Down-dip Variations Research Articles

Subducted carbon weakens the forearc mantle wedge in a warm subduction zone

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

Down-dip variations in a subducting low-velocity zone linked to episodic tremor and slip: a new constraint from ScSp waves

Fluids are thought to play an important role in controlling episodic tremor and slow slip (ETS) in subduction zones. Therefore, constraining the along-dip distribution of fluids is necessary to better understand source mechanism of ETS, and particularly the role played by fluids in ETS generation. Here, we report clear observations of coherent ScSp phases with a dense seismic array in western Shikoku, Japan, where ETS has been most active over the past decade. Using numerical simulations of elastic-wave propagation to reproduce the observed ScSp phases, we demonstrate that, relative to shallower depths, either the Vp/Vs ratio or the thickness of a low-velocity zone (LVZ) within the subducting oceanic crust increases with depth beneath the mantle wedge corner where ETS has been observed. Based on these depth dependences of the structural elements, a wide semi-ductile shear zone appears to be lubricated by high-pressurized fluid in the subducting oceanic crust at ETS source depths, and to be a key factor regulating ETS activity.

Response of A Coal-Bearing Coastal-Plain Succession To Marine Transgression: Campanian Neslen Formation, Utah, U.S.A.

The process regime of low-gradient coastal plains, delta plains and shorelines can change during transgression. In ancient successions, accurate assessment of the nature of marine influence is needed to produce detailed paleogeographic reconstructions, and to better predict lithological heterogeneity in hydrocarbon reservoirs. The Campanian lower Neslen Formation represents a fluvial-dominated and tide- and wave-influenced coastal-plain and delta-plain succession that accumulated along the margins of the Western Interior Seaway, USA. The succession records the interactions of multiple coeval sedimentary environments that accumulated during a period of relative sea-level rise. A high-resolution data set based on closely spaced study sites employs vertical sedimentary graphical logs and stratigraphic panels for the recognition and correlation of a series of stratal packages. Each package represents the deposits of different paleoenvironments and process regimes within the context of an established regional sequence stratigraphic framework. Down-dip variations in the occurrence of architectural elements within each package demonstrate increasing marine influence as part of the fluvial-to-marine- transition zone. Three marine-influenced packages are recognized. These exhibit evidence for an increase in the intensity of marine processes upwards as part of an overall transgression through the lower Neslen Formation. These marine-influenced packages likely correlate down-dip to flooding surfaces within the time-equivalent Iles Formation. The stratigraphic arrangement of these packages is attributed to minor rises in sea level, the effects of which were initially buffered by the presence of raised peat mires. Post-depositional auto-compaction of these mires resulted in marine incursion over broad areas of the coastal plain. Results demonstrate that autogenic processes modified the process response to overall rise in relative sea level through time. Understanding the complicated interplay of processes in low-gradient, coal-bearing, paralic settings requires analysis of high-resolution stratigraphic data to discern the relative role of autogenic and allogenic controls.

Assessment of Backwater Controls On the Architecture of Distributary-Channel Fills In A Tide-Influenced Coastal-Plain Succession: Campanian Neslen Formation, U.S.A.

Abstract The backwater zone of a river is its distal reach downstream of the point at which the streambed elevation reaches sea level. Backwater hydraulics is believed to exert an important control on fluvio-deltaic morphodynamics, but the expressions with which this may be recorded in the preserved stratigraphic record are not well understood. The seaward reaches of modern rivers can undergo flow acceleration and become erosional at high discharges due to drawdown of the in-channel water surface near the river mouth, in relation to the fixed water surface at the shoreline. As coastal-plain distributary channels approach the shoreline they tend to be subject to a reduction in lateral mobility, which could be related to diminished sediment flux at low flow. Current understanding of channel morphodynamics associated with backwater effects, as based on observations from numerical models and modern sedimentary systems, is here used to make predictions concerning the architecture of coastal distributary channel fills in the rock record. On the basis of existing knowledge, distributary-channel fills are predicted to be typically characterized by low width-to-thickness aspect ratios, by a clustering of scour surfaces toward their base, by an aggradational infill style, by a facies organization that bears evidence of drawdown-influenced scour filling, possibly resulting in the overprint of tidal signals toward their base, and by co-genetic sand-prone overbank units of limited occurrence, thickness, and sand content. To test these predictions, fieldwork was carried out to examine sedimentological characters of channel bodies from an interval of the Campanian Neslen Formation (eastern Utah, USA), which comprises a succession of sandstone, carbonaceous mudstone, and coal, deposited in a coastal-plain setting, and in which significant evidence of tidal influence is preserved. Three types of channel bodies are recognized in the studied interval, in terms of lithology and formative-channel morphodynamics: sand-prone laterally accreting channel elements, heterolithic laterally accreting channel elements, and sand-prone aggradational ribbon channel elements. This study concentrates on the ribbon channel bodies since they possess a geometry compatible with laterally stable distributaries developed in the zone of drawdown. Sedimentological and architectural characteristics of these bodies are analyzed and compared with the proposed model of the architecture of distributary-channel fills. Although conclusive evidence of the influence of backwater processes in controlling the facies architecture of distributary-channel fills is not reached, the studied bodies display an ensemble of internal architecture, lithological organization, nature of bounding surfaces, and relationships with other units that conforms to the proposed model to a certain extent. The analyzed ribbon sandbodies are all characterized by erosional cut-banks, very limited proportions of mudstone deposits, a lack of genetically related barform units, clustering of scour fills at their base, and a lack of relationships with co-genetic river-fed overbank sandstones. This work provides a guide to future research, which is required to better understand the role of backwater processes in controlling the architecture of distributary channel bodies, their down-dip variations, and how these are expressed in the stratigraphic evolution of prograding coastal plains.

Tectonic settings of the 2015<italic>M</italic><sub>w</sub>8.3 Coquimbo, Chile earthquake and its implications on megathrust earthquakes

The 2015 M w8.3 Coquimbo, Chile earthquake is a typical megathrust earthquake, whose size and coseismic slip distribution are consistent with the interseismic locking model that was derived from GPS measurements. Our preliminary back-projection results show that the rupture of the M w8.3 Coquimbo earthquake propagates updip from the hypocenter (~25 km in depth). Furthermore, we find frequency-dependent behavior of the radiation power of the rupture, similar to those observed during the 2010 M w8.8 Maule earthquake, which occurred~50 km to the south of the 2015 earthquake rupture area. These observations indicate that there is a systematic downdip variation of properties on the megathrust in southern Chile. The rich observations of the coseismic slip distributions of the 2015 M w8.3 earthquake provide an excellent opportunity to validate numerical simulations of rupture process based on interseismic locking distributions.

Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

AbstractSeismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ~40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ~50 to 95 km from the trench, the plate interface appears as a thin (<400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1–1.5 s) package of reflections, where it appears to intersect the fore‐arc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ~100 to 250 m thick low‐velocity zone, whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers. The thin reflection band is located at the center of the 1938 Mw 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone, correlates with a dipping band of seismicity, and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid‐rich layers, representing a broad transition from stick‐slip sliding to slow slip and tremor.

Newly observed, deep slow slip events at the central Hikurangi margin, New Zealand: Implications for downdip variability of slow slip and tremor, and relationship to seismic structure

AbstractWe present previously undocumented deep (25–45 km depth), moderate‐duration (2–3 months) slow slip events (SSEs) directly downdip of known shallow (<15 km depth), short‐term (2–3 weeks) SSEs at the central Hikurangi margin, New Zealand. The moderate‐duration SSEs produce 2–6 mm of horizontal displacement at continuous Global Positioning System stations in 2006 and 2008, similar to Cascadia SSEs. They involve 2–5 cm of slip over ~5000 km2 of the subduction interface. Tremor occurs just downdip of the deep SSE source area. We also present results from a large, shallow (<12 km) SSE in 2013 just updip of the 2006/2008 SSEs. The location of deep SSEs between shallow, short‐term SSEs and tremor indicates strong downdip variation of subduction interface slip behavior at central Hikurangi. The deep SSEs occur where Qp and Vp/Vs data suggest an accumulation of fluid‐rich underplated sediment at the interface, consistent with the idea that SSEs occur under high fluid pressure.

Subduction of oceanic asthenosphere: A critical appraisal in central Alaska

Song and Kawakatsu (2012) have shown that the sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subduction of the oceanic asthenosphere that has strong radial anisotropy. This model not only explains the non-intuitive trench-parallel splitting pattern in most of subduction zones, but also predicts the trench-normal behavior (fast polarization direction sub-parallel to the absolute plate motion of the incoming plate) observed in several shallow subduction zones. The general validity of such a scenario is crucial to fundamental understandings of the development of mantle anisotropy in sub-lithosphere or/and sub-slab conditions, the nature and formation of oceanic asthenosphere as well as the flow pattern and mass transport near subduction zones. To validate this scenario, we examine SKS splitting patterns observed across the fore-arc in central Alaska. Here the fast splitting direction varies from plate motion sub-parallel near the trench to mostly trench-parallel beyond the 100km slab-isodepth contour, while being strongly variable in between. After taking into account the rotation of anisotropy symmetry in the oceanic asthenosphere with respect to the local plate motion obliquity and down-dip variations in the slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as the back azimuth dependent splitting pattern across the entire fore-arc. The current validation further augments the idea that, apart from anisotropy in the mantle wedge and the subducting slab, subduction of the oceanic asthenosphere is likely to be the dominant source of seismic anisotropy in central Alaska and potentially in many subduction zones. Furthermore, this result also provides alternative views to models of seismic anisotropy in the mantle wedge and on the length scale in which the 3D mantle flow may be important.

Toward a dynamic concept of the subduction channel at erosive convergent margins with implications for interplate material transfer

Convergent plate boundaries accommodate intraplate displacement within a ∼100–1000 m thick shear zone. Marine geophysicists typically define this zone, the subduction channel (SC), as the sedimentary layer between the downgoing oceanic crust and the base of the upper plate. Geologists and modelers, instead, perceive the SC as a specific type of shear zone. The original theory of SCs was developed when the net accretion of marine sediments to the forearc was thought to typify a convergent margin. While erosive margins were briefly mentioned, their mechanics were not discussed in any detail. We now realize that subduction erosion is taking place at roughly half of the modern subduction margins. Here we review and revise the theory of erosive SCs (1) to unify this concept across disciplines, focusing on the meaning of the channel's boundaries; (2) to redefine the portions of the forearc included in the SC concept; and (3) to better idealize this dynamic system where material supply to the channel, fluid content, and the heterogeneity of deformation all influence the SC's upper and lower boundaries. Migration of the channel boundaries controls the downdip variation of tectonic mechanisms that shape the margin. Within the shallow, <15 km deep part of the SC, a gradual change of physical properties defines three zones; zone 1 of rapid fluid dewatering, zone 2 of overpressure, and zone 3 with metamorphic fluid release. A SC is a dynamic feature with along‐strike and downdip variations caused by changes in channel material, in trapped fluid contents, and in interplate boundary geometry.

Along-Strike and Down-Dip Variations in Shallow-Marine Sequence Stratigraphic Architecture: Upper Cretaceous Star Point Sandstone, Wasatch Plateau, Central Utah, U.S.A.

Abstract The sequence stratigraphic architectures of shallow-marine deposits in the upper Cretaceous Star Point Sandstone are analyzed over a large (c. 100 km), nearly continuous outcrop section aligned oblique to depositional strike. The unit consists of five parasequences that predominantly comprise wave-dominated shoreface–shelf deposits. Two parasequences contain riverdominated delta-front deposits locally. Within each parasequence, wave-dominated shoreface–shelf deposits record 7–45 km of ESE- to ENE-directed progradation of a linear to moderately lobate shoreline that was supplied with sediment by longshore drift and subjected to strong offshore sediment transport by storms. Wave-dominated shoreface sandstones in each parasequence thin and wedge out over short distances ( 800 km2) river-dominated delta-front complex (Panther Tongue). This highly focused fluvial sediment flux probably occurred via a structurally controlled sediment entry point between two active thrusts.

３次元ＧＰＳ速度場から推定した南海トラフのすべり欠損分布―推定領域を繰り返しシフトさせる測地インバージョン解析―

The Philippine Sea plate has subducted beneath southwest Japan at the Nankai Trough, causing megathrust earthquakes every 100-140 years. The subduction process shows lateral variations along the trough, such as a bending of the plate interface beneath the Kii Peninsula and rapid changes of strike and dip off the east coast of Kyushu. We invert three-dimensional GPS velocities using ABIC geodetic inversion technique to estimate interseismic slip de.cit distribution at the Nankai Trough. The studied area is roughly from Kii peninsula to eastern Kyushu (131.5-137E, 31-35N). In the ABIC geodetic inversion some initial constraints are needed to stabilize solutions, such that the slip at the outer margin of the fault is set to zero. For the estimation of interseismic slip de.cit on the plate interface, however, these constraints are unrealistic because slip de.cit distribution continues later.ally to the outside of the target area. In this study we propose a repeated inversion method that shifts target area laterally along the plate boundary. This method averages slip de.cit estimates at the same point obtained from more than three consecutive inversions. The estimates obtained near the lateral margin, which may be strongly affected by the non-slip constraints at the outer margin, are excluded from the averaging. As a result final solutions are stabilized regardless the setting of the region. The final slip de.cit distribution at the Nankai Trough shows high contrast of strong and weak plate locking zones. Down-dip variation of the slip de.cit is emphasized more clearly but the lateral variation becomes moderate compared with the single inversion result.

Probable low-angle thrust earthquakes on the Juan de Fuca–North America plate boundary

In 2004, two clusters of earthquakes occurred in the central part of the Cascadia forearc, which displays several characteristics indicative of along-strike and downdip variations in plate coupling. Moment tensor analysis for the main shock in each cluster indicates that both events, which had magnitudes of 4.9 and 4.8, were compatible with low-angle thrust motion on fault planes dipping 6°–15° to the east, consistent with the plate boundary dip of ∼12°. By tracing rays through a high-resolution two-dimensional crustal velocity model to match arrival times of secondary arrivals (pP and PmP), we estimate that the source depth was 9–11 km for the M4.9 event and 15–17 km for the M4.8 event. We conclude that these earthquakes probably represent seismogenic thrust motion on the nominally locked or transitional part of the Cascadia megathrust, updip of where episodic tremor and slip (ETS) have been documented. Continued high-resolution observations of seismicity and improved crustal models are needed to confirm whether apparent temporal correlations between ETS and continued seismic activity in these clusters indicate stress transfer along the megathrust.

Fine-grained Quartz Formed by the Sedimentation of Hydrothermal Precipitates in Mineral Veins: An Example from Tyndrum, Scotland, UK

Sedimented hydrothermal precipitates of silica, now fine-grained quartz, are described from a Pb-Zn-bearing vein system. Their discrimination from the wall-accreted fine-grained quartz that is common in the shallow parts of epithermal systems is discussed. Local fine-grained (<0.3 mm) quartz accumulations occur in a suite of veins with otherwise normal drusiform and crustified infills. This sediment may fill the vein above chokes in steep segments (axial plugs), or form continuous layers within the footwall material where the vein is inclined at low to moderate angles. Pads of fine-grained sediment are deposited on the upwards-facing surfaces of wall-rock clasts and of drusiform crystals growing from the vein walls. The asymmetry of these deposits differentiates them from the more common symmetrical, crustified wall-lining deposits of fine-grained quartz and opaline silica, and requires a gravity-controlled, sedimentary depositional mechanism. Thin section examination shows that much of the fine-grained quartz is present as elongate prisms. The prism habit was present soon after deposition, but it is not possible to identify unequivocally the original sedimented grains. Sedimentation of vein- and wall-rock-derived detritus is also recognized. Deposition of the fine-grained sediment accompanies normal in situ drusiform growth of quartz and other vein minerals. The mammilated and spherulitic textures associated with accretion of amorphous quartz are entirely absent from the Tyndrum veins with sedimented materials. The literature on modern hydrothermal waters and physical chemistry of silica precipitation shows that precipitates of silica suspended in hydrothermal fluids are amorphous and derived from colloidal silica. It is argued that much of the suspended silica was present as quartz at the time of deposition or was converted from amorphous silica almost immediately after deposition. Hydrothermal sediments and their distribution potentially provide information on flow directions and velocity in veins. Plugs of sediment obstruct vertical flow and lead to local flows with large horizontal components. It is shown that sedimentation features occur in other deposits and can involve other minerals. If the features seen here can be scaled up to the size of major veins, they may result in up and downdip variations in mineral assemblages and abundances, which may affect grade.

High-resolution imaging of the Bear Valley section of the San Andreas fault at seismogenic depths with fault-zone head waves and relocated seismicity

SUMMARY Detailed imaging of fault-zone (FZ) material properties at seismogenic depths is a difficult seismological problem owing to the short length scales of the structural features. Seismic energy trapped within a low-velocity damage zone has been utilized to image the fault core at shallow depths, but these phases appear to lack sensitivity to structure in the depth range where earthquakes nucleate. Major faults that juxtapose rocks of significantly different elastic properties generate a related phase termed a fault-zone head wave (FZHW) that spends the majority of its path refracting along the fault. We utilize data from a dense temporary array of seismometers in the Bear Valley region of the San Andreas Fault to demonstrate that head waves have sensitivity to FZ structure throughout the seismogenic zone. Measured differential arrival times between the head waves and direct P arrivals and waveform modelling of these phases provide high-resolution information on the velocity contrast across the fault. The obtained values document along-strike, fault-normal, and downdip variations in the strength of the velocity contrast, ranging from 20 to 50 per cent depending on the regions being averaged by the ray paths. The complexity of the FZ waveforms increases dramatically in a region of the fault that has two active strands producing two separate bands of seismicity. Synthetic waveform calculations indicate that geological observations of the thickness and rock-type of the layer between the two strands are valid also for the subsurface structure of the fault. The results show that joint analysis of FZHWs and direct P arrivals can resolve important smallscale elements of the FZ structure at seismogenic depths. Detailed characterization of material contrasts across faults and their relation to earthquake ruptures is necessary for evaluating theoretical predictions of the effects that these structures have on rupture propagation.

Fluid overpressure and flow in fault zones: field measurements and models

Field studies of small normal faults (throws of metres to tens of metres) show that their fault cores consist of breccias that vary in thickness along the fault plane. Commonly, the down-dip variation in the breccia thickness is 0–1m with a wavelength of 5–10m. The breccia acts mechanically as an inclusion; soft, ductile and sometimes creeping when the fault zone is active, but stiff and brittle when the fault zone is inactive. During interseismic periods, and when the fault has become inactive, the breccia behaves as a very dense, low-permeability material that is a barrier to transverse flow of groundwater. The breccia barrier thus collects water and channels it downdip or updip along the contact between the fault core and the damage zone. For a typical 1-m-thick interseismic breccia, the maximum transmissivity is estimated at Tp∼10−10m2s−1. The field data, however, indicate that during the high strain rates associated with faulting, seismogenic slip may occur either along the breccia, or along its contacts with the damage zone. The resulting fractures with apertures of ∼0.3cm may temporarily increase the transmissivity of the fault core by at least 8 orders of a magnitude, to as much as Tf∼10−2m2s−1. It is suggested that slip of faults of this type is commonly associated with the flow of overpressured water into the fault plane. High water pressure lowers the critical driving shear stress needed for fault slip and may greatly increase the aperture, hence the fluid transport, of the slipping fracture. Theoretical considerations indicate that, other things being equal, fluid flow along strike–slip faults is favoured over flow along dip–slip faults and that, generally, the steeper the dip of the fault, the more effective it is for fluid transport.