Water Migration in the Subduction Mantle Wedge: A Two-Phase Flow Approach

Subduction zones are the main entry points of water into Earth's mantle and play an important role in the global water cycle. The progressive release of water by metamorphic dehydration induces important physical‐chemical processes, including subduction zone earthquakes. Yet, how water migrates in subduction zones is not well understood. We investigate this problem by explicitly modeling two‐phase flow processes, in which fluids migrate through a compacting and decompacting solid matrix. Our results show that water migration is strongly affected by subduction dynamics, which exhibits three characteristic stages in our models: (1) an early stage of subduction initiation; (2) an intermediate stage of gravity‐driven steepening of the slab; and (3) a late stage of quasi steady state subduction. Two main water pathways are found in the models: trenchward and arcward. They form in the first two stages and become steady in the third stage. Depending on the depth of water release from the subducting slab, water migration focuses in different pathways: a shallow release depth (e.g., 40 km) leads the water mainly through the trenchward pathway, a deep release depth (e.g., 120 km) promotes an arcward pathway and a long horizontal migration distance (~300 km) from the trench, and an intermediate release depth (e.g., 80 km) leads water to both pathways. We compare our models with seismic studies from southeast Japan (Saita et al., 2015, https://doi.org/10.1002/2015GL063084) and the west Hellenic subduction zone (Halpaap et al., 2018, https://doi.org/10.1002/2017JB015154) and provide geodynamical explanations for these seismic observations in natural subduction environments.publishedVersio

Localized crustal deformation along the central North Anatolian Fault Zone revealed by joint inversion of P-receiver functions and P-wave polarizations

The North Anatolian Fault Zone (NAFZ) is a major plate boundary that separates the Eurasian Plate to the north from the Anatolian Plate to the south and is associated with powerful damaging earthquakes. Despite numerous studies of the crust and upper mantle across the NAFZ, our understanding of the exact mechanisms and distribution of deformation with depth is still limited. Accurate models of the crustal velocity structure are key to assess seismic hazard associated with strike-slip deformation. Here, we address this need by employing a novel method that jointly inverts receiver function waveforms and P-wave polarizations to recover S-wave velocity structure from the surface to the upper mantle. The method is applied to a dense teleseismic data set collected across a segment of the central NAFZ in Turkey. The results provide important new constraints on the sedimentary thickness, depth to basement and Moho discontinuity beneath the region. Our estimates of uppermost sedimentary thickness range from 0 km in some areas (e.g. in the Central Pontides) to 6 km in the Çankırı Basin. Smaller basins are scattered along the NAFZ. A similar pattern is observed for the basement depth, with values exceeding 10 km beneath the Çankırı Basin, where the Moho is shallowest with a depth of ∼32 km. The Moho reaches a maximum depth of ∼42 km beneath the Central Pontides. Most other areas have an average Moho depth of 35–38 km. The results reveal clear structural–tectonic relationships in the crust: areas of fundamentally different sedimentary and crustal architecture are bounded by faults and suture zones. The NAFZ appears to accommodate small-scale basin and basement-highs, and acts as a thick-skinned (i.e. full crustal-scale) boundary between laterally displaced crustal blocks to the north and south. Seismicity clusters are centred on areas of low Vp/Vs ratios that may be representative of weak zones.publishedVersio

Array-conditioned deconvolution of multiple component teleseismic recordings

We investigate the applicability of an array-conditioned deconvolution technique, developed for analyzing borehole seismic exploration data, to teleseismic receiver functions and data preprocessing steps for scattered wavefield imaging. This multichannel deconvolution technique constructs an approximate inverse filter to the estimated source signature by solving an overdetermined set of deconvolution equations, using an array of receivers detecting a common source. We find that this technique improves the efficiency and automation of receiverfunction calculation and data preprocessing workflow. We apply this technique to synthetic experiments and to teleseismic data recorded in a dense array in northern Canada. Our results show that this optimal deconvolution automatically determines and subsequently attenuates the noise from data, enhancing P-to-S converted phases in seismograms with various noise levels. In this context, the array-conditioned deconvolution presents a new, effective and automatic means for processing large amounts of array data, as it does not require any ad-hoc regularization; the regularization is achieved naturally by using the noise present in the array itself

The crustal structure in the Northwest Atlantic region from receiver function inversion – Implications for basin dynamics and magmatism

The Labrador Sea and Baffin Bay form an extinct Palaeogene oceanic spreading system, divided by a major continental transform, the Davis Strait, with the whole region defined as the Northwest Atlantic. The Davis Strait hosts the Ungava Fault Zone and is the central structural element of the Davis Strait Large Igneous Province (DSIP) that formed broadly coeval with continental breakup to its north and south. While constraints on the crustal structure in this region primarily exist in the offshore, crustal models are limited onshore, which makes an interpretation of regional structures as well as the extent, and therefore origin of the DSIP extremely difficult to ascertain. Here, we have collected all available teleseismic data from the Northwest Atlantic margins and applied a receiver function inversion to retrieve station-wise velocity models of the crust and uppermost mantle. We integrate the outcomes with published controlled-source seismic data and regional crustal models to make inferences about the crustal structure and evolution of the Northwest Atlantic. In particular, we focused on constraining the spatial extent and origin of high velocity lower crust (HVLC), and determining whether it is generically related to the Davis Strait Igneous Province, syn-rift exhumed and serpentinised mantle, or pre-existing lower crustal bodies such as metamorphosed lower crust or older serpentinised mantle rocks. The new results allow us to better spatially constrain the DSIP and show the possible spatial extent of igneous-type HVLC across Southwest Greenland, Northwest Greenland and Southeast Baffin Bay. Similarly, we are able to relate some HVLC bodies to possible fossil collision/subduction zones/terrane boundaries, and in some instances to exhumed and serpentinised mantle.publishedVersio

Mantle wedge temperatures and their potential relation to volcanic arc location

The mechanisms underpinning the formation of a focused volcanic arc above subduction zones are debated. Suggestions include controls by: (i) where the subducting plate releases water, lowering the solidus in the overlying mantle wedge; (ii) the location where the mantle wedge melts to the highest degree; and (iii) a limit on melt formation and migration imposed by the cool shallow corner of the wedge. Here, we evaluate these three proposed mechanisms using a set of kinematically-driven 2D thermo-mechanical mantle-wedge models in which subduction velocity, slab dip and age, overriding-plate thickness and the depth of decoupling between the two plates are systematically varied. All mechanisms predict, on the basis of model geometry, that the arc-trench distance, D, decreases strongly with increasing dip, consistent with the negative D-dip correlations found in global subduction data. Model trends of sub-arc slab depth, H, with dip are positive if H is wedge-temperature controlled and overriding-plate thickness does not exceed the decoupling depth by more than 50 km, and negative if H is slab-temperature controlled. Observed global H-dip trends are overall positive. With increasing overriding plate thickness, the position of maximum melting shifts to smaller H and D, while the position of the trenchward limit of the melt zone, controlled by the wedge's cold corner, shifts to larger H and D, similar to the trend in the data for oceanic subduction zones. Thus, the limit imposed by the wedge corner on melting and melt migration seems to exert the first-order control on arc position

Toward Waveform-Based Characterization of Slab & Mantle Wedge (SAM) Earthquakes

Earthquakes in subduction zones occur in the slab mantle, in the subducting crust, on the subduction plate interface, and, in some cases, in the mantle wedge–regions that are separated by strong seismic discontinuities. These discontinuities are typically imaged with techniques using teleseismic waves, while local earthquakes are located based on arrival times. While this combination of imaging and earthquake location provides a good initial overview of where the earthquakes are located, the uncertainties associated with the two approaches are too large (i.e., few kilometers) to robustly identify on which side of a discontinuity (with thickness urn:x-wiley:21699313:media:jgrb55116:jgrb55116-math-0001100 m) the earthquakes occurred. Here we investigate how the waveforms of local earthquakes, which contain secondary phases arising from wave scattering at discontinuities, can be exploited to determine the source region of subduction zone earthquakes more robustly. Our investigation involves a three-step approach and includes an application to data from western Greece. First, to identify characteristic secondary phases, we analyzed synthetic seismograms from a generic 2-D subduction zone. Second, to enhance the visibility of secondary phases in field data, we implemented a workflow to process three-component seismograms. Third, to identify individual secondary phases in the data, we matched their timing to arrivals computed in a 3-D velocity model. We identified on average two to three secondary arrivals per station. These include P- and S-reflections from the plate interface which indicate hypocenters in the mantle wedge, and P-reflections from the slab Moho which indicate hypocenters on the plate interface and in the subducting crust.publishedVersio

High-Resolution Ps Receiver Function Imaging of the Crust and Mantle Lithosphere Beneath Southern New England and Tectonic Implications

Southern New England exhibits diverse geologic features resulting from past tectonic events. These include Proterozoic and early Paleozoic Laurentian units in the west, several Gondwana-derived terranes that accreted during the Paleozoic in the east, and the Mesozoic Hartford Basin in the central part of the region. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) project involved the deployment of a dense array of 15 broadband seismometers across northern Connecticut to investigate the architecture of lithospheric structures beneath this region and interpret how they were created and modified by past tectonic events in the context of surface geology. We carried out P-to-S receiver function analysis on SEISConn data, including both single-station analysis and common conversion point (CCP) stacking. Our images show that the westernmost part of Connecticut has a much deeper Moho than central and eastern Connecticut. The lateral transition is a well-defined, ∼15 km step-like offset of the Moho over a ∼20 km horizontal distance. The Moho step appears near the surface boundary between the Laurentian margin and the Gondwana-derived Moretown terrane. Possible models for its formation include Ordovician underthrusting of Laurentia and/or modification by younger tectonic events. Other prominent features include a strong positive velocity gradient (PVG) beneath the Hartford basin corresponding to the bottom of the sedimentary units, several west-dipping PVGs in the crust and mantle lithosphere that may correspond to relict slabs or shear zones from past subduction episodes, and a negative velocity gradient (NVG) that may correspond to the base of the lithosphere.publishedVersio

Seismicity modulation due to hydrological loading in a stable continental region: A case study from the Jektvik swarm sequence in Northern Norway

Seismic swarms have been observed for more than 40 yr along the coast of Nordland, Northern Norway. However, the detailed spatio-temporal evolution and mechanisms of these swarms have not yet been resolved due to the historically sparse seismic station coverage. An increased number of seismic stations now allows us to study a nearly decade-long sequence of swarms in the Jektvik area during the 2013–2021 time window. Our analysis resolves four major groups of seismic events, each consisting of several spatial clusters, that have distinct spatial and temporal behaviours. Computed focal mechanism solutions are predominantly normal with NNE–SSW strike direction reflecting a near-vertical maximum principal stress and a NW–SE near-horizontal minimum principal stress, which are controlled by local NW–SE extension. We attribute the swarms to fluid-saturated fracture zones that are reactivated due to this local extension. Over the time period, the activity tends to increase between February and May, which coincides with the late winter and beginning of spring time in Norway. We hypothesize that the seismicity is modulated seasonally by hydrological loading from snow accumulation. This transient hydrological load results in elastic deformation that is observed at local Global Navigation Satellite System stations. The loading is shown to promote failure in a critically stressed normal faulting system. Once a segment is activated, it can then also trigger neighboring segments via stress transfer. Our new results point to a close link between lithosphere and hydrosphere contributing to the occurrence of seismic swarm activity in northern Norway.publishedVersio

A Reappraisal of the H-κ Stacking Technique : Implications for Global Crustal Structure

We thank two anonymous reviewers and editor Michael Ritzwoller for insightful comments which have improved this manuscript. We also thank H. Meek for hard work during the early stages of this project and S. Pilidou, I. Dimitriadis, P. Iosif and their colleagues at the Geological Survey Department of Cyprus for their help establishing the TROODOS network (Bastow et al., 2017). V. Lane and D. Daly (both of SEIS-UK), A. Boyce, M. Liddell and R. Kounoudis were all excellent field assistants in Cyprus. SAC (Helffrich et al., 2013) and GMT (Wessel and Smith, 1991) software were used to process and image seismic data, which were sourced from IRIS DMC and ORFEUS. C.S. Ogden is funded by the Natural Environment Research Council (NERC) Doctoral Training Partnership: Science and Solutions for a Changing Planet, Grant Number NE/L002515/1. S. Rondenay’s contribution to this work was supported by Career Integration Grant 321871 - GLImER from the FP7 Marie Curie Actions of the European Commission, and by the Research Council of Norway FRINATEK programme through SwaMMIS project 231354.Peer reviewedPostprin