Shallow depth, substantial change: fluid-metasomatism causes major compositional modifications of subducted volcanics (Mariana forearc)

Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ~25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K2O contents (median = 4.6 wt.%) and loss on ignition (median = 5.3 wt%, as a proxy for H2O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO2 for all samples, and MgO and Na2O increases at one but the loss of MgO and Fe2O3* at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts.The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K2O, H2O, and fluid-mobile elements to sub-arc depths.These findings demonstrate that mass transfer from subducting lithosphere starts at low P/T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (<30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.

Caribbean slab dynamics beneath northwest South America from SKS and Local S splitting

&lt;p&gt;&amp;#160; &amp;#160; &amp;#160;The southernmost edge of the Caribbean (CAR) plate, a buoyant large igneous province, subducts shallowly beneath northwestern South America (NWSA) at a trench that lies northwest of Colombia. Recent finite frequency P-wave tomography results show a segmented CAR subducting at a shallow angle under the Santa Marta Massif to the Serrania de Perij&amp;#225; (SdP) before steepening while a detached segment beneath the M&amp;#233;rida Andes (MA) descends into the mantle transition zone. The dynamics of shallow subduction are poorly understood. Plate coupling between the flat subducting CAR and the overriding NWSA is proposed to have driven the uplift of the MA. In this study we analyze SKS shear wave splitting to investigate the seismic anisotropy beneath the slab segments to relate their geometry to mantle dynamics. We also use local S splitting to investigate the seismic anisotropy between the slab segments and the overriding plate. The data were recorded by a 65-element portable broadband seismograph network deployed in NWSA and 40 broadband stations of the Venezuelan and Colombian national seismograph networks.&lt;/p&gt;&lt;p&gt;&amp;#160; &amp;#160; &amp;#160;SKS fast polarization axes are measured generally trench-perpendicular (TP) west of the SdP but transition to trench-parallel (TL) at the SdP where the slab was imaged steepening into the mantle, consistent with previous studies. West of the MA the fast axis is again TP but transitions to TL under the MA. This second transition from TP to TL is likely due to mantle material being deflected around a detached slab under the MA. Local S fast polarization axes are dominantly TP throughout the study area west of the Santa Marta Massif and are consistent with slab-entrained flow. Under the Santa Marta Massif the fast axis is TL for reasons we do not yet understand.&lt;/p&gt;

Geochemical study of Cenozoic mafic volcanism in the west-central Great Basin, western Nevada, and the Ancestral Cascades Arc, California

Processes linked to shallow subduction, slab rollback, and extension are recorded in the whole-rock major-, trace-element, and Sr, Nd, and Pb isotopic compositions of mafic magmatic rocks in both time and space over southwestern United States. Eocene to Mio-Pliocene volcanic rocks were sampled along a transect across the west-central Great Basin (GB) in Nevada to the Ancestral Cascade Arc (ACA) in the northern Sierra Nevada, California (∼39°–40° latitude), which are interpreted to represent a critical segment of a magmatic sweep that occurred as a result of subduction from east-northeast convergence between the Farallon and North American plates and extension related to the change from a convergent to a transform margin along the western edge of North America. Mafic volcanic rocks from the study area can be spatially divided into three broad regions: GB (5–35 Ma), eastern ACA, and western ACA (2.5–16 Ma). The volcanic products are dominantly calc-alkalic but transition to alkalic toward the east. Great Basin lavas erupted far inland from the continental margin and have higher K, P, Ti, and La/Sm as well as lower (Sr/P)pmn, Th/Rb, and Ba/Nb compared to ACA lavas. Higher Pb isotopic values, combined with lower Ce/Ce* and high Th/Nb ratios in some ACA lavas, are interpreted to come from slab sediment. Mafic lavas from the GB and ACA have overlapping 87Sr/86Sr and 143Nd/144Nd values that are consistent with mantle wedge melts mixing with a subduction-modified lithospheric mantle source. Eastern and western ACA lavas largely overlap in age and elemental and isotopic composition, with the exception of a small subset of lavas from the westernmost ACA region; these lavas show lower 87Sr/86Sr at a given 143Nd/144Nd. Results show that although extension contributes to melting in some regions (e.g., selected lavas in the GB and Pyramid Lake), chemical signatures for most mafic melts are dominated by subduction-related mantle wedge and a lithospheric mantle component.

New insights into the structural elements of the upper mantle beneath the contiguous United States from S-to-P converted seismic waves

Summary The S-receiver function (SRF) technique is an effective tool to study seismic discontinuities in the upper mantle such as the mid-lithospheric discontinuity (MLD) and the lithosphere-asthenosphere boundary (LAB). This technique uses deconvolution and aligns traces along the maximum of the deconvolved SV signal. Both of these steps lead to acausal signals, which may cause interference with real signals from below the Moho. Here we go back to the origin of the S-receiver function method and process S-to-P converted waves using S-onset times as the reference time and waveform summation without any filter like deconvolution or bandpass. We apply this ‘causal’ SRF (C-SRF) method to data of the USArray and obtain partially different results in comparison with previous studies using the traditional acausal SRF method. The new method does not confirm the existence of an MLD beneath large regions of the cratonic US. The shallow LAB in the western US is, however, confirmed with the new method. The elimination of the MLD signal below much of the cratonic US reveals lower amplitude but highly significant phases that previously had been overwhelmed by the apparent MLD signals. Along the northern part of the area with data coverage we see relics of Archean or younger north-west directed low-angle subduction below the entire Superior Craton. In the cratonic part of the US we see indications of the cratonic LAB near 200 km depth. In the Gulf Coast of the southern US we image relics of southeast directed shallow subduction, likely of mid-Paleozoic age.

A Laurentian margin subduction perspective: Geodynamic constraints from phase equilibria modeling of barroisite greenstones, northern USA Appalachians

Phase equilibria modeling of sodic-calcic amphibole-epidote assemblages in greenstones in the northern Appalachians, USA, is compatible with relatively shallow subduction of the early Paleozoic Laurentian margin along the Laurentia-Gondwana suture zone during closure of a portion of the Iapetus Ocean basin. Pseudosection and isopleth calculations demonstrate that peak metamorphic conditions ranged between 0.65 GPa, 480 °C and 0.85 GPa, 495 °C down-dip along the subducted Laurentian continental margin between ∼20 km and ∼30 km depth. Quantitative petrological data are explained in the context of an Early Ordovician geodynamic model involving shallow subduction of relatively young, warm, and buoyant Laurentian margin continental-oceanic lithosphere and Iapetus Ocean crust beneath a relatively warm and wet peri-Gondwanan continental arc. A relatively warm subduction zone setting may have contributed to the formation of a thin, ductile metasedimentary rock-rich channel between the down-going Laurentian slab and the overriding continental arc. This accretionary channel accommodated metamorphism and tectonization of continental margin sediments and mafic volcanic rocks (greenstones) of the Laurentian margin and provided a pathway for exhumation of serpentinite slivers and rare eclogite blocks. Restricted asthenospheric flow in the forearc mantle wedge provides one explanation for the lack of ophiolites and absence of a well-preserved ultra-high-pressure terrane in central and northern Vermont. Exhumation of the subducted portion of the Laurentian margin may have been temperature triggered due to increased asthenospheric flow following a slab tear at relatively shallow depths.

Rheological properties of the shallow subduction interface: insights from the Chugach Complex, Alaska

&lt;p&gt;The plate interface in subduction zones accommodates a wide range of seismic styles over different depths as a function of pressure-temperature conditions, compositional and fluid-pressure heterogeneities, deformation mechanisms, and degrees of strain localization. The shallow subduction interface (i.e. ~2-10 km subduction depths), in particular, can exhibit either slow slip events (e.g. Hikurangi) or megathrust earthquakes (e.g. Tohoku). To evaluate the factors governing these different slip behaviors, we need better constraints on the rheological properties of the shallow interface. Here we focus on exhumed rocks within the Chugach Complex of southern Alaska, which represents the Jurassic to Cretaceous shallow subduction interface of the Kula and North American plates. The Chugach is ideal because it exhibits progressive variations in subducted rock types through time, minimal post-subduction overprinting, and extensive along-strike exposure (~250 km). Our aims are to use field structural mapping, geochronology, and microstructural analysis to examine a) how strain is localized in different subducted protoliths, and b) the deformation processes, role of fluids, and strain localization mechanisms within each high strain zone. We interpret these data in the context of the relative &amp;#8216;strengths&amp;#8217; of different materials on the shallow interface and possible styles of seismicity.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;Thus far we have characterized deformation features along a 1.25-km-thick melange belt within the Turnagain Arm region southeast of Anchorage.&amp;#160; The westernmost melange unit is sediment poor and consists of deep marine rocks with more chert, shale and mafic rocks than units to the east. The melange fabric is variably developed (weakly to strongly) throughout the unit and is steeply (sub-vertical) west-dipping with down-dip lineations. Quartz-calcite-filled dilational cracks are oriented perpendicular to the main melange fabric.&lt;/p&gt;&lt;p&gt;Drone imaging and structural mapping reveals 3 major discrete shear zones and 6-7 minor shear zones within the melange belt, all of which exhibit thrust kinematics. Major shear zones show a significant and observable strain gradient into a wide (~1 m) region of high strain and deform large blocks while minor shear zones are generally developed in narrow zones (~10-15 cm) of high strain between larger blocks. One major shear zone is developed in basalt and has closely-spaced, polished slip surfaces that define a facoidal texture; the basalt shear zone is ~1 m thick. Preserved pillows are observable in lower strain areas on either side of the shear zone but are deformed and indistinguishable within the high strain zone. The other two major shear zones are developed in shale and are matrix-supported with wispy, closely-spaced foliation and rotated porphyroclasts of chert and basalt; the shale shear zones are ~0.5-2 m thick.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;Abundant quartz-calcite veins parallel to the melange fabric and within shale shear zones record multiple generations of fluid-flow; early veins appear to be more silicic and later fluid flow involved only calcite precipitation. At the west, trench-proximal end of the m&amp;#233;lange unit there is a 5-10 m thick silicified zone of fluid injection that is bound on one side by the basalt shear zone. Fluid injection appears to pre-date or be synchronous with shearing.&lt;/p&gt;

How serpentine peridotites can leak through subduction channels

&lt;p&gt;Serpentinized peridotites are weaker than other mantle rocks, with an internal friction coefficient &amp;#956;&lt;sub&gt;i&lt;/sub&gt;~0.3 vs. ~0.6. Therefore they often promote strain localization. Serpentinite is also considerably lower in density (r=2.5-2.6 g/cm&lt;sup&gt;3&lt;/sup&gt;) than most rocks. In the presence of denser material, its buoyancy can mobilize upwelling masses and aid exhumation. Serpentinized peridotites can therefore influence the evolution of tectonic plate boundaries: their presence enhances shear processes, and serpentinite-hosted faults can evolve into zones of permanent lithospheric weakness that can be reactivated during different tectonic phases. Fault reactivation also provides paths for fluid infiltration and upward remobilization of serpentinized peridotites that can also interact diapirically with overlying rocks.&lt;/p&gt;&lt;p&gt;We have compiled observations that document the near-surface journey of serpentinized peridotites that are exhumed during rifting and continental break-up, reactivated as buoyant material during subduction, and ultimately emplaced as &amp;#8216;ophiolite-like&amp;#8217; fragments within orogenic belts. This lifecycle is particularly well documented in former Tethys margins that now subduct beneath the Calabrian Arc. Here recent studies describe serpentinized peridotites that diapirically rose from a subducting lithospheric slab to be emplaced into the accretionary prism in front of the continental arc. We show that this newly recognized mode of subduction-linked serpentine diapirism from the downgoing lithospheric slab is consistent with the origin of some exhumed mantle rocks in the Apennines, with these assemblages having been ultimately emplaced into their present locations during Alpine Orogenesis. Transfer of serpentinized peridotites from the mantle lithosphere of the subducting slab to the overriding plate motivates the concept of a potentially &amp;#8220;leaky&amp;#8221; subduction channel.&amp;#160; In addition to passing vertically through a shallow subduction channel, weak serpentine bodies may also rise into and preferentially migrate within the intraplate shear zone, leading to strong lateral heterogeneities in its composition, mechanical strength and seismic characteristics.&lt;/p&gt;

Characterizing sediment dewatering and constraining spatially limited fluid flux in accretionary systems. A rock magnetic approach.

&lt;p&gt;The dewatering and subsequent drainage of fluids from porous sediments in forearc regions controls heat flux and the frictional behavior of the plate boundary decollement and all other forearc faults. Here we present new rock magnetic datasets that help to depict the strain history and locus of fluid and gas migration across a shallow subduction thrust near the deformation front of the Hikurangi subduction margin (New Zealand). Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 penetrated hanging-wall, the roughly 60 m thick fault-zone, and footwall sequences of the P&amp;#257;paku fault up to a maximum depth of 504 mbsf.&lt;/p&gt;&lt;p&gt;Rock magnetic investigations include the measurement of Anisotropy of Magnetic Susceptibility (AMS), static three-axis alternating field demagnetization (AFD), magnetic hysteresis, anhysteretic remanence acquisition (ARM) and S-ratio measurement. The datasets are presented for an interval between 275 and 375 mbsf, and encompass both fault-zone and directly adjacent sequences.&lt;/p&gt;&lt;p&gt;Throughout most of the sedimentary sequence, samples yield intensities of the natural remanent magnetization (NRM) between 10&lt;sup&gt;-5&lt;/sup&gt; and 10&lt;sup&gt;-6&lt;/sup&gt; Am&lt;sup&gt;2&lt;/sup&gt;/kg. Magnetic coercivities range from 40 to 60 mT. During static AFD samples acquired a gyroremanent magnetization. These observations indicate the presence of authigenic greigite (Fe&lt;sub&gt;3&lt;/sub&gt;S&lt;sub&gt;4&lt;/sub&gt;). In two intervals, between 304 and 312, and 334 - 351 mbsf, samples yield distinctively lower remanence intensities (~ 10&lt;sup&gt;-7&lt;/sup&gt; Am&lt;sup&gt;2&lt;/sup&gt;/kg) and lower coercivities around 20 mT. The upper interval coincides with the onset of brittle deformation at the top of the fault-zone. In the same interval AMS results change abruptly. We propose that the rock magnetic signature is due to the reduction of ferrimagnetic greigite to paramagnetic pyrite (FeS&lt;sub&gt;2&lt;/sub&gt;). This is most likely caused by the drainage of methane-, and sulfide rich fluids/gas along the upper fault-zone and supports interpretations that the fault zone acts as effective conduit. A continued transport of fluids/gases could have promoted a self-sustaining weakening and strain decoupling with episodic high pore-fluid pressure within localized parts of the fault-zone.&lt;/p&gt;