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Quantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage-displacement scaling as continental faults.

We constrain orientations of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW‐SE convergence and is the site of recurrent slow slip events. The direction of the horizontal maximum stress is E‐W at an active splay thrust fault near the subduction margin trench. This trend changes to NNW‐SSE in a forearc trench slope basin on the offshore accretionary wedge, and to NE‐SW in the onshore forearc. Multiple, tectonic, and geological processes, either individually or in concert, may explain this variability. The observed offshore to onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike‐slip and/or extensional tectonic regime closer to the Taupo Volcanic Zone, further inland. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

The Hikurangi Subduction Margin (HSM) of New Zealand is well-known for its variable seismic behaviour along strike, and across the Pacific-Australian subduction interface. Pacific-Australian plate motion is accommodated by a combination of slow slip and normal seismic events. The mechanics of slow slip earthquakes and their relationship to normal earthquakes are not well constrained, and so they represent a challenge to the development of hazard models for this region of New Zealand.Variability in a number of aspects of the stress state along the HSM may play a role in controlling the observed spatially variable seismic behaviour. Here we present preliminary analysis of stress orientation information from borehole image logs acquired from oil and gas exploration wells, and scientific wells drilled as part IODP Expedition 372. Orientations of borehole breakouts (BOs) and drilling-induced tensile fractures (DITFs) from these image logs are used to determine orientations of the minimum and maximum horizontal stress directions respectively within the upper 3 km of the over-riding Australian Plate (hanging wall of the HSM subduction interface). Our analysis reveals that present day maximum horizontal compressive stress orientation (SHmax) within the subduction hangingwall varies along the HSM strike, from NE-SW (oblique to Pacific-Australian plate motion, parallel to HSM strike) in the northern HSM, to NW-SE (oblique to Pacific-Australian plate motion and HSM strike) in the southern HSM. Some deviation from this trend can be observed in both regions. At the frontal thrust in the northern HSM, SHmax is oriented N-S, and in the southern HSM one well shows a shallow E-W oriented SHmax. The borehole image log SHmax orientations in the northern HSM are consistent with SHmax orientations derived from shallow focal mechanism inversion. In contrast, borehole image logs SHmax orientations in the south of HSM are oblique to focal mechanisms derived shallow SHmax orientations. Borehole SHmax orientations are compatible with the maximum horizontal contraction strain rate direction determined from campaign GPS surface velocities, though in the southern HSM multiple directions are determined from this technique. The difference in stress orientations along the HSM is consistent with the variation in seismic behaviour linked to changes in coupling at the subduction interface. Interestingly, it is the southern HSM, where the subduction interface is considered strongly coupled, that stress field orientations are not consistent with depth and show variability. In the northern HSM, where the subduction interface is considered to be weakly coupled, stress orientations appear broadly consistent with depth.

International audience; nternational Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

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.

Sediments tell a tsunami story Trying to understand where major earthquakes and tsunamis might occur requires analysis of the sediments pouring into a subduction zone. Thick sediments were expected to limit earthquake and tsunami size in the Sumatran megathrust event in 2004, but the magnitude 9.2 earthquake defied expectations. Hüpers et al. analyzed sediments recovered from the Sumatran megathrust. They found evidence of sediment dehydration, which increased fault strength and allowed for the much larger earthquake to occur. Thus, models of other subduction zones, such as the Gulf of Alaska, may underestimate the maximum earthquake magnitude and tsunami risk. Science , this issue p. 841

Recognition of coring-induced disturbance, which is essential for magnetic fabric and paleomagnetic studies of poorly lithified sediments, is generally not straightforward. Here, we report on anisotropy of magnetic susceptibility (AMS) and paleomagnetic data of the sediments from Holes U1480E and U1480H, IODP Expedition 362, west of the Sumatra subduction zone. AMS is characterized by steep minimum principal axes (Kmin) in undisturbed sediments. However, a considerable portion of the recovered sediments are affected by significant coring-induced disturbance. In these cases, we observed three AMS patterns: (1) AMS principal axes are randomly distributed for sediments with mingling and distortion of beds, (2) Kmin axes of sediments with upward-arching beds are deflected out of the splitting face of the working half, and (3) suck-in sediments are characterized by vertical Kmax axes. These deformation-dependent AMS patterns can be attributed to the realignment of mineral particles caused by the coring process and subsequent sampling procedures. Besides a low-coercivity, vertical, drilling-induced overprint, we observed a high-coercivity component that is likely a composite of the primary magnetization with a demagnetization-resistant portion of the drilling overprint. After accounting for the disturbed intervals, several polarity transitions can be identified in the undisturbed sediments which correlate well with the Pleistocene geomagnetic polarity timescale. These observations demonstrate that great caution is required when attributing geological significance to AMS and paleomagnetic data obtained from soft sediment cores, which are highly susceptible to coring-induced disturbance. In addition, AMS measurements provide a potential tool for identifying core deformation for further paleomagnetic studies.

The near-flat subduction of the Cocos Ridge (CR) along the Middle American Trench (MAT) plays a pivotal role in governing the geodynamic evolution of the central American convergent margin. Elucidating the onset of its subduction is essential to understand the tectonic evolution and seismogenesis of the Costa Rican convergent margin, a typical erosive convergent margin and modern example of a flat-slab subduction. Initial subduction of the CR has been previously investigated by examining upper plate deformation that was inferred to have resulted from the initial CR subduction. However, little attention has been paid to the extensive sedimentary archives on the CR that could hold important clues to the initial CR subduction. Drilling on the CR during IODP Expedition 344 discovered a pronounced sedimentary hiatus at Site U1381. Here we present paleomagnetic and rock magnetic results of the Cenozoic sedimentary sequences at this site that bracket the hiatus between ca. 9.61 and 1.52 Ma. We also examine the areal extent, timing, and geologic significance of the hiatus by analyzing sedimentary records from five other ODP/IODP sites on CR and Cocos plate. The analyses show that the hiatus appears to be regional and the presence/absence of the sedimentary hiatus at different locations on CR implies a link to the onset of CR shallow subduction, as a result of either bottom current erosion or CR buckling upon its initial collision at the MAT. Records directly from CR thus provide a new window to unraveling the geodynamic evolution of the central American margin.

The Costa Rica Seismogenesis Project (CRISP) was designed to elucidate processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. The CRISP study area is located offshore the Osa Peninsula where the incoming Cocos Ridge has lifted the seismogenic zone to within reach of scientific drilling. This area is characterized by low sediment supply, a fast convergence rate, abundant plate interface seismicity, and a change in subducting plate relief along strike. In addition to elucidating processes at erosional convergent margins, this project is complementary to other deep fault drilling projects (e.g., The Nankai Trough Seismogenic Zone Experiment and the Japan Trench Fast Drilling Project). Integrated Ocean Drilling Program (IODP) Expedition 344 is the second expedition of CRISP Program A (IODP Proposal 537A-Full5), a first step toward deep riser drilling through the seismogenic zone. The focus of CRISP Program A is on the shallow lithologic, hydrologic, stress, and thermal conditions that lead to unstable slip in the seismogenic zone. Together with IODP Expedition 334, the first expedition of CRISP Program A, these data provide exciting insights into the nature of seismogenesis and erosive plate boundaries.