Michael G. Bostock

An inverted continental Moho and serpentinization of the forearc mantle

Volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge, resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere. Asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back-arc region, but the forearc mantle (in the corner of the wedge) is thought to be significantly cooler. Here we explore the structure of the mantle wedge in the southern Cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers. We find very low shear-wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an ‘inverted’ continental Moho, which reverts to normal polarity seaward of the Cascade arc. This observation provides compelling evidence for a highly hydrated and serpentinized forearc region, consistent with thermal and petrological models of the forearc mantle wedge. This serpentinized material is thought to have low strength and may therefore control the down-dip rupture limit of great thrust earthquakes, as well as the nature of large-scale flow in the mantle wedge.

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction—and indeed plate tectonics—to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson’s ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson’s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

Mantle stratigraphy and evolution of the Slave province

A data set of 1033 three-component, P wave seismograms from five broadband stations at the Yellowknife Array is assembled to investigate mantle structure below the southern Slave province in Canadas Northwest Territories. Following wave field decomposition, seismograms are source-normalized through simultaneous deconvolution to estimate the near-receiver impulse response as a function of epicentral distance and back azimuth. Images of impulse response reveal a well-developed mantle stratigraphy, anisotropic in part, extending from the Mohorovicic discontinuity to the transition zone, A layer of depth-localized anisotropy (±5%), termed H, is situated between ∼70 and 80 km depth with an average shear velocity comparable to that of the ambient mantle and a sharp upper boundary less than 100 m in transition width. The absence of free surface crustal reverberations on the transverse component affords a window into the upper mantle between 100 and 200 km depth. A sequence of at least two layers between 120–150 km depth, collectively termed X, is most clearly evident to the north and is underlain by a second structure L which dips from 170 km in the west to 230 km into the center of the Slave province. The deepest interface above the transition zone W marks a shear velocity inversion near 350 km depth whose signature is restricted to the SV component signalling a dominantly isotropic response. Consideration of these observations in light of data acquired in a recent LITHOPROBE seismic reflection traverse and in petrological studies of kimberlite xenoliths prompts speculation into the role of subduction in craton stabilization. It is suggested that the proto-Slave craton was assembled through processes of shallow subduction resulting in a near-horizontal mantle stratigraphy (i.e., H, X) both compositional and rheological in nature. Interpretation of L as the continuation of dipping reflectors on the seismic reflection profile argues for a final phase of craton assembly involving oblique underplating of subducted lithosphere in the Proterozoic. Subsequent modification of the lithosphere, as manifest by Phanerozoic kimberlite volcanism, may be related to W if an interpretation as the top of a layer containing a dense silicate melt fraction is invoked.

Multiparameter two‐dimensional inversion of scattered teleseismic body waves 1. Theory for oblique incidence

This is the first paper in a three-part series that examines formal inversion of the teleseismic P wave coda for discontinuous variations in elastic properties beneath dense, three-component, seismic arrays. In this paper, we develop the theoretical framework for a migration method that draws upon the tenets of inverse scattering theory and is amenable to practical implementation. The forward problem is formulated for two-dimensional (2-D) heterogeneity in observance of formal sampling requirements and currently accessible instrumentation. A ray theoretic Greens function, corresponding to a line source with axial component of forcing, is employed within the 2-D Born approximation to accommodate planar, incident wave fields at arbitrary back azimuths. Both the forward scattered response generated by the upgoing incident wave field and the backscattered response created by its reflection at the free surface are included within the formulation. In accordance with the high-frequency and single-scattering approximations employed in the forward problem the inverse problem is cast as a generalized Radon transform. The resulting back projection operator is well suited to the teleseismic context in several respects. It is tolerant of irregularities in array geometry and source distribution and allows a full complement of global seismicity to be utilized through its accommodation of oblique incidence. By permitting both independent and simultaneous treatment of different scattering modes (reflections, transmissions, conversions) the inversion formula facilitates a direct appraisal of individual mode contributions to the recovery of structure. In particular, it becomes evident that incorporation of backscattered modes leads to (1) a better localization of structure than possible using forward scattered energy and (2) the imposition of complementary constraints on elastic properties.

Multiparameter two‐dimensional inversion of scattered teleseismic body waves 3. Application to the Cascadia 1993 data set

This is the third paper in a three-part series that examines formal inversion of the teleseismic P wave coda for discontinuous, two-dimensional (2-D) variations in elastic properties beneath dense arrays of three-component, broadband seismometers. In this paper, the method is applied to data from the Incorporated Research Institutions for Seismology-Program for Array Seismic Studies of the Continental Lithosphere (IRIS-PASSCAL) Cascadia 1993 experiment undertaken across central Oregon. Two major features are imaged in the resulting model. The continental Moho becomes evident � 150 km from the coast beneath the Western Cascades and extends through the eastern end of the profile at 35-40 km depth. In the western portion of the model, oceanic crust of the subducting Juan de Fuca plate dips shallowly (12� ) at the coast and more steeply (27� ) below the Willamette Valley and is evident to depths of >100 km beneath the High Cascades. The abrupt increase in plate dip at � 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature. Building on previous work, we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust. Progressive dehydration at lower-grade facies conditions culminates in the transformation to eclogite, producing a pronounced increase in the seismic velocity, density and dip of the subducting plate, and structural complexity in the overlying wedge.

High pore pressures and porosity at 35 km depth in the Cascadia subduction zone

In the Cascadia subduction zone, beneath southern Vancouver Island at 25–45 km depth, converted teleseismic waves reveal an ∼5-km-thick landward-dipping layer with anomalously high Vp/Vs averaging 2.35 ± 0.10 (2σ), interpreted as subducted oceanic crust of the Juan de Fuca plate. This layer is observed downdip of the inferred locked seismogenic zone, in the region of episodic tremor and slip. Laboratory velocity measurements of crystalline rock samples made at 200 MPa confining pressure and elevated pore pressures demonstrate that Vp/Vs increases with increasing fluid-filled porosity. The observed high Vp/Vs values are best explained by pore fluids under near lithostatic pressure in a layer with a high porosity of 2.7%–4.0%. Such large volumes of fluid take ∼1 m.y. to accumulate based on reasonable rates of metamorphic fluid production of ∼10 –4 m 3 /(m 2 yr) in subducting Juan de Fuca crust and mantle. Accordingly, the permeability of the plate interface at these depths must be very low, ∼10 –24 to ∼10 –21 m 2 , or the porous layer must have a permeability –20 m 2 .

Anisotropic upper-mantle stratigraphy and architecture of the Slave craton

The Earths physical properties show a dominantly radial structure which is the result of compositional differentiation, isochemical phase changes and rheological layering. Rheological layering is perhaps the most difficult to investigate using conventional seismological techniques because the seismic manifestation of this property, elastic anisotropy, may closely mimic the effects of isotropic heterogeneity. Nonetheless, an improved characterization of Earths rheological structure promises important insights into such processes as plate dynamics and continental evolution. Here, I present a methodology for effectively characterizing sharp transitions in anisotropic, upper-mantle structure using the coda of teleseismic P-waves. Application to seismograms from the Slave craton reveals a well-developed stratigraphy, at least in part anisotropic, with major boundaries occurring at nominal depths of 75, 135 and 195 km. The geometry and sharpness of these discontinuities suggest a structural origin, perhaps involving shallow subduction.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

[1] Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Lithospheric assembly and modification of the SE Canadian Shield: Abitibi-Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe-Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Quebec-Ontario border. Data from a 600-km-long, N-S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low-velocity, NW-SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S-splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low-velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre-Grenvillian (Archean-Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low-velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian-White Mountain-New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.

A reconnaissance teleseismic study of the upper mantle and transition zone beneath the Archean Slave craton in NW Canada

Abstract The objective of this study is to investigate upper mantle structure below the Archean Slave craton, site of the oldest known rocks on Earth and numerous diamondiferous kimberlites, and thence to gain an understanding of early craton formation and kimberlite genesis. To this aim, a temporary array, consisting of 13 sites equipped with broadband seismometers, recorded teleseisms between November 1996 and May 1998. This data set has been augmented with recordings from the Yellowknife seismic array. Our three most robust observations and their interpretation are: (1) P-wave travel-time tomography reveals the oldest part of the craton, the Central Slave Basement Complex, to be underlain by the fastest seismic velocities, indicating that this block remains distinct well into mantle depths; (2) receiver function analysis requires only the Moho as major S-wave velocity discontinuity and points to a fairly constant crustal thickness throughout the Slave province; and (3) SKS splitting analysis shows little variation in delay times and fast polarization directions across the array, leading us to conclude that the present-day plate motion of North America is the primary cause for mantle fabric beneath the entire array. Furthermore, the data let us rule out the possibility that the Mackenzie plume had any seismologically detectable effect on the Slave lithosphere. More speculative results of our investigation, namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying Lac de Gras kimberlite field, and a possible Archean origin of two shallow low-velocity anomalies, will require further investigation.