is underlain by reduced Vp/Vs material. Structural complexities are revealed by three Mw > 6 events and their aftershocks. The 2004 Mw 6.3 and 2011 Mw 6.4 events are interpreted to reside southeast of the NFZ within the Juan de Fuca plate at depths as shallow as 20 km, and the 2014 Mw 6.6 event is interpreted to reside within oceanic lithosphere of the NFZ at depths >35 km. Vp/Vs structure further indicates that underthrust oceanic lithosphere of the Explorer plate extends to 20 km depth beneath Brooks Peninsula. Our results support the hypothesis that the NFZ represents a structurally independent slice of oceanic lithosphere that exhibits (a) a more northerly trajectory than typically depicted, and (b) increased plate curvature when compared to either the Juan de Fuca or Explorer plates. Plain Language Summary: A newly determined earthquakes catalog is leveraged to study seismic velocity structure in British Columbia. The Nootka Fault zone (NFZ), the boundary that separates the Juan de Fuca and Explorer oceanic plates, is interpreted as a structurally complex region based on earthquake locations and a diagnostic velocity structure observed in subduction zones around the world. In particular, landward‐dipping layering in P‐to‐S velocity ratio parallels earthquake concentrations to depths >30 km beneath the continental slope and suggests that the NFZ comprises a structurally independent sliver of oceanic material. Three M > 6 earthquakes from 2004, 2011, and 2014 and their associated aftershocks provide constraints that support a recent re‐definition of the NFZ – one with a more northerly trajectory that intersects Vancouver Island south of Brooks Peninsula. To the north and south of the NFZ, more shallowly dipping Explorer and Juan de Fuca plates are interpreted from the tomographic model, respectively. Supporting evidence for a steeply dipping NFZ is considered from independent studies offshore Vancouver Island, and we posit that the 2014 Mw 6.6 event may have reactivated a plate‐bending fault in response to increased plate curvature. Key Points: A new seismic event catalog is compiled for southwest British ColumbiaTomographic results suggest the Nootka Fault zone represents a structurally independent region of oceanic lithosphereVp/Vs structure indicates that underthrust Explorer plate is present beneath Brooks Peninsula [ABSTRACT FROM AUTHOR]

Geophysical observations including surface heat flow data indicate the subducting slab becomes fully coupled to the overlying mantle wedge at ∼70–80 km depth. This maximum depth of decoupling (MDD) separates cool, stagnant forearc mantle from warmer, convecting mantle capable of generating arc magmas. Thermodynamic calculations demonstrate that talc is stable in H2O‐undersaturated parts of the mantle wedge where its stability is controlled by the pressure‐dependent, fluid‐absent reaction: talc + forsterite = antigorite + enstatite, which occurs at pressures ∼2–2.5 GPa (∼70–80 km depth) and temperatures <650°C. At shallower depths, H2O‐undersaturated portions of the basal mantle wedge contain talc, which experimental studies show dramatically weakens rocks. At greater depths, talc is restricted to silica‐rich portions of the mantle wedge. The common MDD in subduction zones may reflect the downdip transition from a talc‐present decoupled shear zone to a talc‐absent fully coupled interface along the base of the mantle wedge. Plain Language Summary: In many subduction zones, the relative strength of the plate interface compared to the overlying mantle changes dramatically at 70–80 km depth. At shallower levels, the interface acts as a weak shear zone decoupling the subducting plate from the overlying cool, stagnant mantle. At deeper levels, the strength of the shear zone increases such that subducting plate drives convection in the overlying mantle creating the high temperatures needed for arc magmatism. We propose the change in interface strength may be related to the stability of talc, a very weak mineral, in the forearc mantle. At shallower levels, talc is stable in a wide range of H2O‐undersaturated mantle compositions. At deeper levels, the stability field of talc is dramatically reduced and restricted to silica‐rich regions of the mantle. Key Points: Talc, a very weak mineral, is stable in H2O‐undersaturated ultramafic (mantle) rocks at P < ∼2–2.5 GPa (70–80 km depth) and T < ∼650°CAt pressures greater than ∼2–2.5 GPa, talc breaks down via the P‐dependent reaction: talc + forsterite = antigorite + enstatiteThe breakdown of talc may explain the change in subduction‐interface rheology at 70–80 km depth where full slab‐mantle coupling begins [ABSTRACT FROM AUTHOR]

What controls subduction megathrust seismogenesis downdip of the mantle wedge corner (MWC)? We propose that, in the region of the 2010 Mw = 8.8 Maule, Chile, earthquake, serpentine minerals derived from the base of the hydrated mantle wedge exert a dominant control. Based on modeling, we predict that the megathrust fault zone near the MWC contains abundant lizardite/chrysotile‐rich serpentinite that transforms to antigorite‐rich serpentinite at greater depths. From the MWC at 32–40 km depth to at least 55 km, the predominantly velocity‐strengthening megathrust accommodated dynamic propagation of the 2010 rupture but with small slip and negative stress drop. The downdip distribution of interplate aftershocks exhibits a gap around the MWC that can be explained by the velocity‐strengthening behavior of lizardite/chrysotile. Interspersed velocity‐weakening and dynamic weakening antigorite‐rich patches farther downdip may be responsible for increased abundance of aftershocks and possibly for some of the high‐frequency energy radiation during the 2010 rupture. Plain Language Summary: A subduction megathrust rupture may extend from the trench to beneath populated coastal area. To understand what controls the megathrust seismogenic behavior of its deeper part, we conduct a case study of the 2010 magnitude 8.8 Maule, Chile, earthquake and its aftershocks. With numerical thermal modeling, we find that the deep seisomogenic behavior may be controlled by serpentine materials derived from the base of the overlying hydrated mantle wedge. The deep part of the megathrust ruptured "passively" against increasing resistance (negative stress drop). Lower‐temperature serpentines lizardite and chrysotile that are known to facilitate aseismic slip may be responsible for an observed gap in aftershock distribution in the downdip direction. Higher‐temperature serpentine antigorite that is known to facilitate seismic slip may be responsible for increased aftershocks farther downdip and possibly radiation of high‐frequency energy during the 2010 Maule earthquake. Key Points: Seismogenic behavior of subduction megathrust may be affected by materials derived from the base of the serpentinized forearc mantle wedgeThermopetrologic modeling suggests abundance of lizardite and chrysotile around the mantle wedge corner but antigorite farther downdipSerpentine frictional behavior may explain distributions of stress drop and seismic energy radiation in 2010 rupture and aftershocks [ABSTRACT FROM AUTHOR]

Although the effects of elevated ozone on aboveground carbon (C) assimilation are well understood, its effects on soil C fluxes are less certain. Mesocosms taken from a lowland raised bog in northern England were exposed in open-top chambers for 2 years to ambient air or ambient air plus ozone elevated for 8 h day by an average of 49 ppb in summer and 10 ppb in winter. The effects of elevated ozone on methane emission and ecosystem dark respiration were measured throughout this period, along with soil and plant variables. Methane emissions were significantly reduced, by about 25%, by elevated ozone during midsummer periods of both years, but no significant effect of ozone was found during the winter periods. Dark ecosystem respiration was not significantly affected by elevated ozone. There was no evidence that effects of elevated ozone on methane emissions were mediated through changes in aboveground plant biomass or soil water dissolved organic C concentrations. Our results imply that the increased northern hemisphere background ozone concentrations over the 21st century that are predicted by most models may reduce the rate of increase in methane emissions as the region warms. [ABSTRACT FROM AUTHOR]

High-grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12-15 km. Shear-sense indicators record consistent top-to-the-SE, or left-lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal-plane slip, affirming both the consistency of shear sense and high-grade syn-kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal-plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape-preferred orientation, and (3) the presence of oriented muscovite 'fish' included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left-lateral (top-to-the-SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen-parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen-normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left-oblique convergence in which strain within the orogen was partitioned into deep-level strike slip and shallow-level contraction. [ABSTRACT FROM AUTHOR]

In order to investigate metamorphic reactions and fluid production in subduction zones, a 2-dimensional finite difference model has been constructed. The model simulates heat transfer in a subduction zone with a 27° dip. A spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions; elsewhere the spatial resolution is 5 km. In these calculations, the oceanic crust is assumed to contain 2 wt % bound volatiles, distributed homogeneously within the 7.5-km-thick crust. Fluids are released by continuous and discontinuous end-members of three different dehydration models: pressure-sensitive, temperature-sensitive, and amphibole (negative dP/dT) dehydration. Subduction zone pressure-temperature-time (P-T-t) paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction. In mature subduction zones, and in the absence of significant frictional heating and induced mantle convection, P-T-t paths encounter subsolidus conditions at depths of 100-150 km beneath magmatic arcs, suggesting that slab dehydration reactions play an important role in arc magma genesis. Metamorphic dehydration reactions that consume 50 kJ/kg result in P-T-t paths that are only 5 K cooler than models that neglect such reactions. In both the temperature and amphibole dehydration models, the fluid production region moves to greater depths over time as the subduction zone cools. Within the oceanic crust, fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models. This phenomenon, coupled with the weakening effect of devolatilization reactions, may result in the accretion of oceanic crust to the hanging wall of the subduction zone. For a convergence rate of 3 cm/yr, average fluid fluxes out of the subducting slab range from ∼0.1 kg fluid/(m² yr) for the continuous reaction models to >1 kg fluid/(m² yr) for the discontinuous pressure and amphibole dehydration models. Vertical fluid fluxes on the order of 1 kg fluid/(m² yr) can substantially perturb the thermal structure of overlying rocks and can cause large-scale hydration and metasomatism of the overlying mantle wedge. In contrast, the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere. [ABSTRACT FROM AUTHOR]