Down-dip Direction Research Articles

Deformation across the forearc of the Cascadia subduction zone at Cape Blanco, Oregon

Over the interval 1992–1999 the U.S. Geological Survey measured the deformation of a geodetic array extending N80°E (approximate direction of plate convergence) from Cape Blanco on the Oregon coast to the volcanic arc near Newberry Crater (55 and 350 km, respectively, from the deformation front). Within about 150 km from the deformation front, the forearc is being compressed arcward (N80°E) by coupling to the subducting Juan de Fuca plate. Dislocation modeling of the observed N80°E compression suggests that the main thrust zone (the locked portion of the Juan de Fuca‐forearc interface) is about 40 km wide in the downdip direction. The transverse (N10°W) velocity component of the forearc measured with respect to the fixed interior of North America decreases with distance from the deformation front at a rate of about 0.03 mm yr−1 km−1. That gradient appears to be a consequence of rigid rotation of the forearc block relative to fixed interior North America (Euler vector of 43.4°±0.1°N, 120.0°±0.4°W, and −1.67±0.17 ° (m.y.)−1; quoted uncertainties are standard deviations). The rotation rate is similar to the paleomagnetically measured rotation rate (−1.0±0.2 °(m.y.)−1) of the 15 Ma lava flows along the Columbia River 250 km farther north. The back arc does not appear to participate in this rotation but rather is migrating at a rate of about 3.6 mm yr−1 northward with respect to fixed North America. That migration could be partly an artifact of an imperfect tie of our reference coordinate system to the interior of North America.

Planar, non-planar and refolded sheath folds in the Phulad Shear Zone, Rajasthan, India

Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F 1, F 2 and F 3, with a striping lineation ( L 1) parallel to the hinge lines of F 1. The planar sheath folds of F 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L 1 swerves harmoniously with the curving of F 1 hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F 2 and F 3. Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y-axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X-direction was extremely large, much in excess of 6000 percent.

Evidences for cessation of Indian Plate Subduction in the Burmese Arc Region

The issue of whether subduction is still active in the India‐Burma plate boundary zone has been rather controversial. While the presence of an eastward dipping Indian lithospheric slab is undisputed, different opinions have been voiced regarding the continuance of subduction at present. Analysis of the Harvard CMT data in comparison with major subduction zones of the world demonstrates that the Burmese arc is a unique region where there is a subducted slab but the direction of plate motion is nearly perpendicular to the down‐dip direction. We propose a major right‐lateral shearing of the Indian plate along with its subducted slab past the Burmese plate in the NNE direction.

Facies architecture and geometry of landward‐stepping shoreface tongues: the Upper Cretaceous Cliff House Sandstone (Mancos Canyon, south‐west Colorado)

The Campanian Cliff House Formation represents a series of individually progradational shoreface tongues preserved in an overall landward‐stepping system. In the Mancos Canyon area, the formation consists of four, 50‐ to 55‐m‐thick and 10‐ to 20‐km‐wide sandstone tongues, which pinch out landwards into lower coastal plain and lagoonal deposits of the Upper Menefee Formation and seawards into offshore shales of the Lewis Shale Formation. Photogrammetric mapping of lithofacies along the steep and well‐exposed canyon walls was combined with sedimentary facies analysis and mapping of the detailed facies architecture. Two major facies associations have been identified, one comprising the mostly muddy and organic‐rich facies of lagoonal and lower coastal plain origin and one comprising the sandstone‐dominated facies of shoreface origin. Key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of depositional processes. The stratigraphic surfaces (master ravinement surface, shoreface/coastal plain contact, transgressive surface, maximum flooding surface and the sequence boundary) allow each major sandstone tongue to be divided into a simple sequence, consisting of a basal transgressive system tract (TST) overlain by a highstand system tract (HST). Within each sandstone tongue, a higher frequency cyclicity is evident. The high‐frequency cycles show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion. The complexities of the Cliff House sandstone tongues are believed to reflect changes in the rate of sea‐level rise combined with the responses of the depositional system to these changes. Synsedimentary compaction, causing a thickness increase in the sandstone tongues above intervals of previously uncompacted lagoonal/coastal plain sediments, also played a role. This study of the facies architecture, geometry and sequence stratigraphy of the Cliff House Formation highlights the fact that there may be some problems in applying conventional sequence stratigraphical methods to landward‐stepping systems in general. These difficulties stem from the fact that no single stratigraphic surface can easily be identified and followed from the non‐marine to the fully marine realm (i.e. from the landward to the basinward pinch‐out of the sandstone tongues). In addition, the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface.

The Variation of Stresses due to Aseismic Sliding and its Effect on Seismic Activity

Numerical simulation of recurring large interplate earthquakes in a subduction zone is conducted to explore the effects of aseismic sliding on the variation of stresses and the activity of small earthquakes. The frictional force obeying a rate-and state-dependent friction law is assumed to act on the plate interface in a 2-D model of uniform elastic half-space. The simulation results show that large earthquakes repeatedly occur at a constant time interval on a shallow part of the plate interface and that aseismic sliding migrates from the upper aseismic zone as well as from the lower aseismic zone into the central part of the seismogenic zone before the occurrence of a large interplate earthquake. This spatiotemporal variation of aseismic sliding significantly perturbs the stresses in the overriding plate and in the subducting oceanic plate, leading to the precursory seismic quiescence in the overriding plate and the activation of the intermediate-depth earthquakes of down-dip tension type. After the occurrence of a large interplate earthquake, the activity of the intermediate-depth earthquakes of down-dip compression type in the subducting slab is expected to increase and migrate downward. This is because the downward propagation of postseismic sliding causes the downward migration of compressional-stress increase in the down-dip direction of the plate interface. The simulation result further indicates that episodic events of aseismic sliding may occur when the spatial distributions of friction parameters are significantly nonuniform. The variation of stresses due to episodic sliding is expected to cause seismicity changes.

Mechanics of low‐stress forearcs: Nankai and Cascadia

The Nankai and Cascadia subduction zones have low‐stress forearcs, where the margin‐normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms. In the absence of active back‐arc spreading, the forearc margin‐normal stress is controlled mainly by the total shear force along the subduction fault (or the plate coupling force) and the gravitational force. The plate coupling force creates compression, and the gravitational force, in the presence of margin topography, creates lateral tension. We give a simple expression that relates the forearc stress to topography and average fault strength, with the latter represented by an effective coefficient of friction μ′. We also use a finite element model of two converging plates in frictional contact to model the forearc stresses. The analyses indicate that a low‐stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth. For Nankai and Cascadia, where the coupled fault areas are narrow and shallow because of the young and hot subducting plates, μ′ is about 0.05 or less and in any case less than 0.09. The low fault strength is probably not a unique property of low‐stress forearcs, since the same low μ′ values combined with wider and deeper coupled areas can be shown to create large margin‐normal compression. In addition, there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces. Great earthquakes occur at subduction zones having low‐stress forearcs such as Nankai and Cascadia.

Shelf-to-basin facies distributions and sequence stratigraphy of a steep-rimmed carbonate margin; Capitan depositional system, McKittrick Canyon, New Mexico and Texas

ABSTRACT Shelf-to-basin outcrop studies in steep-rimmed, shelf-margin settings are uncommon because continuous shelf-to-basin transects are rarely exposed in a single outcrop. Discontinuous or absent stratigraphic marker beds across the shelf margin further complicate outcrop studies in the shelf-margin setting. This paper discusses the results of a high-resolution investigation of the shelf-to-basin profile along the north wall of North McKittrick Canyon, New Mexico and Texas. In McKittrick Canyon, carbonate-dominated sedimentary rocks associated with the steep-rimmed, Upper Permian Capitan depositional system are exposed along a continuous 5-km outcrop face. Measured sections, lateral transects, scintillometer readings, and geochemical data were synthesized into a digital database and interpreted in conjunction with a digital photomosaic of the entire canyon wall. Results of this work include a shelf-to-basin facies map and sedimentologic interpretation of the north wall of North McKittrick Canyon, and indicate that the dominant bathymetric profile during Capitan deposition was a marginal mound. In this model, the Capitan reef facies was deposited at the shelf-slope break in water depths ranging from 15 to 75 m, but always downdip from the topographically higher shelf crest. This model is supported by the following observations and interpretations: (1) a facies progression from the shelf crest to the shelf margin interpreted to represent a shallow-to-deeper-water succession; (2) proportional expansion of beds in a downdip direction; (3) presence of oriented (transported) fusulinid grainstones downdip from in situ fusulinid wackestones and packstones updip; (4) siltstones that thin and pinch out towards the shelf margin; (5) a decrease in dolomite from the shelf crest to the shelf margin; and (6) the absence of true toplap stratal geometries. In reality, a static paleobathymetric model cannot characterize the depositional system, because the facies distributions, facies proportions, stratal geometries, and quantified depositional parameters vary systematically from the Seven Rivers through the Tansill. In order to understand the observed variations, emphasis was placed on quantifying key depositional parameters such as progradation, aggradation, offlap angle, outer-shelf dip, water depth, distance to the shelf margin and toe of slope, and facies-tract width. The systematic variations in these parameters, in conjunction with the facies distribution map and stratal geometries, helped to define the sequence-stratigraphic framework, and allowed for comparative evaluation of such things as sediment accumulation rates and sites, and stratigraphic evolution. The Capitan depositional system is represented by three composite sequences, each containing four high-frequency sequences. Two and one half of these composite sequences are exposed in McKittrick Canyon. The overall depositional system is interpreted to have evolved predictably from a deeper-water margin in the Seven Rivers composite sequence, to a shallow-water margin in the Tansill composite sequence. The subtidal outer-shelf and shelf-margin facies tracts were sites of major sediment production. Accumulation rates across the shelf margin indicate a relatively continuous growth history, with periods of nondeposition or erosion limited to the terminal phase of each composite sequence. As a result, the preserved sedimentary record of high-frequency and composite sequences in the outer-shelf to upper-slope position is equally proportioned between transgressive and highstand systems tracts. This symmetric outer-shelf to upper-slope record of carbonate accumulation is significantly different from the asymmetric, highstand-dominated middle-shelf accumulation record reported previously for this and many other carbonate shelves.

Timing and temperature of decollement on hydrocarbon source rock beds in cyclic lacustrine successions

The generation of hydrocarbons causes lubrication of source rock bedding planes which can enhance deformation during basin subsidence and subsequent tectonic events. Deformation includes decollement, i.e. gravitational sliding of rocks above the lubricated horizon. In cyclic sequences, such as occur in many lacustrine basins, decollement is generally restricted to the organic-rich beds of the cycle. Case studies in lacustrine successions in the Devonian Orcadian Basin, Scotland, and the Triassic–Jurassic Hartford–Deerfield Basin, USA, show that movement of material associated with the decollement surface occurred in the current down-dip direction. These extensional basins experienced rapid subsidence and a high heat flow such that hydrocarbon generation, and therefore decollement, occurred early in the basin histories. During extension, half-graben rotation ensured that the beds were dipping during hydrocarbon generation, facilitating decollement. Inversion of the Orcadian Basin reactivated decollement surfaces, involving thrust motion in some cases. Minerals precipitated in tension cavities associated with decollement yield fluid inclusion homogenization temperatures in the ranges 96–139°C and 85–96°C for the Orcadian and Hartford–Deerfield basins, respectively. These data are consistent with burial into the oil window at depths of between 1.5 and 2.0 km.

Extensional layer-parallel shear and normal faulting

An extensional fault system in Bare Mountain, Nevada, U.S.A., contains abundant evidence of layer-parallel shear deformation contemporaneous with faulting. Layer-parallel shear is manifest by deformation of pre-faulting fabrics and cleavage at low angles to bedding that indicate shear in the down-dip direction, perpendicular to fault-bedding intersections. Layer-parallel shear along discrete bedding planes locally offsets normal faults, and shear distributed within layers reorients block-bounding normal faults. In simple rigid block models of extension accommodated by normal faults above a low-angle detachment or décollement, extension causes faults to rotate to progressively shallower dips, while originally horizontal beds rotate to steeper dips. These rotations reorient faults out of originally optimum conditions for slip into orientations of a lower slip tendency, whereas bedding rotates to steeper dips with progressively higher slip tendency. The timing or amount of rotation before the initiation of layer-parallel shear depends on the frictional resistance to sliding or resistance to shearing within layering in fault blocks. Offset or deflection of block-bounding normal faults may cause faults to lock as extension increases. Alternatively, bedding and faults may become simultaneously active, progressively lowering dips of faults and bedding until neither is well oriented for slip, at which point new faults are required to accommodate additional extension. At Bare Mountain, early extension within the fault system was accomplished by fault slip and associated block rotation. Continued extension took place by slip along bedding within fault blocks.

Stress pattern in the double seismic zone beneath Cook Inlet, south-central Alaska

Relocation of south-central Alaskan earthquakes recorded by the regional seismograph network, using the joint hypocenter determination method, revealed a double seismic zone in the underthrusting Pacific plate beneath the Cook Inlet area. The lower zone is separated from the upper one by a decrease in seismic activity within a 5–10 km wide band, and extends from a depth of 50 km to about 90 km. Stress tensor inversion for earthquakes in the upper and lower zones shows different stress regimes. In the upper zone minimum principal stress is oriented in the down-dip direction and maximum principal stress is normal to the Wadati-Benioff zone (WBZ). In the lower zone both maximum and minimum principal stresses are horizontal and rotated by 40–60° from the down-dip direction of the WBZ. Below 80–90 km depth, stress tensor inversion results show down-dip orientation of the minimum principal stress and the maximum stress aligned in the direction of convergence of Pacific plate.

Asymmetrically Propped Hydraulic Fractures

SummaryMapping during and after mining of four hydraulic fractures placed in the German Creek coal seam of the Bowen Basin in Queensland, Australia, revealed that in all four cases the fractures were propped asymmetrically. Three treatments resulted in vertical fractures and one resulted in a T-shaped but mostly horizontal fracture. In all cases, the proppant extended to a greater distance in the down-dip than in the up-dip direction. The transport of proppant in the down-dip direction by gravity currents is consistent with the field and laboratory data and is able to explain the asymmetry.

Regional-Scale Flow of Formation Waters in the Williston Basin

The Williston basin is a structurally simple intracratonic sedimentary basin that straddles the United States-Canada border east of the Rocky Mountains and that contains an almost continuous stratigraphic record since the Middle Cambrian. Based on the wealth of data generated by the oil industry, the regional-scale characteristics of the flow of formation waters were analyzed for the Canadian side of the basin, and integrated with previous studies performed on the American side. Several aquifers and aquifer systems identified in the basin were separated by intervening aquitards and aquicludes. The Basal, Devonian, and Mannville (Dakota) aquifers are open systems, being exposed at the land surface in both recharge and discharge areas. Recharge takes place in the west-southwest at relatively high altitude in the Bighorn and Big Snowy mountains and at the Black Hills and Central Montana uplifts, whereas discharge takes place in the east and northeast at outcrop along the Canadian Precambrian shield in Manitoba and the Dakotas. The Mississippian and Pennsylvanian aquifer systems are semi-open, cropping out only in the west-southwest where they recharge, but discharging in the northeast into adjacent aquifers through confining aquitards. The Lower Cretaceous Vikin aquifer is a partially closed system, being confined by Cretaceous aquitards except for a small recharge area exposed at the Black Hills uplift and narrow discharge area in the Dakotas. The Upper aquifer is unconfined, with groundwater flow being driven by local topography, whereas the flow in all the other aquifers is regional in nature, being driven by basin-scale topography and characterized by normal hydraulic heads. The intervening aquitards seem to be strong, allowing little cross-formational flow. On regional and geological scales, the entire system seems to be at steady-state, although locally transient flow is present in places due to water use and hydrocarbon exploitation, and to some erosional rebound in the uppermost confining shales. Fresh meteoric water is present in the western and shallower parts of the basin, and brines are found in the Paleozoic aquifers in the central and eastern parts of the basin. This shows that the basin has not been completely flushed by meteoric water. Some cross-basinal inflow from the Alberta basin is apparent along the northwestern margin of the Williston basin, particularly for the Devonian and Viking aquifers. Hydrocarbons generated in the deeper, thermally mature part of the Williston basin have generally migrated within the same units updip north and northeastward, their buoyancy-driven secondary migration being enhanced by the northeastward flow of formation waters. On the western flank of the basin, the interplay between the northeastward structural downdip direction and the n rtheastward flow of formation waters creates conditions favorable for hydrodynamic oil entrapment.

Reconstructing Fluvial Macroform Architecture from Two-Dimensional Outcrops: Examples from the Castlegate Sandstone, Book Cliffs, Utah

ABSTRACT The Castlegate Sandstone is part of the Upper Cretaceous (Campanian-Maastrichtian) Mesaverde Group, a clastic wedge that prograded eastward from the Sevier Orogen of central Utah across the Western Interior Basin. The unit comprises the lowstand part of a regional fourth-order stratigraphic sequence. Earlier studies showed that the Castlegate Sandstone was deposited by braided rivers that deepened downstream from Price to east of Green River. Outcrop mapping employing photomosaics to document facies architecture, bounding surfaces, and paleocurrent information has been used to reconstruct the geometry of bars (macroforms) and bar complexes in the Castlegate rivers. The most common bar types are those that accreted laterally and downstream relative to local channel orientation. Gradations between these end-member accretion geometries may be present within the same depositional element. In one case, curved accretion surfaces are concave in map view, with the concavity oriented in a downdip direction, suggesting an origin as a counterpoint bar. Such bars are rare in braided systems. Accretion directions are determined relative to local paleoflow and are the basis for classification of bars as downstream- or laterally-accreted types. However, local paleoflow may be oriented at a high angle to regional or total-outcrop mean flow direction, indicating locally high channel sinuosity. The results of this study indicate that megascopic heterogeneity in the Castlegate Sandstone has a lateral dimension in the order of 100 m. It is suggested that this might be a suitable length dimension for scaling of grid blocks in simulation models of petroleum production of other, similar sandy braided fluvial systems.

Kinematic data for the Coast Range fault and implications for exhumation of the Franciscan subduction complex

Kinematic data from two segments of the Coast Range fault are used to test the hypothesis that the Franciscan subduction complex was exhumed by extensional faulting. The data reveal a consistent geometry for the principal directions of brittle strain. The maximum extension direction has an east-west orientation and lies subperpendicular to the present attitude of the Coast Range fault, which dips steeply to the east. The maximum shortening direction is subvertical and lies at a low angle to the present down-dip direction of the fault. Given that the Coast Range fault probably formed with a gentle dip, our results indicate subhorizontal crustal shortening in a northeast direction, which is at variance with interpretations that invoke regional-scale extension between the Franciscan subduction complex and the overlying Coast Range ophiolite and Great Valley Group. We favor a model involving out-of-sequence thrust faults. Such a model is consistent with fault-parallel contraction and the pronounced discontinuity in metamorphic grade across the Coast Range fault, as well as substantial thinning of the Coast Range ophiolite. This interpretation implies that erosion, not extensional faulting, was the major process that exhumed the Franciscan subduction wedge during the Late Cretaceous.

Depth distribution of coseismic slip along the Nankai Trough, Japan, from joint inversion of geodetic and tsunami data

Coseismic slip distribution on the fault plane, particularly in the downdip direction, associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data. Two large earthquakes, the 1944 Tonankai earthquake (Mw=8.1) and the 1946 Nankaido earthquake (Mw=8.3), occurred on the Nankai trough, southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The source areas of these events extended over both land and ocean. Coseismic crustal movements on land were measured by leveling, while those in ocean were recorded as tsunami waveforms on tide gauges. The coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts. This indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low. The result has implications for other subduction zones having a similar tectonic environment such as for the Pacific Northwest region of the United States. Although the possibility of a large earthquake there is still debated, should a large subduction earthquake occur in this region, coseismic slip on the shallow part could be large, and the potential for large tsunamis is high.

Downdip Water Incursion and Gas Trapping Styles Along the Southwest Flank of the Piceance Basin

Gas is commonly found downdip of water in low-porosity and low-permeability gas reservoirs. Several authors have discussed the physical mechanism and geological characteristics of this phenomenon. However, much of the previous work that documents this style of gas entrapment has been diagrammatic. For the few cases where these diagrams are supported by actual well data, they are generally of a basin-wide scale and do not show detail of gas and water distribution. This paper provides a detailed log analysis of selected wells along the southwest flank of the Piceance basin. The study fringes the southern and southwestern margin of the basin-centered Mesaverde Group gas package from the Plateau field through the Shire Gulch, Debeque, and Logan Wash fields. Vertical profiles of each analyzed well show the vertical distribution of gas and water through the section. Five distinct zones are identified by their interpreted interstitial fluids. The composition of these fluids with increasing depth are as follows: fresh water of meteoric origin, fresh water mixed with connate water, connate water, gas-water transition, and finally, gas. Cross sections show the gradations of these same zones laterally. Both the gas zone and freshwater zone occur in higher stratigraphic position and in lower structuralmore » position in the downdip direction. These relations suggest that fluid movement crosscuts stratigraphy. The dominant migration path of both gas and fresh water appears to be lateral. The gas-water transition zone creates a temporal, dynamic reservoir seal as a result of the low effective gas permeability within this zone. However, buoyancy forces are important to updip gas migration, particularly toward the margin of the basin where gas-water contacts are common. Examples are presented of significant gas accumulations in conventional structural-stratigraphic traps along the southwest margin of the basin.« less

Reactivated interstratal karst—example from the Late Silurian rocks of western Lake Erie (U.S.A.)

Interstratal karst developed in the Late Silurian rocks of western Lake Erie that, after a long interruption, was exhumed and reactivated. The dissolution front of the G evaporite of the Salina Group receded in the downdip direction during these two well-documented periods of subaerial exposure. The karst features that developed in the overlying Bass Islands Dolomite (Pridolian) consist of a large tabular body of collapse breccia and a number of smaller features including breccia pipes, partially filled pipes, blister caves and collapse dolines. The tabular breccia body and the breccia pipes, which originated penecontemporaneously during post-Silurian and pre-Middle Devonian subaerial exposure, occur along the updip edge of the present outcrop belt of the dolostone. They are monolithologic, fragment-supported rubble breccias, with the pipes exhibiting a greater fragment displacement, rotation and rounding, and a smaller fragment size. The matrix sediment of the tabular body is a quartz sand, an equivalent of the basal sandstone that filtered down from the erosion surface. The presence in the matrix sediment of nodular celestite, a later replacement of evaporites that formed when the sediment was still soft, indicates that a sabkha environment existed at the time the breccia was infilled. The partially filled pipes, which form cylindrical caves that are lined with late diagenetic celestite, are believed to be cogenetic with the collapse breccias. The blister caves and dolines occur downdip from the breccias, postdating Pleistocene glaciation and predating isostatic rebound. These caves are isolated, crescent- or oval-shaped openings with domed roofs, averaging about 60 m in width and 4 m in height. The hydration and resulting expansion of lenticular bodies of anhydrite along the receding solution front of the G unit is believed to be the cause of doming. The numerous crescentic caves, originating from the dissolution of this gypsum and the subsequent collapse of the domed roofs, are expressed at the surface as shallow dolines.

Mapping magma sources in the east Sunda-Banda arcs, Indonesia: Constraints from helium isotopes

We report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from Flores in the east Sunda arc through the inactive segment between the arcs to Banda Island at the extreme of the contiguous Banda arc. In the east Sunda arc, 3He 4He ratios ( R) vary between 4.5 R A ( R A = air 3He 4He ) for the leucitic Batu Tara volcano to a remarkable low of pure radiogenic helium (0.0075 R A ) for Werung at the southern tip of Lomblen Island. Lavas from the inactive zone, which represents the locus of collision of the Australian continent with the arc, have a narrower range in R R A —from 3.9 for Kisu in the straits of Pantar to 1.0 for Romang Island. Our one locality (Banda Island) in the Banda arc gives the highest R R A ratio (3.1) observed along this arc to date. The results are consistent with the involvement of crustal material in magma genesis throughout the east Sunda/Banda arcs, as far west as Iya in central Flores. We combine these helium isotope results with published and on-going strontium isotope studies, and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the Australian continent; rather, degassing of Australian continental crust appears to be the dominant process controlling the helium budget. The He-Sr isotope systematics also provide a framework to account for the areal pattern of 3He 4He in this part of the arcs: the radiogenic crustal component is diluted with mantle helium both in a down-dip direction and with increasing lateral distance from the collision zone. These factors result in an excellent first-order relationship between the 3He 4He ratio, degree of He Sr enrichment (relative to the postulated mantle endmember), and alkalinity of the erupted lavas. Such a relationship has a direct bearing on models of the tectonic evolution of the collision zone, and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the Banda arc.

Pliocene-Pleistocene Depocenter Distribution and Shelf Break Progradation, Northwest Offshore Louisiana

ABSTRACT Interpretation of 4000 line miles of seismic data, covering an area between 91° 60W to 94° 00W longitude and 28° 00N to 29° 00N latitude, reveals the sedimentation patterns underneath the continental shelf. Seven Plio-Pleistocene paleohorizons were mapped using the first occurrence paleotops: Trimosina A, Hyalinea balthica, Angulogerina B, Lenticulina 1, Globoquadrina altispira, Buliminella 1 and Textularia X. The sediment distribution in this area is controlled by different structural features. Between 93°W and 93°40W, three East-West trending fault zones are the most pronounced structural features. The master fault is located between 28°20N and 28°30N and extends about 60 miles in the downdip direction. It controls sediment distribution for the interval from Lent 1 to Trim A. The expansion index approaches 5. The fault toe merges with a broad shale uplift near the Trim A and Hyal B shelf edge. Largest depocenters from Ang B to Lent 1 are located on the downthrown side of this fault. The second fault zone trend is located between 27°50N and 28°10N. The maximum thickness in the Hyal B - Trim A interval occurs at the downthrown side of this fault. The third fault trend is located between 28°30N and 28°40N. This fault is much smaller than the first fault zone and it mainly controls depocenters from Lent 1 to Glob Alt. Salt diapirism was active during this interval. A series of salt spines occur in the region between 93°W and 92°W. Large scale growth faults did not develop here. Maximum sediment deposition occurs in interdiapiric basins as a result of the salt withdrawal. This region contrasts with the region between 93°W and 93°40W, which is dominated by salt anticlines and salt sheets, and where large growth faults are well developed. Around 92°W, another large growth fault with a long glide plane is observed with a depocenter on its downthrown side. The distribution of depocenters of different periods coincides with the trends of prograding paleoshelf edges and growth fault trends related to failure at shelf edges. A portion of Trim A paleoshelf edge is controlled by a landward fault zone. Hyal B and Ang B paleoshelf edges are related to the fault trend between 27°50N and 28°10N. Both this fault trend and the second fault trend control the location of Lent 1 paleoshelf edge. The Bulli 1 paleoshelf edge is along a belt of salt diapirs. Thickest Plio-Pleistocene depocenters are in middle shelf - lower slope environments. Depocenters from Trim A to Lent 1 are in an outer shelf environment; and Glob Alt - Tex X are in upper slope environments. End_of_Record - Last_Page 829-------

Waveform effects of a metastable olivine tongue in subducting slabs

We constructed velocity models of subducting slabs with a kinetically‐depressed olivine → β‐ and γ‐spinel transition, and examined the effect that such structures would have on teleseismic P waveforms using a full‐wave finite‐difference method. These two‐dimensional calculations yielded waveforms at a range of distances in the downdip direction. The slab models included a wedge‐shaped, low‐velocity metastable olivine tongue (MOTO) to a depth of 670 km, as well as a plausible thermal anomaly; one model further included a 10‐km‐thick fast layer on the surface of the slab. The principal effect of MOTO is to produce grazing reflections at wide angles off the phase boundary, generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take‐off angle. The amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross‐section. Careful analysis of waveforms from earthquakes with depths near 400 km, simple sources, and adequate station coverage in appropriate geometries will be required to resolve whether MOTO is present.