Roland von Huene

Generic model of subduction erosion

Erosion by high stress abrasion of convergent margins from horsts and grabens on the subducting plate is not shown in seismic images. In a proposed model, the frontal sediment prism is a dynamic mass that elevates pore-fluid pressure. Overpressured fluid invades fractures in the upper plate and separates fragments that are dragged into a subduction channel along the plate interface. Removed fragments are smaller than surface ship seismic techniques have resolved and beyond the reach of past scientific ocean drilling; however, current drill capability and downhole geophysics can test the model.

Long-term subduction-erosion along the Guatemalan margin of the Middle America Trench

A new analysis of Deep Sea Drilling Project (DSDP) Leg 84 data demonstrates that the dominant process controlling the Guatemala margin tectonic evolution since ca. 25 Ma is subduction-erosion. Data from benthic foraminifera, assemblages from upper-slope DSDP Sites 568, 569, and 570 indicate long-term, progressive subsidence from upper to middle bathyal depths (600–1000 m) ca. 19 Ma to modern abyssal depths (>2000 m). Rapid subsidence migrated landward starting at the Oligocene-Miocene boundary time under the current middle slope, where it increased sharply ca. 19 Ma, reached the current upper slope by ca. 15 Ma, and arrived at the uppermost slope ca. 2 Ma. Subsidence indicates crustal thinning by basal tectonic erosion of mass from the underside of the upper plate. Under the assumption that, in the Miocene, the morphology of the forearc was similar to that of today, landward migration of the trench was at a rate of 0.8–0.9 km/m.y. This linear rate corresponds to a tectonic erosion rate of the submerged forearc of 11.3–13.1 km3·m.y.−1·km−1. The evolution of arc magmatism and superfast spreading at the East Pacific Rise since early Miocene time may have caused slab shallowing and tectonic erosion that readjusted the forearc geometry.

Interplate earthquakes as a driver of shallow subduction erosion

Basal erosion is a prevalent process at subduction zones and plays an important role in the mass balance of global plate tectonics. In contradiction with the theoretical expectation that basal erosion requires high basal friction and hence compression in the upper plate, extensional faulting is commonly observed in submarine wedges that undergo such erosion. Here we propose a model to explain this apparent paradox in terms of stress fl uctuations during earthquake cycles. In this model, basal erosion occurs during large earthquakes when the shallow, rate-strengthening part of the plate interface strengthens and its overlying wedge weakens, but extension occurs during interseismic relaxation of wedge stress. The mechanics of basal erosion provide important information on the nature of the updip limit of the megathrust seismogenic zone in margins dominated by basal erosion.

When Seamounts Subduct

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Convergent Margin Structure in High-Quality Geophysical Images and Current Kinematic and Dynamic Models

Understanding the mechanics of convergent margins is fundamental to assessing risks from earthquakes and trans-oceanic tsunamis. Marine observations of the past decade have advanced that understanding. A once commonly inferred accreted wedge extending from trench axes to shelves is now resolved into 3 domains of different mechanics in space, that vary during an earthquake cycle. The frontal prism increases weight on subducting materials elevating pore fl uid pressure and reducing interplate friction. The middle prism is moderately stable and merges into the more stable margin framework of the inner prism beneath the upper slope and shelf. Significant accretion occurs as material from the frontal prism is added to the middle prism. Accretion is common along thickly (>1 km) sedimented trenches and slowly converging margins. Rapid convergence enhances the effi ciency of sediment subduction and subduction erosion. The subduction channel on the lower plate accepts a fi nite amount of trench sediment and any excess is added to the frontal prism on the upper plate. Erosion beneath the middle slope contributes material to the subduction channel. Erosion and accretion can be coeval, for instance, subducted seamounts erode the upper plate as adjacent sediment accretes. The change in strain during interseismic locking that is released during coseismic slip, changes the dynamics of each segment in time. This helps explain extensional normal faults in a converging plate environment. Recent observations provide information for a unifying framework concept to aid interpretations of both accreting and eroding margins.

Interplate seismicity at the CRISP drilling site: The 2002 Mw 6.4 Osa Earthquake at the southeastern end of the Middle America Trench

We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1-D and a 3-D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth-migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

In situ stress state from walkaround VSP anisotropy in the Kumano basin southeast of the Kii Peninsula, Japan

To reveal the stress state within the Kumano basin, which overlies the Nankai accretionary prism, we estimated seismic anisotropy from walkaround vertical seismic profiling (VSP) data recorded at Site C0009 during Integrated Ocean Drilling Program (IODP) Expedition 319. We obtained the following anisotropic parameters: (1) P wave velocity anisotropy derived from azimuthal normal moveout (NMO) velocity analysis, (2) P wave amplitude variation with azimuth, and (3) axes of symmetry of S wave splitting. Azimuthal variations of P wave velocity by ellipsoidal fitting analysis showed that P wave velocity anisotropy within sediments of the Kumano basin was ∼5%. Both the directions of fast P wave velocity and strong amplitude are aligned with the convergence vector of the Philippine Sea plate. Furthermore, S wave splitting analysis indicated that S wave polarization axes were parallel to and normal to the direction of plate subduction. These results indicate that the maximum horizontal stress at Site C0009 in the Kumano basin is in the direction of plate subduction. The horizontal differential stress estimated from the P wave velocity anisotropy (2.7∼5.5 MPa) indicates that the maximum horizontal stress is similar in magnitude to (or a little higher than) the vertical stress.

Potential of 3‐D vertical seismic profiles to characterize seismogenic fault zones

The potential of a 3-D vertical seismic profile (VSP) to improve resolution of seismogenic plate interfaces was explored with synthetic modeling. The 3-D VSP modeled is at a proposed site for a 1 to 1.5 km deep open hole that provides background for riser drilling. Three-dimensional VSP images could resolve 30–60 m spaced reflective horizons in a Costa Rican subduction zone. It can record a great amount of high-fidelity S wave data to invert for physical properties, directions of strain, and pore pressure above and below the plate interface fault. A 6 km × 12 km grid of shots with a surface ship will illuminate a ∼4 km × 7 km area of the plate interface fault zone with a high data density. Acquisition adds 5 to 9 days to drill ship time on site and a shooting ship. Seismic image resolution falls between that of borehole information and 3-D surface ship seismic images. A multiple-kilometer 3-D volume of high-fidelity S wave data is an exceptional addition not available with other techniques.

Focusing on proto‐seismogenic zone of erosional convergent margin

Great earthquakes in subduction zones occur after stable slip in the proto-seismogenic zone transitions to the unstable slip that characterizes seismogenic zones. Subducted material input to seismogenic zones affects this transition. Material structure, lithology and physical properties change progressively during subduction, and according to current hypotheses, specific material transformations trigger the stable to unstable slip transition.Where accretion dominates a convergent margin, material input is trench sediment that is easily drill-sampled. However, where erosion dominates a margin, material input is unknown because it originates along the base of the upper plate and alters differently. The depth at which material is eroded lies beyond the sampling capabilities of past scientific ocean drilling, so the protoseismogenic zone transformed material has never been drill-sampled; nor does geophysics resolve its structure, lithology, and physical properties. The Japanese riser drill ship Chikyu in the Integrated Ocean Drilling Program (IODP) overcomes this difficulty. Preparing a site for deep drilling is a much greater task than preparing the shallower sites of past programs, so this is accomplished during workshops.