Frontal Prism Research Articles

Influence of outer-rise faults on shallow décollement heterogeneity and sediment flux at the Japan trench

We investigate the impact of outer-rise normal fault subduction on the structural evolution of the décollement and frontal prism in a portion of the Japan trench that hosted the 2011 Tohoku earthquake. We use seismic reflection data to map the relative occurrence of sediment accretion, sediment subduction, and frontal tectonic erosion in the shallow portion of the subduction zone and correlate these deformation styles to the magnitude of outer-rise fault throw and incoming plate sediment thickness. These data reveal spatial heterogeneity in the modes of deformation over distances of 5-10 km that necessitate correlative heterogeneity in the geometry and composition of the shallow décollement over similar length-scales. We find that sediment accretion predominantly occurs in regions where incoming plate sediment thickness is greater than fault throw. In these areas, the décollement appears to be non-planar and compositionally homogenous. Conversely, frontal tectonic erosion and slope failures are predominantly observed in regions where fault throw is greater than sediment thickness. In these areas, the décollement may be planar but compositionally heterogeneous. Additionally, spatial variations in near trench slip appear to correlate with the dominant deformation modes, suggesting that both sediment thickness and outer-rise fault throw may be important controls on shallow megathrust behavior.

Seamount and Ridge Subduction at the Java Margin, Indonesia: Effects on Structural Geology and Seismogenesis

AbstractThe Java ‐ Lesser Sunda margin, which features multi‐scale subducting oceanic basement relief, is classified as neutral (Lombok and Sumbawa) to erosional (Central Java to Bali) in comparison to its accretionary counterpart offshore Sumatra. However, a comprehensive analysis of how plate boundary and upper plate structure across the neutral to erosional transition are modulated by the subduction of oceanic basement relief is lacking to date. To shed light on the tectonic parameters that push the margin into the neutral or erosional domain, we combine multi‐channel reflection seismic images derived through a grid‐based P‐wave velocity inversion, and multibeam bathymetric maps. The data document how different scales of subducting topography modify seafloor morphology, upper plate structure, and décollement position. Large‐scale subducting features cause a landward shift of the deformation front, shortening of the accretionary wedge, and seafloor steepening at the relief's trailing edge. Small‐scale subducting ridges primarily impact the frontal prism resulting in over‐steepening at the trench and localized slope failure. Ahead of subducting relief, deformation of the accretionary wedge encompasses enhanced compression and a reduction in seafloor slope but appears independent of the size of the relief. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P‐wave velocity models indicate that the rigidity of the upper plate's base along the eastern Sunda margin is significantly lower than the worldwide trend. We conclude that this favors the genesis of tsunami earthquakes that have occurred on the Java margin.

Abrupt water temperature increases near seafloor during the 2011 Tohoku earthquake

We investigated temperature records associated with seafloor pressure observations at eight stations that experienced the 2011 Mw 9 Tohoku earthquake near its epicenter. The temperature data were based on the temperature measured inside the pressure transducer. We proposed a method to estimate ambient water temperature from the internal temperature using an equation of heat conduction. The estimated seafloor water temperature showed remarkable anomalies, especially increases several hours after the Mw 9 earthquake. A station of P03 (sea depth of 1.1 km) showed an abrupt temperature increase of + 0.19 °C that occurred ~ 3 h after the earthquake, which lasted for several hours. At stations of GJT3 (sea depth of 3.3 km) and TJT1 (sea depth of 5.8 km), there were abrupt temperature anomalies of + 0.20 °C and + 0.10 °C that began to occur 3–4 h after the earthquake. These anomalies both decayed to their original levels over a few tens of days. During the decay processes, only TJT1 showed several intermittent temperature rises. A water temperature anomaly within + 0.03 °C was found up to ~ 500 m above TJT1 2 weeks after the earthquake. There was no significant anomaly at the other five stations. Processes to cause these seafloor temperature anomalies were discussed. The temperature anomaly of P03 was reasonably caused by a tsunami-generated turbidity current, as also suggested by a previous study. Meanwhile, we proposed a scenario that the abrupt temperature anomalies of GJT3/TJT1 and the intermittent anomalies of TJT1 were caused by warm water discharges from the subseafloor. The pathways of the warm water were probably composed of the branch normal fault between GJT3 and TJT1, the reverse fault near TJT1, the backstop interface, and perhaps reverse faults at the frontal prism. The proposed scenario was almost compatible with other studies based on epicentral observations. We estimated the heat properties of the initial temperature anomalies of GJT3/TJT1. The estimated heat source might be explained by that most of the geothermal fluids trapped in those fault pathways were discharged to the seafloor immediately after the earthquake. The onsets of the subsequent intermittent anomalies of TJT1 were possibly activated by low or falling ocean tidal loading.

Impact of the Iquique Ridge on structure and deformation of the north Chilean subduction zone

The subduction of seamounts and basement ridges affects the structure, morphology, and physical state of a convergent margin. To evaluate their impact on the seismo-tectonic setting of the subduction zone and the tectonic development of the lower subducting and upper overriding plate, it is essential to know the precise location of subducted topographic features under the marine forearc. Offshore Northern Chile, the Iquique Ridge represents a broad zone of complex and heterogeneous structure of variable width on the oceanic Nazca Plate, which complicates attempts to project it beneath the forearc of the Chilean subduction zone. Here we use a state-of-the-art seismic reflection data processing approach to map structures related to ridge subduction under the marine forearc with unprecedented accuracy and resolution and evaluate their impact on the deformation of both the plate boundary and the upper plate. We show that significant ridge-related topography is currently subducting south of 20.5 °S and that the combined effect of horst and graben subduction with subduction of Iquique ridge-related thickened and elevated crust causes an upward bulging of the entire upper plate from the plate interface up to the seafloor as well as the presence of kilometer-scale anticlinal structures observed in multibeam bathymetric data that are approximately aligned with horsts seaward of the trench. In the area affected by the subducting ridge, a frontal prism is absent, which may relate to frontal subduction erosion caused by the excess lower plate topography. In contrast farther towards the north, where only isolated seamounts subduct, a small frontal prism and a slope/apron sediment cover down to 3000 m water depth are found.

Crustal Structure of the Northern Hikurangi Margin, New Zealand: Variable Accretion and Overthrusting Plate Strength Influenced by Rough Subduction

AbstractExploring the structure of convergent margins is key to understanding megathrust slip behavior and tsunami generation. We present new wide‐angle and marine multichannel seismic data that constrain the crustal structure and accretion dynamics of the northern Hikurangi margin. The top of the basement of the Hikurangi Plateau is overlain by a rough, 2–3 km thick layer of volcanic cover with P‐wave velocities (VP) between 3 and 5 km/s. This volcanic cover contributes significantly to seismic reflectivity beneath the shallow subduction plate boundary. The frontal prism structure varies along‐strike from ∼25 km wide with imbricate thrust faults where accretion of trench sediments is undisrupted, to narrower (∼14 km) with slumps and branching, irregular thrust fault geometries, which may reflect lower sediment supply or past seamount collisions. A large thrust fault network in the inner prism with a seismically fast hanging wall indicates a mechanical boundary between a seismically faster deforming backstop and the seismically slower frontal prism. Near the coastline, VP increases between 2.8 and 4 km/s at 2–8 km depth and is 0.5–1.7 km/s slower than the southern Hikurangi margin. Low seismic wavespeeds and low vertical velocity gradients in the inner prism support the hypothesis that a weak overthrusting plate contributes to historic tsunami‐earthquakes and long duration seismic ground motion.

Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

AbstractIn 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time.Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism.Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

The western Durkan Complex (Makran Accretionary Prism, SE Iran): A Late Cretaceous tectonically disrupted seamounts chain and its role in controlling deformation style

The Durkan Complex is a key tectonic element of the Makran accretionary prism (SE Iran) and it has been interpreted as representing a continental margin succession. We present here a multidisciplinary study of the western Durkan Complex, which is based on new geological, stratigraphic, biostratigraphic data, as well as geochemical data of the volcanic and meta-volcanic rocks forming this complex. Our data show that this complex consists of distinct tectonic slices showing both non-metamorphic and very low-grade metamorphic deformed successions. Stratigraphic and biostratigraphic data allow us to recognize three types of successions. Type-I is composed by a Coniacian – early Campanian pelagic succession with intercalation of pillow lavas and minor volcaniclastic rocks. Type-II succession includes a volcanic sequence passing to a volcano-sedimentary sequence with Cenomanian pelagic limestones, followed by a hemipelagic sequence. This succession is characterized by abundant mass-transport deposits. Type-III succession includes volcanic and volcano-sedimentary sequences, which are stratigraphically covered by a Cenomanian platform succession. The latter is locally followed by a hemipelagic sequence. The volcanic rocks in the different successions show alkaline geochemical affinity, suggesting an origin from an oceanic within-plate setting. Our new results indicate that the western Durkan Complex represents fragments of seamounts tectonically incorporated in the Makran accretionary wedge during the latest Late Cretaceous–Paleocene. We propose that incorporation of seamounts in the frontal prism caused a shortening of the whole convergent margin and possibly contributed to controlling the deformation style in the Makran Accretionary Wedge during Late Cretaceous–Paleocene times.

СУБДУКЦИОННАЯ ЭРОЗИЯ НА КОНВЕРГЕНТНЫХ ОКРАИНАХ ТИХООКЕАНСКОГО ТИПА

The paper presents a review of processes of subduction or tectonic erosion at the Pacific-type convergent margins (PTCM) including definition of “tectonic erosion”, its triggers, driving forces and consequences. We review examples of tectonic erosion at the Circum-Pacific PTCMs and at the fossil PTCMs of the Paleo-Asian Ocean (PAO) currently hosted by the Central-Asian Orogenic Belt (CAOB). Recent geological and stratigraphic studies have shown two types of PTCMs: accreting and eroding. Accreting PTCMs consist of older deposits of accretionary and frontal prisms and grow oceanward, i.e. the trench retreats. Eroding PTCMs are characterized by the destruction of the prism, approaching arc and trench and typically form during shallow-angle and fast subduction of an oceanic slab with oceanic floor topographic highs. The mechanism of tectonic erosion includes destruction of oceanic slab, island arcs, accretionary prism, fore-arc and related prism. Tectonic erosion is a common phenomenon at many Circum-Pacific PTCMs, e.g., in South America, Tonga and Nankai troughs, Alaska. Accretion and subduction of oceanic rises contributes greatly to the processes of formation, transformation and destruction of continental crust at PTCM. The episodes of tectonic erosion can be also reconstructed for an ancient ocean, for example, for the PAO, which evolution and suturing formed the CAOB. Many CAOB foldbelts (Altai, Tienshan, eastern Kazakhstan, Transbaikalia, Mongolia) carry signs of disap-pearance of big volumes of continental crust (arcs). Studying processes responsible not only for the formation of continental crust, but also for the disappearance of big volumes of crustal mate-rial is important for correct evaluation of the nature of intra-continental orogenic belts, e.g., CAOB, and development of reliable tectonic models.

The Alaska Convergent Margin Backstop Splay Fault Zone, a Potential Large Tsunami Generator Between the Frontal Prism and Continental Framework

AbstractThe giant tsunami that swept the Pacific from Alaska to Antarctica in 1946 was generated along one of three Alaska Trench instrumentally recorded aftershock areas following great and giant earthquakes. Aftershock areas were investigated during the past decade with multibeam bathymetry, ocean bottom seismograph wide‐angle seismic, reprocessed legacy, and new seismic reflection images. Summarized and updated here are previous papers and additional data. Tectonic structures collocated with aftershock area boundaries indicate possible lengths of rupture in future great earthquakes. NE aftershock area boundaries relate to subducted lower plate structures whereas the SW zone upper plate retains Beringian structural relicts. The lower to middle slope transition separating a stronger continental framework rock from a weaker accreted prism occurs along splay fault zones previously interpreted as backstops in seismic images. Damage zones along splay faults are generally 1‐km‐wide dipping typically 21°. Splays form slip paths from the plate interface to the seafloor much shorter than the 3–4° dipping plate interface beneath the frontal prism. Associated seafloor vent structures indicate overpressured fluids at depth. Splay fault dip and its rigid hanging wall impart greater seafloor uplift than the accreted prism per unit of slip making them effective tsunami generators. Backstop splay fault zones (BSFZs) run along the entire Alaska Trench. Beneath the frontal prism, active bend faults add rugosity to the plate interface and km high relief is commonly imaged in reprocessed legacy and new seismic data. The 1946 Unimak great (M8.6) earthquake epicenter is located near the BSFZ.

Seismic structure and tectonics of the continental wedge overlying the source region of the Iquique Mw8.1 2014 earthquake

On April 1, 2014, a large earthquake (Mw = 8.1) ruptured the central part of a historic seismic gap in northern Chile. In order to study the relationship between the co-seismic rupture characteristics and the crustal structure of the subduction zone, we processed a trench-perpendicular seismic reflection profile acquired across the zone of maximum slip and generated a P-wave velocity model. The results show a frontal prism in the continental wedge characterized by low velocities that increase rapidly towards the shore and acted as a barrier for trench-ward propagation of aftershocks. Landward, a transition zone with increasing upper crust velocity (4–5 km/s) concentrates most of the aftershocks. In addition, a trench-ward dipping set of fault zones is observed along the continental wedge associated to the Iquique forearc basin formation (1.5 km thick at the depocenter on this profile). We identify three stratigraphic units within the basin. A landward tilt and thickness increase is detected in each stratigraphic unit, along with growth strata and domino structures, suggesting landward migration of syn-extensional deformation in response to basal subduction erosion. By extrapolating our results to the plate boundary and based on published focal mechanisms of intra-crustal seismicity, we find a strong spatial correlation between the Iquique basin and the highest slip area for the 2014 earthquake, suggesting long-term extensional deformation due to coseismic tensional stresses.

Correlation of frontal prism structures and slope failures near the trench axis with shallow megathrust slip at the Japan Trench

Since the giant 2011 Tohoku earthquake and tsunami, much research has focused on the distribution of coseismic slip at shallow depths during this subduction megathrust event. Here we present seismic images obtained in the immediate vicinity of the trench axis, that show thrust faults and fold-and-thrust type deformation structures near the epicenter of the 2011 Tohoku earthquake where the large coseismic slip has been inferred, and chaotic structure and the absence of thrust faults in northern and southern source areas. Seismic profiles show evidence of slope failures of the trench inner wall in a proposed tsunami source region around 39°–40° N, where the slips estimated from previous studies are in disagreement. Our results show that structural characteristics in the trench axis may be related to the occurrence of shallow megathrust slip and tsunamigenesis in the Japan Trench.

Subduction megathrust heterogeneity characterized from 3D seismic data

Megathrust roughness and structural complexity are thought to be controls on earthquake slip at subduction zones because they result in heterogeneity in shear strength and resolved stress. However, because active megathrust faults are difficult to observe, the causes and scales of complexity are largely unknown. Here we measured the in situ properties of the megathrust of the Middle America subduction zone in a three-dimensional seismic reflection volume to determine how fault properties vary. We quantify spatial variability in the megathrust roughness, overburden and rock physical properties. Heterogeneity in the megathrust roughness exists at length scales of a few kilometres because the megathrust is dissected by active lower-plate normal faults, which offset the megathrust and renewed fault roughness. Spatial variations in the rock physical properties at the plate interface are characterized by correlation length scales of hundreds of metres. Frontal prism taper, historical seismicity and the variation in earthquake stress drop values local to the megathrust are all affected by the heterogeneity at these length scales. Both geometric and rheological complexities may therefore control the mechanical behaviour of the subduction plate interface, which includes earthquake rupture characteristics. Geometric and rheological complexities may control the mechanical behaviour of megathrusts, according to an analysis of the heterogeneity in roughness and rock properties of the Middle America megathrust from 3D seismic reflection data.

Probing the Northern Chile Megathrust With Seismicity: The 2014 M8.1 Iquique Earthquake Sequence

AbstractWe used data from >100 permanent and temporary seismic stations to investigate seismicity patterns related to the 1 April 2014 M8.1 Iquique earthquake in northern Chile. Applying a multistage automatic event location procedure to the seismic data, we detected and located ~19,000 foreshocks, aftershocks, and background seismicity for 1 month preceding and 9 months following the mainshock. Foreshocks skirt around the updip limit of the mainshock asperity; aftershocks occur mainly in two belts updip and downdip of it. The updip seismicity primarily locates in a zone of transitional friction on the megathrust and can be explained by preseismic stress loading due to slow‐slip processes and afterslip driven by increased Coulomb failure stress due to the mainshock and its largest aftershock. Afterslip further south also triggered aftershocks and repeating earthquakes in several EW striking streaks. We interpret the streaks as markers of surrounding creep that could indicate a change in fault mechanics and may have structural origin, caused by fluid‐induced failure along presumed megathrust corrugations. Megathrust aftershocks terminate updip below the seaward frontal prism in the outer continental wedge that probably behaves aseismically under velocity‐strengthening conditions. The inner wedge locates further landward overlying the megathrust's seismogenic zone. Further downdip, aftershocks anticorrelate with the two major afterslip patches resolved geodetically and partially correlate with increased Coulomb failure stress, overall indicating heterogeneous frictional behavior. A region of sparse seismicity at ~40‐ to 50‐km depth is followed by the deepest plate interface aftershocks at ~55‐ to 65‐km depth, which occur in two clusters of significantly different dip.

Meteoric 10Be Reveals a Young, Active Accretionary Prism and Structurally Complex Décollement in the Vicinity of the 2011 Tohoku Earthquake Rupture

AbstractWe present new meteoric 10Be concentration data from marine sediment cores recovered during Integrated Ocean Drilling Program Expedition 343 (JFAST) that help constrain the age, origin, and internal structure of the frontal prism at the Japan trench in the vicinity of the 2011 Tohoku‐oki M9 earthquake rupture. 10Be sediment ages from the lower portion of the frontal prism range from ~0–10 Ma, with >60% of analyzed samples above the décollement yielding young ages <2 Ma. Repetition and inversion of high‐over‐low 10Be‐concentration sediments indicate the presence of stratigraphic inversions that correspond to faulting and imbrication of late Miocene and Quaternary sediments. The density of faults inferred indicates that the frontal prism has a fault spacing on the order of 10 s of meters, and the identification of faults in the underthrust section suggests that the plate boundary décollement may be a zone with multiple slip surfaces. Comparison of 10Be concentrations in the frontal prism with those of the incoming and forearc slope sediments indicates that the majority of the prism is sourced from accretion of Pacific Plate sediments, rather than from reworked frontal prism or slope sediments. These data suggest that over at least the past ~1–2 Ma, the décollement preferentially has localized at or near the base of the incoming sediment section, with relatively efficient sediment accretion occurring even in the presence of subducted horst‐and‐graben topography.

Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

AbstractThe submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

Texture Development of Clay‐Rich Sediments Across the Costa Rica Subduction Zone

AbstractDuring sedimentation, burial, and deformation at active continental margins, clay‐rich sediments develop crystallographic preferred orientations (textures) due to the progressive alignment of phyllosilicates. Such textures help to interpret sedimentation and compaction conditions as well as tectonic processes at convergent margins. At the Costa Rica Trench, subduction and plate boundary deformation between the downgoing oceanic Cocos Plate and the overriding Caribbean Plate was investigated during Integrated Ocean Drilling Program Expeditions 334 and 344 within the Costa Rica Seismogenesis Project. Samples of varying depths from the Cocos Plate, the frontal prism, and the slope of the Caribbean Plate were analyzed regarding their composition and texture. Composition is quite similar for all sample locations of the hemipelagic section across the trench as determined by X‐ray powder analysis. Texture analysis reveals that phyllosilicates in samples from the incoming plate show in general weaker textures than those from upper and middle slope of the overriding plate. Samples from the frontal accretionary prism, however, mostly correspond to the incoming plate fabric according to their oceanic origin. Texture intensity depends on the internal parameters grain size and shape, porosity, and composition as well as compaction and tectonics. In samples from the continental wedge and the frontal accretionary prism, we are able to distinguish tectonically undisturbed compacted sediments from core sections that suffered faulting and folding due to subduction‐related deformation. This helps to constrain a more detailed image of sedimentary compaction and localized as well as distributed deformation across the active continental margin offshore Costa Rica.

Subduction Erosion, Crustal Structure, and an Evolutionary Model of the Northern Yap Subduction Zone: New Observations From the Latest Geophysical Survey

AbstractThe Yap subduction zone is a distinctive erosive margin with an extremely slow convergence rate. The high relief of the subducting plate, generated by horst and graben structures and seamounts, leads to attenuation of the crust. In this study, we present the latest geophysical data, collected by the Chinese research vessel Kexue, to investigate subduction erosion at the Yap subduction zone and develop subduction models for Yap subduction zone structures. We show an anomalous distance between the Yap Trench and the adjacent volcanic arc, the steep slope of the trench arcward, a frontal prism, and rare sediment in the trench, all typical features of erosive margins. We propose that the high‐relief subducting plate has led to erosion of the overlying plate and that different subduction processes, controlled locally by the topography of the subducting plate, have modified the Yap subduction zone. Numerous normal faults increase the relief of the seafloor entering the trench, and an anomalously large slope angle along the trench reflects uplift of the fore‐arc high via seamount subduction. In addition, the thin crust of the subducted horst and graben structures, along with the normal faults, might have eroded the fore‐arc crust and subsequently eroded the Yap arc crust during subduction. These subduction erosional processes at the Yap Trench provide one of the best cases of an erosive margin in the world, particularly for a subduction zone with an extremely slow convergence rate.

In-situ mechanical weakness of subducting sediments beneath a plate boundary d\xe9collement in the Nankai Trough

The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP) Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at ~ 800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from ~ 10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~ 800–1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments.

Paleomagnetism of IODP Site U1380: Implications for the Forearc Deformation in the Costa Rican Erosive Convergent Margin

The destructive nature of subduction erosion poses challenges to fully understanding the evolution of erosive convergent margins that are critical to understanding crustal recycling and seismogenesis. Forearc deformation holds important clues to the evolution of erosive convergent margins. Here we present detailed paleomagnetic and structural analyses of IODP Site U1380 cores from the middle slope of the forearc of the Costa Rican erosive convergent margin. The analyses reveal a strong deformation zone from ~490 to ~550 mbsf that is characterized by abundant fissility/foliations shallower than the bedding. Similar relatively strong deformation zones are recognized from the frontal prism and upper slope sites, and are broadly correlative, forming a zone of strong deformation across the forearc. This zone spans ~2.0 to 1.83 Ma and the deformation likely occurred briefly at ~1.80 Ma. The widespread, short-lived, and strong deformation is interpreted as a result of intense subhorizontal shear following the rapid forearc subsidence driven by the dramatic subduction erosion associated with the abrupt onset of the Cocos Ridge subduction. Given the typical occurrence of forearc subsidence by subduction erosion, similar styles of deformation are probably common in other erosive convergent margins as well.

Thermal Alteration of Pyrite to Pyrrhotite During Earthquakes: New Evidence of Seismic Slip in the Rock Record

AbstractSeismic slip zones convey important information on earthquake energy dissipation and rupture processes. However, geological records of earthquakes along exhumed faults remain scarce. They can be traced with a variety of methods that establish the frictional heating of seismic slip, although each has certain assets and disadvantages. Here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep‐sea sediments cored from the Japan Trench—one of the seismically most active regions around Japan—during the Integrated Ocean Drilling Program Expedition 343, the Japan Trench Fast Drilling Project. Fault zone sediments and adjacent host sediments were analyzed mineral magnetically, supplemented by scanning electron microscope observations with associated energy dispersive X‐ray spectroscopy analyses. The presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at ~697, ~720, and ~801 m below sea floor in the frontal prism sediments, while it is absent in the adjacent host sediments. Elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite. The presence of pyrrhotite in combination with pyrite‐to‐pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800°C. The integrated mineral‐magnetic, microscopic, and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature.