Marginal Wedge Research Articles

A combination of modified Kuhnt-Szymanowski and Celsus-Hotz techniques for correction of entropion and overlong lower eyelids in dogs (40 eyes).

To describe a surgical approach to correct entropion and overlong lower eyelids in dogs by combining the Celsus-Hotz with the modified Kuhnt-Szymanowski technique. Medical records of patients undergoing the described surgical procedure were reviewed. A semilunar-shaped piece of lower eyelid was excised and combined with an angled incision at the most lateral aspect of the wound. Adjacent to this incision a skin flap was mobilized to expose the subcutaneous tissue in the ventral aspects. A four-sided wedge resection was used to shorten the lid margin in variable positions. Following closure of the eyelid margin wedge resection, a wedge of equal width was removed from the lateral skin flap. Skin and subcutaneous tissues were closed in a routine fashion. All surgeries were performed by an ECVO diplomate or resident. The surgery was performed unilaterally in four and bilaterally in 18 dogs. Most common breeds were English Bulldog (n=7), Saint Bernard (3), Rottweiler (2) and Cane Corso Italiano (2). Median age was 22 months (range 5-100 months). Median follow-up was 30 days (range 9-987 days). A single surgical procedure was sufficient to correct the entropion in 97.5% (39/40) of eyes. The combination technique described is a suitable surgical procedure to simultaneously correct lower lid entropion and excessive eyelid length, with the added benefits of a stepped wound closure and a flexible lid margin wedge positioning.

Characterization of Cenomanian-Turonian oxic facies in Tunisia: sequence stratigraphy and correlation of recorded bioevents

On the southwestern Tethyan Margin of Tunisia, the well-known anoxic black shale facies of the Cenomanian-Turonian transition contrasts with oxic fossiliferous carbonates, first characterized here as lateral equivalents. In terms of sequence stratigraphy, the analysis of four wisely sampled sections in Central and Southern Tunisia led to interpret these deposits as a transgressive interval (TST). This interval spans the Whiteinella archaeocretacea Zone of foraminifera and is capped with a glauconite-rich highly bioturbated maximum flooding surface (MFS). In the reference section of Oued Bahloul and the Kalaat Senan area of North-Central Tunisia, these deposits overlay a Shelf Margin Wedge made of conglomeratic and bioclastic limestones. In South-Central and Southern Tunisia, the TST is characterized by the onset of oxic facies relaying laminated carbonates with local emergence surfaces. The analysis of both oxic and anoxic facies from the Cenomanian-Turonian transition allows identifying five successive bioevent markers, known elsewhere within the Tethyan domain: the extinction of the foraminifera genus Rotalipora (or Thomasinella), the Heterohelix shift, the Whiteinella proliferation, the filament event, and the appearance of Helvetotruncana helvetica. Among these bioevents, the Heterohelix shift coincides with the transgressive surface, while the filament event announces the maximal flooding surface. These Cenomanian-Turonian transition bioevents are of a particular relevance for regional and long-distance high-resolution correlations.

Metamorphic Conditions of Neotethyan Meliatic Accretionary Wedge Estimated by Thermodynamic Modelling and Geothermobarometry (Inner Western Carpathians)

Metamorphic evolution of an accretionary wedge can be constrained by a reconstructed P–T conditions of the oceanic and continental margin fragments. This paper deals with the metamorphic overprinting of the Inner Western Carpathians (IWC) Meliatic Triassic–Jurassic paleotectonic units after the closure of the Neotethyan Meliata Basin. Medium to high-pressure and lower temperature conditions were estimated by Perple_X pseudosection modelling, combined with garnet–phengite, calcite–dolomite and chlorite thermometers and chlorite–phengite and phengite barometers. The Late Jurassic subductional burial to a maximum 50 km depth was estimated from the Bôrka Unit continental margin fragments at 520 °C and 1.55 GPa. This is compatible with the metamorphic peak garnet–glaucophane–phengite assemblage of blueschist facies in metabasites. The Jaklovce Unit oceanic fragments were subducted to maximum 35–40 km at 390–420 °C and 1.1–1.3 GPa. Metabasalts and metadolerites contain winchite, riebeckite, actinolite, chlorite, albite, epidote and phengite. A glaucophane-bearing metabasalt recorded an intra-oceanic subduction in blueschist-facies conditions. Rare amphibolite-facies metabasalts of this unit indicate the base of an inferred oceanic crust sliver obducted onto the continental margin wedge. The Meliata Unit oceanic/continental margin flysch calciclastic and siliciclastic metasediments suggest the burial to approximately 15–20 km at 250–350 °C and 0.4–0.6 GPa. This is indicated by a newly formed albite, K-feldspar, illite–phengite and chlorite associated with quartz and/or calcite and dolomite in these rocks. Magnesio-hastingsite to magnesio-hornblende bearing metagabbro with newly formed metamorphic magnesio-riebeckite and actinolite is an inferred detached Meliatic block tectonically emplaced in a Permian salinar mélange in the Silica Nappe hanging wall. Reconstructed P–T paths indicate variable metamorphic conditions from the medium-pressure to high-pressure subduction of the Bôrka and Jaklovce units to the Meliata Unit shallow burial in an accretionary wedge during Late Jurassic to Early Cretaceous Meliaticum evolution. Mélange blocks of Meliaticum incorporate different juxtaposed Meliatic paleotectonic units exposed in nappe outliers overlying the IWC Gemeric and Veporic superunits.

The Esmeraldas Canyon: A Helpful Marker of the Pliocene‐Pleistocene Tectonic Deformation of the North Ecuador‐Southwest Colombia Convergent Margin

AbstractDeciphering the migration pattern of the Esmeraldas submarine Canyon (EC) and its history of cut‐and‐fill allows constraining the Pliocene‐Pleistocene tectonic evolution of the Ecuador‐Colombia convergent margin. Swath bathymetry, multichannel seismic reflection, and chronological data show that the EC is a 143‐km‐long, shelf‐incising, river‐connected canyon that started incising slope apron deposits in the Manglares fore‐arc basin ~5.3 Ma ago. The EC inception appears contemporaneous with the subduction of the Carnegie Ridge that is believed to have initiated 5–6 Myr ago and is considered an indirect cause of the EC formation. During its two‐stage left‐lateral migration, the EC upper‐half scoured deep incisions providing evidences for uplift episodes in the Manglares Basin that are correlated with mid‐Pliocene and Pleistocene regional tectonic events. Glacioeustatic variations contributed significantly to shape the EC and its upslope tributaries by increasing the rate of canyon incision during rapid sea level falls. Faults, folds, and diapirs have structurally controlled the location of the EC and of its tributary canyons, including the Ancon Canyon, which served as the main spillway of the Manglares Basin prior to be cut from its source ~170 kyr ago by the growth of a fault‐related anticline. The margin wedge that hosts the EC is highly unstable as it is cut by active faults and shaken by large subduction earthquakes. Several mass transport deposits have dammed the EC, one of them between >~65 and ~37 kyr causing an impoverishment of detrital material in the trench sedimentation and a possible interruption of the paleoseismological record.

Pneumonectomy with carinal resection in patients with non-small cell lung cancer: short and long-term results

Objective. With to compare short- and long-term treatment outcomes of different carinal resection techniques in patients with non-small cell lung cancer (NSCLC) and carina involvement. Materials and methods. We performed retrospective nonrandomized clinical trial with following groups of patients: patients underwent pneumonectomy with sleeve carinal resection (47 patients); patients underwent pneumonectomy with marginal and wedge carinal resection (18 patients). Statistical analysis was made using Statistic 6.0 program. Case-control analysis of the both groups was performed to evaluate short- and long-term treatment outcomes of different carinal resection techniques in patients with NSCLC and carina involvement. Results. Frequency of non-surgical and surgical complications was 27.7% and 18.5% correspondingly. Postoperative mortality was 8.5% in sleeve carinal resection group and 11.1% in marginal and wedge carinal resection group. Five-year survival rates were 32.6% in sleeve carinal resection group and 11.1% in marginal wedge carinal resection group.

High-resolution stratigraphic forward modeling of a Quaternary carbonate margin: Controls and dynamic of the progradation

The relationships between the margin sedimentary regime and the platform progradation are studied using forward stratigraphic numerical simulations on the Leeward (Western) prograding margin of the Great Bahama Bank (GBB) during the Quaternary (1.7–0 Ma). The corresponding sedimentary regime in the slope and the platform is well known from the ODP leg 166 and Bahamas Drilling Project wells located along the “Western line” seismic transect. However the sedimentary regime on the margin is not well established: the coral reefal margin observed before between 1.7 and 0.8 Ma in the well Clino is not active anymore at present-day, and the Holocene sedimentary regime is geometrically unable to account alone for the progradation. This study is based on three 2D high-resolution forward stratigraphic numerical modeling experiments with the software DionisosFlow that include the platform, margin and slope domains on the “Western Line Section” in the same sedimentary models. The results are compared to the six sedimentary cores and to the present day bathymetry in order to identify the more realistic scenario. The three experiments test different models of carbonate sediment production and transport. Experiment 1 shows that the highstand shedding of the fine-grained uncemented platform production is unable to reproduce the progradation and the present-day profile. Experiments 2 and 3 incorporate cemented facies in the margin, with the best results obtained with the cemented marginal wedges produced in Experiment 2 during platform emersion. From these results a high-resolution interpretation of the margin seismic section is proposed. This study shows that the platform progradation can be decoupled from the highstand shedding of the fine-grained platform production. It is dependent on the accumulation in front of the steep margin of coarse or cemented material. Before 0.8 Ma this corresponds to the coral reef identified in Clino. The transition after 0.45 Ma to 100-kyr large eustatic cycles with total platform flooding created two distinct marginal regimes: (1) during platform flooding aggrading accumulation of non-skeletal sands, and (2) during platform emersion prograding cemented marginal wedges produced in-situ.

A recent phase of accretion along the southern Costa Rican subduction zone

Abstract In 2011 we acquired a 3D seismic reflection volume across the Costa Rica margin NW of the Osa Peninsula to investigate the complex structure and the development of the seismogenic zone within the Costa Rican subduction zone in the vicinity of recent International Ocean Drilling Program (IODP) drilling. In contrast to previous interpretations, these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments, similar to materials recovered from IODP drilling, that have been thrust and thickened into thrust-bounded folded sequences. These structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model. We interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the Cocos–Nazca–Caribbean triple junction, and accreted during recent margin subduction that also accelerated with passage of the triple junction. The margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last ∼5 Ma that was probably preceded throughout the Neogene by periods of non-accretion or erosion.

Horizontal principal stress orientation in the Costa Rica Seismogenesis Project (CRISP) transect from borehole breakouts

AbstractThe Costa Rica Seismogenesis Project (CRISP) drilled the Pacific margin of the Middle America Trench just north of where the Cocos Ridge enters the subduction zone, resulting in basal erosion of the upper plate. Here we report the orientations of the maximum horizontal principal stress (SHmax) from borehole breakouts detected by logging‐while‐drilling and wireline downhole measurements. All SHmax directions were estimated in the sediment cover of the margin, above the deeper rocks of the deformed margin wedge. We observe three overall SHmax orientations: NNE‐SSW (25° azimuth) in the deepest interval drilled at the upper slope Site U1379; ENE‐WSW (82°) in the rest of Site U1379 and in Site U1413, also drilled in the upper slope; and NNW‐SSE (157°) in the mid‐slope Site U1378. Our preferred interpretation is that the deepest interval of Site U1379 records the stress conditions in the underlying margin wedge, as SHmax is parallel to the direction of the Cocos‐Caribbean plate convergence and of the compressional axes of plate boundary fault earthquakes. The variable SHmax directions observed elsewhere are likely due to the effect of a network of normal faults that subdivide the sediment cover into a number of independently deforming blocks. In addition, the observed SHmax directions may be influenced by the subducting Cocos Ridge, which acts as an indenter causing oblique deformation, and by the transition to seismogenic subduction along the plate boundary fault.

Three-dimensional velocity structure of the outer fore arc of the Colombia-Ecuador subduction zone and implications for the 1958 megathrust earthquake rupture zone

In 2005, an onshore, offshore 3-D refraction and wide-angle reflection seismic experiment was conducted along the convergent margin at the border between Colombia and Ecuador, over the rupture zone of the 1958, Mw 7.6 subduction earthquake. A well-defined Vp velocity model of the plate boundary and upper and lower plates was constructed, down to 25 km depth, using first arrival traveltimes inversion. The model reveals a several kilometers thick, low-velocity zone in the upper plate, located immediately above the interplate contact. This low-velocity zone might be related to alteration and fracturing of the mafic and ultramafic rocks, which composed the upper plate in this area by fluids released by the lower plate with possible contributions from sediment underplating. Near the toe of the margin, the model shows a low-velocity gradient in the outer wedge, which is interpreted as highly faulted and fractured rocks. This low-velocity/low-gradient region appears to limit the oceanward extension of the rupture zones of the 1958 and 1979 earthquakes, possibly because coseismic deformation and uplift of the outer margin wedge dissipates most of the seismic energy.

Seismic swarms, fluid flow and hydraulic conductivity in the forearc offshore North Costa Rica and Nicaragua

At the continental margin of north Costa Rica and Nicaragua, the strongly hydrated Cocos Plate subducts beneath the Caribbean Plate. From the downgoing Cocos plate fluids are released through extensional fractures in the overriding plate. At the seafloor, they form fluid seeps, mounds and other types of fluid expulsion. Using an offshore temporary seismic network, we investigated seismicity possibly related to these processes and observed several swarms of earthquakes located on the continental slope trenchward of the seismogenic zone of S Nicaragua. The seismicity occurred within the downgoing plate, near the plate interface and in the overriding plate. We interpret these swarm events as an expression of pore pressure propagation under critical stress conditions driven by fluid release from the downgoing plate. In order to estimate hydraulic diffusivity and permeability values, we applied a theory developed for injection test interpretation to the spatio-temporal development of the swarms. The resulting diffusivity and permeability values are in the ranges of 28–305 m²/s and 3.2 × 10−14 m²–35.1 × 10−14 m², respectively, applying to the continental and oceanic crust near the plate interface. These values are somewhat larger than observed in drill logs on the margin wedge off north Costa Rica, but of comparable magnitude to values estimated for the Antofagasta 1995 earthquake aftershock sequence.

A synthesis of heat flow determinations and thermal modeling along the Nankai Trough, Japan

We review marine heat flow data along the Nankai Trough and show that observations >30 km seaward of the deformation front are 20% below conductive predictions (129–94 mW m−2) but consistent with the global heat flow average for oceanic crust of the same age (16–28 Ma). Heat flow values ≤30 km seaward of the deformation front are generally 20% higher than conductive predictions. This heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench, >200 and >140 mW m−2, and steep landward declines to values of approximately 60 mW m−2 over distances of 65 and 50 km along the Muroto and Kumano transects, respectively. Along the Ashizuri transect, the lack of heat flow data precludes a definitive interpretation. We conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25°C higher near the toe of the margin wedge and 50–100°C lower near the downdip limit of the seismogenic zone than estimated by purely conductive models.

Upper-plate deformation in response to flat slab subduction inboard of the aseismic Cocos Ridge, Osa Peninsula, Costa Rica

Along the Middle America Trench in southern Costa Rica, flat slab subduction of the aseismic Cocos Ridge has uplifted and exposed the outer forearc, shortened the Terraba forearc basin sequence in the inner forearc (i.e., the Fila Costena thrust belt), and uplifted the magmatic arc. The Osa Peninsula, an outer forearc high ∼20 km inboard of the Middle America Trench and ∼3 km to ∼10 km above the plate interface at its trenchward edge, is deforming in response to variations in the bathymetry of the subducting aseismic Cocos Ridge where relief locally exceeds 1 km. Modern topography of the Osa Peninsula, elevation of the basement rocks (Early to Middle Tertiary Osa melange), elevations of Quaternary marine deposits (Marenco formation), and distribution of late Quaternary uplift rates directly mirror the bathymetry on the Cocos Ridge outboard of the Middle America Trench. Rates of late Quaternary uplift are calculated from eight new radiocarbon ages, five new optically stimulated luminescence ages, and 10 previously published radiocarbon ages. Rates of uplift range from 1.7 m/k.y. to 8.5 m/k.y. The Osa Peninsula is fragmented into small (∼5 km), independently deforming blocks bounded by trench-parallel and trench-perpendicular, subvertical, normal and reverse faults that extend down to the plate interface, allowing for greatly different deformation histories over short distances. Quaternary deformation on the Osa Peninsula is modeled as a thin, outer-margin wedge that deforms in response to subduction of short-wavelength, high-relief asperities on the downgoing plate. Permanent deformation is largely accomplished by simple shear on a complex array of subvertical faults that allow the upper plate to adjust to variations in the slope of incoming ridges and seamounts. Currently, permanent deformation of the outer forearc does not appear to involve significant subhorizontal shortening of the margin wedge, although the global positioning system velocity field records elastic shortening related to locking of the plate interface. Permanent uplift and uplift rates in the outer forearc in southern Costa Rica are driven, to the first order, by the bathymetry associated with the subducting Cocos Ridge and not by the basal shear stress on the plate interface.

Application of Facies Associations, Integrated Prediction Error Filter Analysis, and Chemostratigraphy to the Organic-Rich and Siliceous Cenomanian-Turonian Sequence, Bargou Area, Tunisia: Integrated Sequence Stratigraphic Analysis

Facies associations, integrated prediction error filter analysis (INPEFA) of spectral Gamma-ray data, Sr/Ca and Mn chemostratigraphy, and sequence stratigraphy of the organic-rich and siliceous Cenomanian-Turonian Bahloul formation have been studied in Bargou section, located in north-central Tunisia. The studied section is subdivided into seven facies evolving from platform to basin deposits. Based on basin geometry, facies distribution, spectral Gamma-ray INPEFA curves, Sr/Ca and Mn profiles patterns, and the sequence was also subdivided into shelf margin wedge (uppermost Fahden Formation-lowermost Bahloul), basal transgressive systems tract (remainder Bahloul Formation) and early high-stand systems tract (Kef Formation). The Sr/Ca ratios, Mn profile, sequence stratigraphic, and cyclostratigraphic interpretations reveal the existence of four transgressive parasequences deposited throughout 420 kyr within the organic rich and siliceous Bahloul facies.

Thermal effects of hydrothermal circulation and seamount subduction: Temperatures in the Nankai Trough Seismogenic Zone Experiment transect, Japan

[1] We examine the thermal effects of seamount subduction. Seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position. Prior to subduction, seamounts provide high-permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust. As the seamount is subducted, the high-permeability pathway is closed, shutting down the advective transfer of heat. If significant fluid flow occurs, it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system. Additionally, subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete. Shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge. We apply these features of seamount subduction to a thermal model for the Nankai Trough Seismogenic Zone Experiment transect on the margin of Japan. Models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench. They yield temperatures of ∼100°C−295°C for the rupture area of the 1944 Tonankai earthquake. Temperatures in the region of episodic tremor and slip are estimated at ∼290°C–325°C, ∼70°C cooler than a model with no fluid circulation.

Chapter 35 Regional seismic interpretation of crustal framework, Canadian Arctic passive margin, Beaufort Sea, with comments on petroleum potential

Abstract Three new regional 2D seismic surveys in the Beaufort Sea provide a broad, deeper view of basins and crustal structure of the Meso-Cenozoic Canadian Arctic passive margin. The seismic profiles cover 16 270 km from inner shelf to over 2000 m water depth and include areas previously unsurveyed due to sea-ice limits. The surveys cover offshore Mackenzie Delta and extend north to Latitude 74°N off Banks Island. Geological interpretation is tied to exploratory wells in the Beaufort–Mackenzie Basin that has yielded 48 oil and gas discoveries in Cretaceous–Oligocene sandstones. The entire Meso-Cenozoic passive margin wedge beyond the depocentre ties into this established petroleum system. Three segments of the margin are defined by orientation and structural styles, from typical passive margin geometry to wrench structures to a compressional foldbelt superposed on the western segment. Outboard seismic profiles image oceanic crust and the inferred extinct spreading centre of the Canada Basin. The continent–ocean boundary is interpreted using gravity data supported by deep seismic reflectors. The regional deep imaging of the tectonic architecture, structural sequence and tectonostratigraphy of each segment are major results, and advance the understanding of the geological framework of known and potential petroleum basins of the Arctic margin of Canada.

Tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion: The case of the Ligurian wedge of the northern Apennines, Italy

Subduction can be associated either with accretion or with removal of material from the overriding plate. These two processes can either coexist or alternate in time along the same margin. Their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out-of-sequence thrusts, normal and strike-slip faults, or large submarine landslides in the frontal part of the subduction zone. In this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern Apennines (Ligurian subduction complex). New structural data suggest that in the early Neogene (Aquitanian), the removal and underthrusting of the toe of the wedge, formed by both the accreted oceanic sediments and the overlying wedge-top basin fill (i.e., the Subligurian units), implied a process of frontal tectonic erosion. The presence, on top of the subduction complex, of a complete succession of mid-late Eocene to late Miocene slope-apron sediments—i.e., the Epiligurian succession—facilitates a reconstruction of the sedimentary response to this event. In the Aquitanian, large areas of the wedge were denuded from the lower-slope sedimentary cover through extensive gravitational mass movements. The subsequent deposition of a thick body of submarine debris flows is documented. The mass-wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the Ligurian subduction complex formed by the Subligurian units.

Margin architecture and seismic attenuation in the central Costa Rican forearc

Seismic attenuation across the central Costa Rican margin wedge is determined from amplitude analysis of wide-angle seismic data. Travel time and amplitude modeling are applied to ocean bottom hydrophones along two trench-parallel profiles, located 30 km (P21) and 35 km (P18) landward of the deformation front northeast of Quepos Plateau. Tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench. A 1–1.5 km thick décollement zone with seismic velocities of 3.5–4.5 km/s is sandwiched between the margin wedge and the subducting Cocos plate. For strike line P21, amplitude modeling indicates a Qp value of 50–150 for the upper margin wedge with seismic velocities ranging from 3.9 km/s to 4.9 km/s. Along strike line P18, Qp values of 50–150 are determined with velocities of 4.3–5.0 km/s in the upper margin wedge, increasing to 5.1–5.4 km/s in the lower margin wedge. Quantitative amplitude decay curves support the observed upper plate Qp values. In conjunction with earlier results from offshore Nicoya Peninsula, our study documents landward decreasing attenuation across the margin wedge, consistent with a change in lithology from the sediment-dominated frontal prism to the igneous composition of the forearc middle prism.

Three-dimensionalP-wave velocity structure on the shallow part of the Central Costa Rican Pacific margin from local earthquake tomography using off- and onshore networks

The Central Costa Rican Pacific margin is characterized by a high-seismicity rate, coincident with the subduction of rough-relief ocean floor and has generated earthquakes with magnitude up to seven in the past. We inverted selected P-wave traveltimes from earthquakes recorded by a combined on- and offshore seismological array deployed during 6 months in the area, simultaneously determining hypocentres and the 3-D tomographic velocity structure on the shallow part of the subduction zone (<70 km). The results reflect the complexity associated to subduction of ocean-floor morphology and the transition from normal to thickened subducting oceanic crust. The subducting slab is imaged as a high-velocity perturbation with a band of low velocities (LVB) on top encompassing the intraslab seismicity deeper than ∼30 km. The LVB is locally thickened by the presence of at least two subducted seamounts beneath the margin wedge. There is a general eastward widening of the LVB over a relatively short distance, closely coinciding with the onset of an inverted forearc basin onshore and the appearance of an aseismic low-velocity anomaly beneath the inner forearc. The latter coincides spatially with an area of the subaerial forearc where differential uplift of blocks has been described, suggesting tectonic underplating of eroded material against the base of the upper plate crust. Alternatively, the low velocities could be induced by an accumulation of upward migrating fluids. Other observed velocity perturbations are attributed to several processes taking place at different depths, such as slab hydration through outer rise faulting, tectonic erosion and slab dehydration.

Crustal structure of the central Costa Rica subduction zone: implications for basal erosion from seismic wide-angle data

On the Pacific margin off central Costa Rica, an anomalous lens-shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front. This feature was previously recognized in reflection seismic data when it was termed 'megalens'. Its origin and seismic velocity structure, however, could not unambiguously be derived from earlier studies. Therefore during RV SONNE cruise SO163, seismic wide-angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the 'megalens', intersecting on the middle slope. The P-wave velocities and structure of the subducting oceanic Cocos Plate and overriding Caribbean Plate were determined by modelling the wide-angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms. The margin wedge is defined by high seismic velocities (4.3-6.1 km s(-1)) identified within a wedge-shaped body covered by a slope sediment drape. It is divided into two layers with different velocity gradients. The lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the 'megalens'. Seismic velocities of the 'megalens' are lower (3.8-4.3 km s(-1)) relative to the margin wedge. We propose that the 'megalens' represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate. Upward-migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing, thus causing material transfer from the upper plate to the lower plate. Results from amplitude modelling support that the 'megalens' observed off central Costa Rica is bound by a low-velocity zone documenting fluid drainage from the plate boundary to the upper plate.

Thermal effects of fluid circulation in the basement aquifer of subducting ocean crust

Most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment, the plate boundary fault zone, and margin wedge likely transports only a minor amount of heat. We model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can greatly influence subduction zone temperatures. Fractured basaltic basement is several orders of magnitude more permeable than a typical plate boundary fault zone or marine sediments, allowing fluid circulation to redistribute and extract heat from a subduction zone. Fluid circulation within the basement aquifer suppresses temperatures along the subducting slab relative to cases with no fluid transport. Heat is extracted from under the margin wedge and transported into the ocean crust near the trench. This circulation has a large effect on subduction zone temperatures when topographic wavelength‐dominated convection occurs (&gt;5 times the critical Rayleigh number). With the exception of very cold subduction zones (i.e., those with very fast convergence or very small taper) topographic wavelength‐dominated convection occurs with aquifer permeability ≥10−11 m2 (in the range typically determined for upper ocean crust), suggesting such fluid circulation may be important in many subduction zones. Because fluid circulation more effectively transports heat in warmer systems, hydrothermal circulation moderates the effects of convergence rate as a control on subduction zone temperatures.