Search

Accreted basaltic rocks are expected to provide information on intraplate volcanism in the oceans. Basaltic rocks, originating from mid‐ocean ridges, plateaus, and seamounts, have been reported from exhumed accretionary complexes. However, basaltic rocks related to petit‐spot volcanism remain poorly documented. We examined basaltic intrusive rocks in an exhumed accretionary complex on Amami‐Oshima Island, Ryukyu Arc. Basaltic sills intruded mélange composed of seamount‐derived micritic limestone and basaltic rocks, which deformed together with the mudstone‐dominated mélange in the subduction zone. A separate basaltic sill intruded pelagic chert before being incorporated as a mélange block in the mudstone matrix during deformation in the subduction zone. Geochemical discrimination diagrams and trace element patterns indicate that the accreted dolerites and nonintrusive basalts likely originate from mid‐ocean ridges and hotspots, respectively. On the other hand, the geochemical characteristics of basaltic sills are compatible with alkaline basalt with enriched in incompatible trace elements relative to other basaltic rocks of hotspot origin and may represent a low degree of partial melting from a deeper mantle source (>90 km). A higher ratio of gadolinium to ytterbium in the basaltic sills relative to hotspot or mid‐ocean ridge‐related basalt indicates both a deeper melting depth, and a magma source which upwelled through thick oceanic lithosphere far from the mid‐ocean ridge. Based on the timing of basaltic intrusion and geochemical features, we suggest that the basaltic intrusive rocks could record past petit‐spot volcanism in the oceanic plate. Plain Language Summary: Basaltic rocks in exhumed accretionary complexes are crucial for understanding volcanism in oceanic plates. Petit‐spot volcanoes on modern seafloor have been found in the region seaward of the trench, derived from rising basaltic magma from the deep mantle along fractures from the bending of oceanic plates. However, past petit‐spot volcanism on already subducted oceanic plates remains poorly understood. This paper reports on the geological features and geochemical element analyses used to determine that basaltic rocks found in the Chichibu accretionary complex on Amami‐Oshima Island, Ryukyu Arc, originated from the deep mantle and intruded oceanic sedimentary rocks and seamount‐related rocks. We suggest that the basaltic intrusion resulted from petit‐spot volcanism. Our results highlight that basaltic rocks found in outcrops exhumed from subduction zones could contribute to understanding past petit‐spot volcanism in the oceanic plate that has completely subducted to the mantle. Key Points: Alkaline basaltic rocks intruded mélange in an exhumed accretionary complex on Amami‐Oshima IslandGeochemical features suggest that alkaline basaltic intrusive rocks originated from low‐degree partial melts generated in the deep mantleTiming and geochemical signatures of the alkaline basalt intrusions suggest they represent past petit‐spot volcanism [ABSTRACT FROM AUTHOR]

Accretionary processes at mid‐ocean ridge segments with low magma input have seldom been investigated over the long term. The evolution of such magma‐starved segments over time is still largely unknown. We present a study on the structure and evolution of the southernmost intra‐transform ridge segment of the St. Paul Transform Fault System in the Equatorial Mid‐Atlantic Ridge, based on new bathymetry, gravity, and rock sampling data. We show that this area evolves differently from previously described tectonics along ridge segments of similar spreading rate. On the flanks of the axial ridge segment, we observe a succession of structures exhumed by detachment faulting, evolving from east‐facing, long‐lived, corrugated oceanic core complexes (∼6 Ma ago), to short‐lived detachment faults exposing lower crust and mantle rocks and facing alternatively east and west in the more recent part of the segment. The oldest detachment faults have been repeatedly split and partially transferred to the opposite flank through the formation of new detachments into the footwall. The terminations of three old, east‐facing detachments are observed on the east flank of the segment. The westward relocations of the plate boundary appear to compensate for the asymmetry of accretion through detachment faulting, overall creating the same amount of lithosphere on both flanks of the ridge. We interpret the observed changes in the time of the accretionary processes to reflect a decrease of the melt supply over the last ∼6 Myr. Plain Language Summary: The generation of new seafloor at mid‐ocean ridges where cold underlying mantle delivers low magma supply has not been investigated over the long term. Here we present the analysis of new bathymetry, gravity, and rock sampling data over such a ridge segment located within the St. Paul Transform Fault system in the Equatorial Mid‐Atlantic Ridge, which allowed us to bring constraints on its structure and evolution over the last ∼6 Myr. We show that this area evolves differently from previously described ridge segments of similar spreading rate. We observe the remnants of very large normal faults called detachment faults, which have been active for very long times, forming domes called oceanic core complexes. The fault surfaces have been dissected by further extensional deformation, until the plate motion became accommodated along a new detachment fault formed west of the previous plate boundary. The emergence lines of the detachment faults are observed on the eastern flank of the ridge segment. We also observe a change in time of the structures, from typical oceanic core complexes to shorter ridges formed by the exhumation of mantle rocks. We interpret these changes to possibly reflect a decrease in the melt supply in the last 6 Myr. Key Points: The southern intra‐transform ridge segment in the St. Paul transform fault system, over the last ∼6 Myr is dominated by detachment faultingSeveral detachments have been split and partly transferred to the opposite ridge flank through ridge axis relocationThe change in the accretionary processes, from Oceanic Core Complexes to mantle exhumation, suggests a decrease in the time of the melt supply [ABSTRACT FROM AUTHOR]

Subducted sediment plays a key role in modulating pore pressure and seismic behavior at subduction zones. We investigated the seismic character of incoming sediments to test how sediment and basement variations relate to the along‐strike changes within the accretionary prism and plate boundary conditions at the Nankai Trough. High‐resolution seismic data reveal for the first time the presence of countourite mounded drifts in the Shikoku Basin. These features have probably introduced permeability heterogeneities into an otherwise homogenous mud‐dominant unit. Additionally, we found that normal faults in this unit are more extensive than previously documented, which probably enhances along‐strike fluid transport. The wedge taper is more correlative with the thickness of the mud‐dominant facies than the turbidite thickness. This may be due to the permeability heterogeneities associated with contourite deposits and normal faults, or due to the absence of thick turbidite deposits. Turbidite deposits can either aid the drainage of the margin where they are not confined by basement topography, or contribute to high pore fluid pressures where they are confined by less permeable mudstone or basement topography. When confined turbidite deposits and contourite mounded drifts are subducted, they may contribute to localized compartments of excess pore pressure which provide the necessary conditions for slow slip behavior. We determined that along‐strike variations in seismic behavior are likely related to the subducting basement topographic and sediment characteristics, which vary on a local (<10 km) scale. Plain Language Summary: Subduction zone faults, such as those off the coast of southwest Japan, have been shown to slip at a range of speeds. The ruptures range from typical earthquakes, which occur within seconds or minutes, to slow earthquakes, which can rupture over the course of days or weeks. The sediment characteristics of the incoming plate play a key role in determining the type of slip behavior along a margin. When sediments are subducted, they are compressed and expel the fluid trapped in the sediment. If the fluid is unable to escape, the fluid pressure builds up, and can reduce the strength of the plate boundary fault. The type of sediment, as well as whether it has been faulted or modified by the ocean currents, can control how well fluid can escape. The sediment characteristics and corresponding plate boundary strength vary greatly on a sub‐10 km scale. Slow earthquakes in our study area are co‐located with subducted seamounts. We suggest that this is because the sediments commonly deposited on the sides of large seamounts are conducive to forming high pore fluid pressures upon subduction. This result in a locally weaker plate boundary fault, which may be a necessary condition for slow earthquakes. Key Points: High‐resolution seismic reflection data reveal for the first time asymmetrical contourite mounded drifts in the Shikoku BasinBasement topography and paleocurrents influenced the location of turbidite and contourite deposits, as well as how well sealed the deposits areContourites, turbidites, and normal faults result in localized heterogeneities in the hydrogeologic properties of the subducting sediment [ABSTRACT FROM AUTHOR]

Characterization of the porosity evolution across the sedimentary section entering subduction zones and accreted sediments provide valuable information for understanding the deformation history at accretionary margins and the physico‐chemical processes in and around fault zones. International Ocean Discovery Program Expeditions 372 and 375 drilled, logged, and cored the reference section on the incoming plate (Site U1520) and across the active Pāpaku thrust (Site U1518), which is a <30° westward‐dipping splay fault in the frontal accretionary wedge at the north Hikurangi margin in a region where tsunami earthquakes and recurrent slow slip events are documented. We observe strong variations of physical properties across the thrust fault. In particular, interstitial porosity increases by ∼10% through the fault zone while it exponentially decreases with depth, showing lower values in the hanging‐wall (HW; on average, ∼36%) than in the footwall (∼42%). Based on comparisons of porosity with the compaction curve at Site U1518 with that of reference Site U1520, we infer an overcompaction in the HW and a nearly normal compaction in the footwall of the Pāpaku thrust. We suggest that the porosity pattern across the thrust infers differences in the maximum burial depth of sediments, characterized by an overcompacted HW that has been uplifted, thrusted, and concomitantly tectonically eroded above the footwall. Porosity data indicate hydrostatic conditions around the thrust fault contrasting with commonly assumed excess pore pressure at the plate interface. Key Points: Strong contrasts of physical properties including porosity occur across the Pāpaku thrust fault at IODP Site U1518The hanging‐wall (HW) is overcompacted and shows lower interstitial porosity and higher P‐wave velocity and resistivity than the footwallPorosity suggests differences in maximum burial depth with an uplifted, thrusted, and concomitantly eroded HW above the footwall [ABSTRACT FROM AUTHOR]

Three‐dimensional (3‐D) seismic reflection data and sediment cores record ~2.87 million years of structural and depositional history of a trench slope basin along the outer Nankai accretionary prism, southwest Japan. Numerous mass transport deposits (MTDs) and fault structures are present in the basin. Here, we investigate the links between slope failures, slope basin development, and movement along a prominent out‐of‐sequence thrust (OOST) fault and development of a large anticline. Three two‐dimensional (2‐D) cross sections are restored to ~2.2 Ma using stratigraphic and structural relationships interpreted in the 3‐D data. The restorations are then compared and combined to provide a 3‐D perspective of basin development. We find that total shortening across all faults and folds was accommodated by different displacement styles along multiple branches, with 5.3, 5.5, and 7.3 km of shortening from northeast to southwest over the past ~2.2 Ma. We believe that the majority of this displacement occurred prior to ~1.7 Ma, followed by a dramatic decrease in slip rate within the study area as slip was transferred to the more seaward portions of the prism. In the northeast, deformation is primarily accommodated by the main branch of the OOST and anticlinal faulting, while deformation in the southwest is primarily along deeper branches of the OOST. This differential motion explains the occurrences of various mass wasting events, and lateral differences in trench slope basin geometry within the study area. Plain Language Summary: Accretionary prisms form at subduction zones where one tectonic plate dives beneath another. Sediments from the downgoing plate are scraped off and added to the overriding plate over millions of years to form the prism. Here, we use three‐dimensional (3‐D) seismic reflection data and sediment cores recovered during drilling into the prism to restore the movement along the faults back through time. Numerous submarine landslide deposits, faults, and other geologic structures are present in the data. To understand the links between submarine landslides, sediment basin development, and fault and geologic structure development, three two‐dimensional (2‐D) cross sections within the 3‐D data are restored back through time ~2.2 million years. The restorations are then compared and combined to provide a 3‐D perspective of sediment basin development. We find more displacement along faults near the seafloor in the northeastern part of the study area while fault displacement is much deeper under the seafloor in the southwest. Total shortening across the faults is partitioned differently across the area, with total displacement being the greatest in the southwest and gradually decreasing to the northeast. These differential motions explain the occurrences of various landslide events and seafloor variations within the study area. Key Points: Faulting that generated a slope basin on the Nankai prism evolved in the context of an accretionary prism maintaining critical taperTrench slope basin development was influenced by an underlying regional anticline and local slope failuresShortening in the midslope region slowed by ~1.7 Ma and uplift of the outer fore arc was accommodated on faults in the upper slope region [ABSTRACT FROM AUTHOR]

We 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. Plain Language Summary: We help constrain the physical and mechanical properties of the materials near the trench that hosted the 2011 Tohoku M9 earthquake and tsunami and test whether sediments from the Pacific Plate are scraped off or subducted as they enter the subduction zone. We use sediments from cores recovered during the Integrated Ocean Drilling Program Expedition 343. Because traditional sedimentologic and biostratigraphic techniques alone are not sufficient to determine sediment source region, we apply a new approach using the radioactive decay of the 10Be isotope. We use 10Be to date sediments, to identify sediments that have been faulted out of chronologic order, and to correlate sediments between sites. Our results show numerous faults in the system, likely resulting from multiple stages of deformation during slip over rough seafloor topography. Our data also indicate that 50–90% of the sediments on the incoming Pacific Plate are scraped off and 10–50% of the incoming sediments are subducted. The subducted sediments consist of muds and frictionally weak clays that might promote slip during subduction earthquakes. Similar approaches may provide insight on the composition and evolution of faults that host large earthquakes and tsunami at other plate boundaries. Key Points: Meteoric 10Be provides high‐resolution age control in the frontal prism of the Japan trench at IODP Exp 343 (JFAST) Site C0019The frontal prism and plate boundary contain faults at decameter scale spacing, possibly due to subduction of horst‐and‐graben topographyThe majority of incoming sediments at this location have been accreted to the overriding plate during at least the Quaternary period [ABSTRACT FROM AUTHOR]

Sand‐shale mélanges from the Kodiak accretionary complex and Shimanto belt of Japan record deformation during underthrusting along a paleosubduction interface in the range 150 to 350 °C. We use observations from these mélanges to construct a simple kinetic model that estimates the maximum time required to seal a single fracture as a measure of the rate of fault zone healing. Crack sealing involves diffusive redistribution of Si from mudstones with scaly fabric to undersaturated fluid‐filled cracks in sandstone blocks. Two driving forces are considered for the chemical potential gradient that drives crack sealing: (1) a transient drop in fluid pressure ∆Pf, and (2) a difference in mean stress between scaly slip surfaces in mudstones and cracks in stronger sandstone blocks. Sealing times are more sensitive to mean stress than ∆Pf, with up to four orders of magnitude faster sealing. Sealing durations are dependent on crack spacing, silica diffusion kinetics, and magnitude of the strength contrast between block and matrix, each of which is loosely constrained for conditions relevant to the seismogenic zone. We apply the model to three active subduction zones and find that sealing rates are fastest along Cascadia and several orders of magnitude slower for a given depth along Nicaragua and Tohoku slab‐top geotherms. The model provides (1) a framework for geochemical processes that influence subduction mechanics via crack sealing and shear fabric development and (2) demonstration that kinetically driven mass redistribution during the interseismic period is a plausible mechanism for creating asperities along smooth, sediment‐dominated convergent margins. Plain Language Summary: Geophysical monitoring of active subduction zones has revealed plate boundary slip behaviors such as creep, slow slip events, and earthquakes that vary spatially and temporally for different plate boundaries and downdip along a given boundary. Fault rocks exposed on land from paleosubduction plate boundaries provide a record of the deformation processes that likely occur during slip along active boundaries, so we review the characteristics of these ancient rocks to develop insight into slip behavior in subduction zones. We find that plate boundary deformation in these cases occurs within a wide fault zone through processes that involve redistribution of silica from shearing mudstones to cracking sandstone blocks. We use a geochemical model to calculate how long it would take to seal a crack by this process and conclude that cracks seal at rates that could influence the earthquake cycle, with rates of crack healing dependent on the temperature structure and the depth where slip occurs. Our results suggest that processes of frictional failure and geochemical healing in downgoing sediments may influence the slip stability along the subduction interface. Key Points: Microstructural observations of subduction mélange record dissolution along a scaly fabric and diffusion to cracks in sandstone blocksA silica kinetics model depicts crack healing that is highly dependent on temperature and slab‐top geothermKinetic healing of cracks could modulate fault zone strength during the interseismic period [ABSTRACT FROM AUTHOR]