J. Casey Moore

Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress

Off southwest Japan the seaward limit of coseismic displacement (or updip limit of the seismogenic zone) of the 1946 Mw 8.3 thrust earthquake reaches to 4 km depth and ∼40 km landward of the trench. This limit coincides with the estimated location of the 150 °C isotherm, and has been linked to changes in physical properties associated with the smectite to illite clay-mineral transition. Here we show that this limit correlates with a suite of diagenetic to low-grade metamorphic processes characterized by (1) declining fluid production and decreasing fluid pressure ratio (λ*) and (2) active clay, carbonate, and zeolite cementation and the transition to pressure solution and quartz cementation. These diagenetic to low-grade metamorphic changes cause the onset of velocity weakening during thrust faulting, an increase in effective stress, and strengthening of the hanging wall, which together combine to produce recordable earthquakes.

Tectonics and hydrogeology of accretionary prisms: role of the décollement zone

Abstract At convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony, by changes in stress orientation and by a discontinuity in plate velocity. The decollement zone initiates in a weak sediment layer, typically a low-permeability hemipelagic mud lying below a more rapidly deposited, stronger, more permeable trench turbidite sequence. At the deformation front, trench turbidites tend to be offscraped, whereas the finer-grained hemipelagic and pelagic sediments are more likely to be underthrust. Offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried, undergo high-pressure metamorphism, and are more likely to be preserved in the stratigraphic record. Burial rates associated with underthrusting are high, exceeding 20 km/my. With high rates of underthrusting, sediment descending below the decollement zone is probably buried faster than it can dewater, resulting in emplacement of relatively high porosity deposits at depth. Fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources. The episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone. Possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it. The thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering, or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism. Deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence.

Landward vergence and oblique structural trends in the Oregon margin accretionary prism: Implications and effect on fluid flow

Abstract The central Oregon margin spans a regional transition in accretionary structures from seaward-verging in the south to landward-verging in the north. New multichannel seismic (MCS) data image both landward- and seaward-vergent provinces along the central and northern Oregon margin. Landward-vergence is characterized by a deep decollement, with deformation distributed across a broad lower continental slope in a coherent structural style. In the landward-vergent area, virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet, forming fault-bend folds and a distinctive ridge/trough morphology. This style of landward-vergence is not explained by existing models. In contrast, seaward-vergence correlates with a shallower decollement, approximately 1.4 km above the oceanic crust, and a more intense style of deformation within a narrower slope. Initial thickening of the trench sediments occurs across a well-developed protothrust zone. The frontal thrust forms a ramp-anticline that is cut by a prominent backthrust. Previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high-amplitude, reversed-polarity reflections suggestive of enhanced porosity along the faults. Potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin. Abrupt changes in structural style occur both along strike and updip, and are bounded by two sets of oblique-slip faults. Three NW-striking left-lateral faults are imaged in both MCS and SeaBeam data. Plunging anticlines developed along the NW-striking faults are venting fluids and were previously interpreted as mud volcanoes. The deformation front is locally disrupted where these faults intersect the prism, but they appear to have limited influence on the structural evolution of the prism. In contrast, the NE-striking right-lateral faults are confined to the deforming sediments of the upper plate. These faults interact with the thrusts within the prism, forming a rhomboidal pattern of three-dimensional deformation.

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Seismically inferred dilatancy distribution, northern Barbados Ridge decollement: Implications for fluid migration and fault strength

A 5 x 25 km, three-dimensional seismic survey of the lower part of the northern Barbados Ridge accretionary prism creates a three-dimensional image of a major active decollement fault. The fault is usually a compound negative-polarity reflection modeled as a low-velocity, high-porosity zone less than ∼14 m thick. This thickness is significantly less than that defined by drilling of a >40 m zone of deformation at Ocean Drilling Program (ODP) Site 671B, located within the surveyed area. We infer that the seismically defined fault is a thin, high-porosity zone and is thus an undercompacted, high-fluid-pressure dilatant section. If these inferences are correct, then map-view variations in seismic-reflection waveform and amplitude illustrate complex patterns of fault-zone fluid content and fluid migration paths. The amplitude map suggests kilometre-wide channels of locally high porosity and thus focused fluid flow. These paths are only subparallel to the expected minimum head, as inferred from the shape of the overlying sediment wedge; other factors must modify fluid concentrations and ultimately migration. Several areas of positive-polarity fault reflections define square-kilometre-sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault. One, coincident with Site 671B, may explain the success of drilling through the fault here. All other holes drilled in the area were within the negative-polarity regions and were unsuccessful in penetrating through the entire fault zone, possibly because of instability associated with high fluid pressures and a weak fault. ODP Leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures.

Structure and composition of the plate-boundary slip zone for the 2011 Tohoku-Oki earthquake.

Deep Drilling for Earthquake Clues The 2011 Mw 9.0 Tohoku-Oki earthquake and tsunami were remarkable in many regards, including the rupturing of shallow trench sediments with huge associated slip (see the Perspective by Wang and Kinoshita). The Japan Trench Fast Drilling Project rapid response drilling expedition sought to sample and monitor the fault zone directly through a series of boreholes. Chester et al. (p. 1208) describe the structure and composition of the thin fault zone, which is predominately comprised of weak clay-rich sediments. Using these same fault-zone materials, Ujiie et al. (p. 1211) performed high-velocity frictional experiments to determine the physical controls on the large slip that occurred during the earthquake. Finally, Fulton et al. (p. 1214) measured in situ temperature anomalies across the fault zone for 9 months, establishing a baseline for frictional resistance and stress during and following the earthquake. The Tohoku-Oki earthquake occurred along a thin, clay-rich fault zone in the basal strata of the subducting plate. The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.

Structural style and kinematics of an underplated slate belt, Kodiak and adjacent islands, Alaska

The Kodiak Formation, composed of coherent Maastrichtian turbidites, is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest Cretaceous. It consists of about 80% coherent landward-dipping thrust packets; zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder. About half of these disrupted sandstone zones are related to pre-accretion deformation, and the rest formed along late-stage, strike-slip faults that postdate development of slaty cleavage. The formation is divided into three structural belts. The landward and seaward belts include steeply dipping structures, and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain. The central belt probably acted as a low-angle, southeast-verging, floor thrust zone beneath the landward belt. The structural history of the Kodiak Formation includes (1) early soft-sediment disruption; (2) tectonic stratal disruption; (3) thrust faulting, slaty cleavage (S 1 ) development, and folding (F 1 ); (4) intrusion of granodioritic plutons, dikes, and sills and associated normal faulting; (5) development of crenulations (F 2 ) and crenulation cleavage (S 2 ); (6) thrust faulting; and (7) development of right-lateral, strike-slip faults. Event 3 produced the dominant northwest-dipping structural grain and caused the greatest amount of shortening; the timing of event 7 relative to events 5 and 6 is not certain. The dominant structures of the Kodiak Formation are interpreted as developing during underthrusting, underplating, and intra-wedge shortening during latest Cretaceous time within an accretionary wedge. The maximum time from deposition to emplacement was about 12 m.y., and the timing is constrained by the maximum depositional age (74 m.y.) and the age of plutonic rocks (62 m.y.) that crosscut the dominant fabric. Pre-cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting. Slaty cleavage, thrusts, and F 1 folds developed during underplating that also resulted in the formation of duplexes. Crenulations are probably related to post-underplating subhorizontal shortening of sediments within the accretionary wedge. Syn-deformation dynamic recrystallization of quartz, minimum syn-accretion metamorphic temperatures between 205 and 250 °C, and pressures of at least 2.65 kb recorded during the earliest stages of deformation suggest underplating at >10 km.

Thickening of fault zones: A mechanism of melange formation in accreting sediments

Sediments accreted at subduction zones undergo stratal disruption and form a type of melange. The thickness of the disrupted zones grows with progressive deformation. This suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment. Factors causing the abandonment of fault surfaces during continuing deformation include (1) strengthening owing to porosity loss during consolidation, (2) localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits, and (3) reorientation of fault surfaces. The disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass.

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

In situ stress state in the Nankai accretionary wedge estimated from borehole wall failures

We constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures (borehole breakouts and drilling-induced tensile fractures) detected in four vertical boreholes (C0002, C0001, C0004, and C0006 from NW to SE) drilled in the Nankai accretionary wedge. The directions of the maximum horizontal principal stress (SHmax), indicated by the azimuths of borehole wall failures, are consistent in individual holes, but those in C0002 (margin-parallel SHmax) are nearly perpendicular to those in all other holes (margin-normal SHmax). Constrained stress magnitudes in C0001 and C0002, using logged breakout widths combined with empirical rock strength derived from sonic velocity, as well as the presence of the drilling-induced tensile fractures, suggest that the stress state in the shallow portion of the wedge (fore-arc basin and slope sediment formations) is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike-slip faulting or possible reverse faulting. Thus, the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults. The margin-perpendicular tectonic stress components in the two adjacent sites, C0001 and C0002, are different, suggesting that tectonic force driven by the plate pushing of the Philippine Sea plate does not uniformly propagate. Rather, the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation-driven extension in the fore arc and thrusting and bending within individual geologic domains.