Subduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the
- NSF-PAR ID:
- 10218108
- Date Published:
- Journal Name:
- Bulletin of the Seismological Society of America
- ISSN:
- 0037-1106
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract X EOLX ELEK ‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest aM 6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments. -
In the eastern San Gabriel Mountains, located north of Los Angeles, California, the late Cenozoic Cucamonga thrust has uplifted and exposed the lower crustal root of the Mesozoic Southern California Batholith. We use structural data and U-Pb zircon analyses from these exposures to document changes in the style of intra-arc deformation in the batholith as the Laramide Orogeny began during the Late Cretaceous (at or after ~90 Ma). At the base of the uplifted section, a 4 km-thick package of metasedimentary rock records the intrusion of amphibolite, charnokite and other dikes of probable Jurassic to Early Cretaceous age. The oldest gneissic fabrics (S1, S2) in these rocks record Early Cretaceous partial melting, granulite-facies metamorphism, and top-to-the-S and -SE (present day reference frame) reverse motion on surfaces that dip moderately to the N and NW. These structures define a D1/D2 thrust system that formed on the trench side of the arc and was active during the Early Cretaceous. From 89-77 Ma this thrust system was reactivated by oblique-slip shear zones (D3) that record sinistral-reverse displacements on N- and NW-dipping surfaces. The timing of deformation in these latter shear zones is indicated by the age of 90-85 Ma syn-kinematic intrusions of the Tonalite of San Sevaine Lookout. After emplacement of the tonalite, the lower crustal section was deformed by a series of S-vergent, overturned folds. The emplacement of granodioritic dikes into the axial planes of some of these folds suggests that they formed during the latest stages of D3 transpression and tonalite emplacement. Superimposed on all these structures are a series of ductile-to-brittle thrust faults and folds that appear to be related to formation of the late Cenozoic Cucamonga thrust fault at the southern edge of the San Gabriel mountains. These data show that the Southern California Batholith in the San Gabriel Mountains records a tectonic transition from Early Cretaceous reverse faulting and crustal imbrication on the trench side of the arc to Late Cretaceous transpression and oblique sinistral-reverse deformation during a magmatic flare-up from 89-77 Ma. Another major episode of shortening and crustal imbrication occurred during the late Cenozoic when the Cucamonga thrust uplifted the San Gabriel block.more » « less
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SUMMARY The dynamics of accretionary prisms and the processes that take place along subduction interfaces are controlled, in part, by the porosity and fluid overpressure of both the forearc wedge and the sediments transported to the system by the subducting plate. The Hikurangi Margin, located offshore the North Island of New Zealand, is a particularly relevant area to investigate the interplay between the consolidation state of incoming plate sediments, dewatering and fluid flow in the accretionary wedge and observed geodetic coupling and megathrust slip behaviour along the plate interface. In its short geographic extent, the margin hosts a diversity of properties that impact subduction processes and that transition from north to south. Its southernmost limit is characterized by frontal accretion, thick sediment subduction, the absence of seafloor roughness, strong interseismic coupling and deep slow slip events. Here we use seafloor magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data collected along a profile through the southern Hikurangi Margin to image the electrical resistivity of the forearc and incoming plate. Resistive anomalies in the shallow forearc likely indicate the presence of gas hydrates, and we relate deeper forerarc resistors to thrust faulting imaged in colocated seismic reflection data. Because MT and CSEM data are highly sensitive to fluid phases in the pore spaces of seafloor sediments and oceanic crust, we convert resistivity to porosity to obtain a representation of fluid distribution along the profile. We show that porosity predicted by the resistivity data can be well fit by an exponential sediment compaction model. By removing this compaction trend from the porosity model, we are able to evaluate the second-order, lateral changes in porosity, an approach that can be applied to EM data sets from other sedimentary basins. Using this porosity anomaly model, we examine the consolidation state of the incoming plate and accretionary wedge sediments. A decrease in porosity observed in the sediments approaching the trench suggests that a protothrust zone is developing ∼25 km seaward of the frontal thrust. Our data also imply that sediments deeper in the accretionary wedge are slightly underconsolidated, which may indicate incomplete drainage and elevated fluid overpressures of the deep wedge.
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Booth, A.M. (Ed.)The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out the possibility that such materials were removed via subduction and/or strike slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice Formation–filled basin demonstrably occurred adjacent to North America; and (3) the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench assemblages underplated >100 km inboard of the subduction margin, presumably during a previously unrecognized phase of shallow-angle subduction. In aggregate, these relations suggest that the Klamath Mountains and adjacent Franciscan complex represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic subduction zone, wherein the downgoing oceanic plate took a variety of trajectories into the mantle. We speculate that the downgoing plate contained alternating tracts of smooth and dense versus rough and buoyant lithosphere—the former gliding into the mantle (facilitating slab rollback and upper plate extension) and the latter enhancing basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific Ocean, with subduction of the Australian plate beneath New Guinea and adjacent island groups providing perhaps the best analog.more » « less
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Abstract The timing of crustal shortening and strike‐slip faulting along the East Kunlun Shan provides insight into the history of surface uplift and may constrain the time at which the Tibetan Plateau reached high elevations. We investigate a series of extensional basins and restraining bends along the Xidatan strand of the Kunlun strike‐slip fault, which provide an ideal setting to unravel the tectonic history of the northern plateau margin. We present new apatite (U‐Th)/He, apatite fission track, and zircon (U‐Th)/He ages and QTQt thermal modeling,40Ar/39Ar fault gouge dating, and structural mapping from the central East Kunlun Shan. Our data suggest that the East Kunlun Shan experienced slow to negligible exhumation until late Cretaceous time, followed by an increase in rate by 65–50 Ma. Along with a ~47 Ma fault gouge age, we posit that the Paleocene–early Eocene was a time of crustal shortening along the northern plateau. Rapid exhumation along transpressional portions of the Xidatan fault initiated by 23–20 Ma, which we interpret as the local onset of strike‐slip faulting. An early Miocene transition from north‐south crustal shortening to left‐lateral shear along the East Kunlun Shan, the onset of normal and strike‐slip faulting in central and southern Tibet by 18 Ma, and lower crustal flow in eastern Tibet by 13 Ma suggest the establishment of orogen‐wide east‐west oriented extension and extrusion by the middle Miocene. The plateau‐wide shift in stress accommodation implies that high gravitational potential energy, and likely high elevation, was attained by the middle Miocene.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1785/0120200204
