Han
Shuoshuo
Han
Shuoshuo
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ArticleA 2-D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey(John Wiley & Sons, 2016-08-14) Horning, Gregory W. ; Canales, J. Pablo ; Carbotte, Suzanne M. ; Han, Shuoshuo ; Carton, Helene ; Nedimovic, Mladen R. ; van Keken, Peter E.We report results from a wide-angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two-dimensional velocity model of the crust and upper mantle is derived from a joint reflection-refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages >1 Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5–2.1 wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8 wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (<0.3 wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.
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ArticleStructure and evolution of northern Juan de Fuca Crust and uppermost mantle over the last 8 Ma from an active‐source seismic tomography study(American Geophysical Union, 2022-09-13) Boulahanis, Bridgit ; Carbotte, Suzanne M. ; Canales, Juan Pablo ; Han, Shuoshuo ; Nedimović, Mladen R.We present results from a two-dimensional wide-angle controlled source seismic transect designed to characterize the velocity structure of the oceanic crust and uppermost mantle spanning the northern Juan de Fuca (JdF) plate from near Endeavor ridge to the Cascadia margin. Reflection and refraction travel time inversion is used to derive a tomographic Vp model of sediments, crust, and upper mantle. Velocity model results are compared to baseline reference Vp values for unaltered crustal and upper mantle rocks at temperatures assuming plate cooling. Effective medium theory is used to infer the degree of hydration of the crust and mantle. Results indicate a somewhat fractured and hydrated upper crust (≤2.5 wt% water), a near dry lower crust (≤0.7 wt%) and upper mantle (≤0.5 wt%) west of the deformation front, and an ∼75 km wide region of modestly lower velocity in the mid-plate. Comparison with prior results from a complementary transect offshore Oregon indicates significant differences in Vp of the upper crust, with lower Vp-inferred porosity along most of the Washington transects that may reflect different extents of fault-related alteration and sediment burial histories. Approaching the deformation front, Washington transect Vp structure indicates dryer conditions than offshore Oregon, consistent with differences in extent of subduction bend faulting found in reflection imaging studies. On both transects, quasi-abrupt changes in plate properties at ages of 8, 6, and 3.4/4 Ma are found. Distinct crustal accretion modes are recognized, aligning with changes in JdF plate motion and recent history of Cobb-Eickelberg hotspot influence on crustal accretion at the JdF ridge.
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ArticleAlong-trench structural variations of the subducting Juan de Fuca Plate from multichannel seismic reflection imaging(John Wiley & Sons, 2018-04-22) Han, Shuoshuo ; Carbotte, Suzanne M. ; Canales, J. Pablo ; Nedimovic, Mladen R. ; Carton, HeleneTo characterize the along‐strike structural variations of the Juan de Fuca (JdF) Plate as it enters the Cascadia subduction zone, we present prestack time migrated multichannel seismic reflection images of the JdF Plate along a 400‐km‐long trench‐parallel transect extending from 44.3°N to 47.8°N. Beneath the 1.8–3.0‐km‐thick sediment cover, our data reveal basement topographic anomalies associated with a 1.2‐km‐high seamount and in the vicinity of propagator wakes (390–540‐m relief). Weak Moho reflections are imaged beneath the propagator wakes and coincide with reduced Vp in the lower crust and/or uppermost mantle. The inferred locations of propagator wakes in the downgoing plate collocate with some of the boundaries of episodic tremor and slip events. We propose that the structural and hydration heterogeneities associated with these features could lead to anomalous plate interface properties and contribute to episodic tremor and slip segmentation. Intracrustal reflections with apparent dips (20°–30°) consistent with subduction bending normal faults change near 45.8°N, from northward dipping reflections confined to the middle crust in the north to antithetic reflections through the crust in the south, coinciding with a Vp reduction in the lower crust. These observations indicate more extensive faulting deformation and associated hydration of the JdF Plate south of 45.8°N, which likely results from variations of slab dip and resistance to subduction across 46°N. Basement offsets and abrupt depth/amplitude changes in Moho reflections are imaged beneath the four major WNW trending strike‐slip faults that cross the Cascadia deformation front, providing strong evidence of a lower plate origin for these faults.
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ArticleSeismic reflection imaging of the Juan de Fuca plate from ridge to trench : new constraints on the distribution of faulting and evolution of the crust prior to subduction(John Wiley & Sons, 2016-03-21) Han, Shuoshuo ; Carbotte, Suzanne M. ; Canales, J. Pablo ; Nedimovic, Mladen R. ; Carton, Helene ; Gibson, James C. ; Horning, Gregory W.We present prestack time-migrated multichannel seismic images along two cross-plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper-middle crust. The regional difference in subduction bend-faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate-depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°–40°) lower crustal reflections are imaged in ~6–8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.
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ArticleVp/Vs ratio of incoming sediments off Cascadia subduction zone from analysis of controlled-source multicomponent OBS records(American Geophysical Union, 2020-05-28) Zhu, Jian ; Canales, J. Pablo ; Han, Shuoshuo ; Carbotte, Suzanne M. ; Arnulf, Adrien F. ; Nedimovic, Mladen R.P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vs versus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.