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Ongoing oroclinal bending in the Cascadia forearc and its relation to concave-outboard plate margin geometry
The concave-inboard (concave toward the overriding plate) geometry of most convergent margins is considered a natural consequence of the depression of the edge of a thin spherical cap, whereas concave-outboard margin segments commonly form around indenters on the subducting plate. At the Cascadia subduction zone, the apex of a > 500-km-long concave-outboard bend in the trench presently shows no obvious subduction of an indenter, but does coincide with the axis of an outboard-facing concavity in upper-plate rocks arched around the Olympic Peninsula in northwestern Washington State, USA. Here we synthesize paleomagnetic and structural data together with new analyses of Global Navigation Satellite System data to show that the upper plate at Cascadia has been folded from the Miocene to the present into an orocline with an axial trace that bisects the Olympic Peninsula. The processes that accommodate bending, which we suggest include (1) folding by flexural slip on the orocline limbs and (2) shortening, uplift, and escape within the core of the fold at the Olympic Mountains, have the combined result of relative motion of the forearc towards the arc at the core of the orocline, and sustained opposing rotations of the upper plate on the orocline limbs. We propose that oroclinal bending is promoted and maintained by along-strike variations in plate-boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of concave-outboard margins without the need for a subducting indenter
Timing and magnitude of shortening within the inner fore arc of the Japan Trench
New structural data and kinematic modeling provide evidence for Plio-Quaternary, inner fore-arc shortening inboard of the Japan Trench, northeastern Honshu, accommodated by the Futaba fault, a high-angle, basement-involved fault that bounds the Abukuma massif on the east. Significant throw along the Futaba fault associated with exhumation of the massif is implied by a regionally extensive footwall syncline, the absence of Neogene sediments in the hanging wall, and high relief in the hanging wall adjacent to the fault. Kinematic fault-related fold modeling best reproduces fold geometry with 2.0-3.1 km of dip slip along a 40°-55° west dipping reverse fault. At the southern tip of the fault, tephra horizons of known age within units that predate and postdate deformation bracket the onset of deformation to 3.95-5.6 Ma and are used to calculate an average slip rate of 0.5-0.7 mm/yr, a throw rate of 0.3-0.5 mm/yr, and a shortening rate of 0.3-0.5 mm/yr. The northeastern Japan subduction zone is viewed as a classic example of an erosive margin, where offshore subsidence records have been used to argue for Neogene basal erosion of the upper plate. Tectonic erosion rates have been estimated from reconstructions of the paleomargin that assume no upper plate deformation and temporally constant fore-arc taper. Evidence presented here for Neogene fore-arc shortening, however, suggests that the upper plate is deformable and implies that that offshore subsidence records may reflect a combination of tectonic erosion and upper plate shortening
Relationship between outer forearc subsidence and plate boundary kinematics along the Northeast Japan convergent margin
Tectonic erosion along convergent plate boundaries, whereby removal of upper plate material along the subduction zone interface drives kilometer-scale outer forearc subsidence, has been purported to explain the evolution of nearly half the world's subduction margins, including part of the history of northeast Japan. Here, we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern Japan. A synthesis of newly updated analyses of outer forearc subsidence, the timing and kinematics of upper plate deformation, and the history of plate convergence along the Japan trench demonstrate that the onset of rapid fore-arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the Sea of Japan and with an acceleration in convergence rate at the trench. In Plio-Quaternary time, relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate. The coincidence of these changes across the forearc, arc, backarc system appears to require an explanation at the scale of the entire plate boundary. Similar observations along other western Pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule, rather than the exception. We suggest that a significant component of forearc subsidence at the northeast Japan margin is not the consequence of basal tectonic erosion, but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics. If correct, this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins. Key Points Forearc subsidence coeval with upper plate extension, increased convergence rate Regional forearc subsidence/uplift likely reflects plate boundary-scale dynamics Results imply basal tectonic erosion is less efficient than previously assume
Climate change resilience of a globally important sea turtle nesting population
This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordFew studies have looked into climate change resilience of populations of wild animals. We use a model higher vertebrate, the green sea turtle, as its life history is fundamentally affected by climatic conditions, including temperature-dependent sex determination and obligate use of beaches subject to sea level rise (SLR). We use empirical data from a globally important population in West Africa to assess resistance to climate change within a quantitative framework. We project 200 years of primary sex ratios (1900–2100) and create a digital elevation model of the nesting beach to estimate impacts of projected SLR. Primary sex ratio is currently almost balanced, with 52% of hatchlings produced being female. Under IPCC models, we predict: (a) an increase in the proportion of females by 2100 to 76%–93%, but cooler temperatures, both at the end of the nesting season and in shaded areas, will guarantee male hatchling production; (b) IPCC SLR scenarios will lead to 33.4%–43.0% loss of the current nesting area; (c) climate change will contribute to population growth through population feminization, with 32%–64% more nesting females expected by 2120; (d) as incubation temperatures approach lethal levels, however, the population will cease growing and start to decline. Taken together with other factors (degree of foraging plasticity, rookery size and trajectory, and prevailing threats), this nesting population should resist climate change until 2100, and the availability of spatial and temporal microrefugia indicates potential for resilience to predicted impacts, through the evolution of nest site selection or changes in nesting phenology. This represents the most comprehensive assessment to date of climate change resilience of a marine reptile using the most up-to-date IPCC models, appraising the impacts of temperature and SLR, integrated with additional ecological and demographic parameters. We suggest this as a framework for other populations, species and taxa.Portuguese Foundation for Science and TechnologyMAVA FoundationRufford Foundatio
Slip Inversion Along Inner Fore-Arc Faults, Eastern Tohoku, Japan
The kinematics of deformation in the overriding plate of convergent margins may vary across timescales ranging from a single seismic cycle to many millions of years. In Northeast Japan, a network of active faults has accommodated contraction across the arc since the Pliocene, but several faults located along the inner fore arc experienced extensional aftershocks following the 2011 Tohoku-oki earthquake, opposite that predicted from the geologic record. This observation suggests that fore-arc faults may be favorable for stress triggering and slip inversion, but the geometry and deformation history of these fault systems are poorly constrained. Here we document the Neogene kinematics and subsurface geometry of three prominent fore-arc faults in Tohoku, Japan. Geologic mapping and dating of growth strata provide evidence for a 5.6–2.2 Ma initiation of Plio-Quaternary contraction along the Oritsume, Noheji, and Futaba Faults and an earlier phase of Miocene extension from 25 to 15 Ma along the Oritsume and Futaba Faults associated with the opening of the Sea of Japan. Kinematic modeling indicates that these faults have listric geometries, with ramps that dip ~40–65°W and sole into subhorizontal detachments at 6–10 km depth. These fault systems can experience both normal and thrust sense slip if they are mechanically weak relative to the surrounding crust. We suggest that the inversion history of Northeast Japan primed the fore arc with a network of weak faults mechanically and geometrically favorable for slip inversion over geologic timescales and in response to secular variations in stress state associated with the megathrust seismic cycle
Green turtles highlight connectivity across a regional marine protected area network in west Africa
Networks of marine protected areas (MPAs) are invaluable for the protection of species
with high dispersal capacity, yet connectivity within networks is poorly understood. We
demonstrate the connectivity within the regional MPA network in West Africa (RAMPAO),
mediated by the largest green turtle population in the eastern Atlantic Ocean. We
equipped with satellite tags 45 female green turtles nesting in the Bijagó
s Archipelago,
Guinea-Bissau, and tracked them during internesting, migration, and foraging to quantify
the degree of coverage the RAMPAO network provides during each of these critical
periods. During the internesting period, turtles were largely concentrated around the
nesting islands, with a mean of 94.8% (SD 0.1%, range: 46% - 100%, n = 40 turtles) of
tracking positions falling within MPA limits. Among the 35 turtles successfully tracked into
the foraging period, we identified variable migratory strategies, with 12 turtles remaining
near-resident at distances of 40-90 km from breeding sites, 10 turtles migrating 300-400
km to The Gambia and Senegal, and 13 turtles traveling >1000 km to northern Mauritania.
Of the 35 foraging turtles, 26 used MPAs, with a mean of 78.0% (SD 34.8%, range: 3.7% -
100%) of their tracking positions falling within the limits of RAMPAO MPAs, across
Guinea-Bissau, Senegal and Mauritania. Migration corridors with high concentrations of
passing turtles were mostly located nearshore, and 21% of these high passage areas fell
within the MPA network. Overall, we found that this population connects five RAMPAO
MPAs, yet some foraging sites (e.g., in the Bijagó
s) and important migration areas (e.g.,
Cap-Vert peninsula) described here are currently unprotected. These results are relevant
to any considerations of MPA extension or establishment within the regional network,
which would contribute towards meeting the Convention on Biological Diversity targets for
national marine protected area estate coverage. By documenting biological connectivity
across RAMPAO, this study represents an important example of the relevance of
international protected area networks for green turtle conservation and for wider
conservation action at a regional scale.Fundação para a Ciência e Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio
IODP workshop: Core-Log Seismic Investigation at Sea – Integrating legacy data to address outstanding research questions in the Nankai Trough Seismogenic Zone Experiment
The first International Ocean Discovery Program (IODP) Core-Log-Seismic
Integration at Sea (CLSI@Sea) workshop, held in January–February 2018,
brought together an international, multidisciplinary team of 14 early-career
scientists and a group of scientific mentors specialized in subduction zone
processes at the Nankai Trough, one of the Earth's most active
plate-subduction zones located off the southwestern coast of Japan. The goal
of the workshop was to leverage existing core, log, and seismic data
previously acquired during the IODP's Nankai Trough Seismogenic Zone
Experiment (NanTroSEIZE), to address the role of the deformation front of the
Nankai accretionary prism in tsunamigenic earthquakes and slow slip in the
shallow portion of the subduction interface. The CLSI@Sea workshop was
organized onboard the D/V Chikyu concurrently with IODP Expedition
380, allowing workshop participants to interact with expedition scientists
installing a long-term borehole monitoring system (LTBMS) at a site where the
workshop's research was focused. Sedimentary cores from across the
deformation front were brought onboard Chikyu, where they were made
available for new description, sampling, and analysis. Logging data, drilling
parameters, and seismic data were also available for investigation by
workshop participants, who were granted access to Chikyu laboratory
facilities and software to perform analyses at sea.Multi-thematic presentations facilitated knowledge transfer between the
participants across field areas, and highlighted the value of
multi-disciplinary collaboration that integrates processes across different
spatiotemporal scales. The workshop resulted in the synthesis of existing
geophysical, geologic, and geochemical data spanning IODP Sites C0006,
C0007, C0011 and C0012 in the NanTroSEIZE area, the identification of key
outstanding research questions in the field of shallow subduction zone
seismogenesis, and fostered collaborative and individual research plans
integrating new data analysis techniques and multidisciplinary approaches.</p
Low Coseismic Friction on the Tohoku-Oki Fault Determined from Temperature Measurements
The frictional resistance on a fault during slip controls earthquake dynamics. Friction dissipates heat during an earthquake; therefore, the fault temperature after an earthquake provides insight into the level of friction. The Japan Trench Fast Drilling Project (Integrated Ocean Drilling Program Expedition 343 and 343T) installed a borehole temperature observatory 16 months after the March 2011 moment magnitude 9.0 Tohoku-Oki earthquake across the fault where slip was ~50 meters near the trench. After 9 months of operation, the complete sensor string was recovered. A 0.31°C temperature anomaly at the plate boundary fault corresponds to 27 megajoules per square meter of dissipated energy during the earthquake. The resulting apparent friction coefficient of 0.08 is considerably smaller than static values for most rocks
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