Explore the vast range of titles available.

The water cycle at subduction zones remains poorly understood, although subduction is the only mechanism for water transport deep into Earth. Previous estimates of water flux 1 – 3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres. The main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle. Previous active-source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate-bending region near the oceanic trench 4 – 7 . However, these studies do not constrain the depth extent of hydration and most investigate young incoming plates, leaving subduction-zone water budgets for old subducting plates uncertain. Here we present seismic images of the crust and uppermost mantle around the central Mariana trench derived from Rayleigh-wave analysis of broadband ocean-bottom seismic data. These images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the Moho discontinuity. Combined with estimates of subducting crustal water, these results indicate that at least 4.3 times more water subducts than previously calculated for this region 3 . If other old, cold subducting slabs contain correspondingly thick layers of hydrous mantle, as suggested by the similarity of incoming plate faulting across old, cold subducting slabs, then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates 3 . Because a long-term net influx of water to the deep interior of Earth is inconsistent with the geological record 8 , estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards 9 . Seismic images of Earth’s crust and uppermost mantle around the Mariana trench show widespread serpentinization, suggesting that much more water is subducted than previously thought.

This dissertation examines the incoming Pacific plate and the overriding forearc at the Mariana Subduction Zone using passive and active source seismology. The incoming plate, with water bound in the plate sediment, crust and mantle, is of interest to help constrain the global water cycle. The seismogenic zone at Mariana is noted for being an aseismic end-member, while extensional faults and active serpentine mud volcanoes characterize the forearc. I use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study the seismicity and shallow structure. The passive source component of the deployment consists of 20 broadband OBSs, 5 hydrophones, and 7 land stations on the volcanic arc deployed for one year. The active source component used in this study consists of two ~400 km transects that span both the outer forearc and Pacific Plate, with co-located multi-channel seismic (MCS) reflection and wide-angle refraction data collected. 59 short period OBSs and hydrophones were deployed for the first month to collect the refraction data, while a 646 channel, 8-km long streamer was used to collect the reflection data. Using all of the deployed OBSs, land stations, and hydrophones, 1,692 earthquakes were detected and located in the study area. Of these, the largest 17 were used to invert for earthquake focal mechanisms, and clusters of earthquakes were relocated using a double-difference method. Using the wide-angle refraction and MCS reflection data, P-wave velocity and reflection profiles were constructed for the two transects. Earthquakes in the incoming plate occur to ~35 km below the seafloor, and focal mechanisms indicate normal faulting due to the plate bending. Most of the seismicity occurs within 70 km of the trench, indicating the greatest deformation due to bending occurs near the trench axis. This is consistent with the normal faults identified in the reflection profiles, with increasing fault offset and occurrence near the trench. The P-wave velocity in the mantle also reflects this change, with increasing reduction of velocity nearing the trench. The mantle velocity reduction is indicative of hydration by serpentinization and corresponds to a maximum of 23 vol% bulk serpentinization. However, considering the case in which hydration is limited to the normal faults identified in the reflection profiles, the estimate of water bound in the incoming plate mantle is greatly reduced. Earthquakes occurring arc-ward of the trench reflect the seismogenic character of the megathrust. A thrust sequence at 10 km depth and 20 km west of the trench indicates the seismogenic zone extends to nearly the trench. The forearc is marked by a heterogeneous distribution of low magnitude seismicity, which may be attributed to incoming plate roughness and/or serpentinization. The reflection profiles highlight the variable forearc morphology. Compressional features and a steep trench slope are observed in the northern line, while the southern line hosts a serpentine mud volcano and gradually slopes towards the trench. The difference may be due to the influence of subducted topography. The serpentine mud volcano, Turquoise Seamount, is underlain by a wide and deep velocity anomaly, suggesting a wide influence by seamount building. The outermost forearc is characterized by low velocities, and may indicate tectonic erosion.