Ridge Collision Research Articles

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust

AbstractUnderstanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate‐induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench‐parallel variations in exhumation have a direct impact on exploration paradigms for porphyry‐related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates.

Kinematics of the western Caribbean: Collision of the Cocos Ridge and upper plate deformation

AbstractSubduction of the Cocos plate and collision of the Cocos Ridge have profound effects on the kinematics of the western Caribbean, including crustal shortening, segmentation of the overriding plate, and tectonic escape of the Central American fore arc (CAFA). Tectonic models of the Panama Region (PR) have ranged from a rigid block to a deforming plate boundary zone. Recent expansion of GPS networks in Panama, Costa Rica, and Colombia makes it possible to constrain the kinematics of the PR. We present an improved kinematic block model for the western Caribbean, using this improved GPS network to test a suite of tectonic models describing the kinematics of this region. The best fit model predicts an Euler vector for the counterclockwise rotation of the CAFA relative to the Caribbean plate at 89.10°W, 7.74°N, 1.193° Ma−1, which is expressed as northwest‐directed relative block rates of 11.3 ± 1.0–16.5 ± 1.1 mm a−1 from northern Costa Rica to Guatemala. This model also predicts high coupling along the Nicoya and Osa segments of the Middle American subduction zone. Our models demonstrate that the PR acts as a single tectonic block, the Panama block, with a predicted Euler vector of 107.65°W, 26.50°N, 0.133° Ma−1. This rotation manifests as northeast migration of the Panama block at rates of 6.9 ± 4.0–7.8 ± 4.8 mm a−1 from southern Costa Rica to eastern Panama. We interpret this motion as tectonic escape from Cocos Ridge collision, redirected by collision with the North Andes block, which migrates to the northwest at 12.2 ± 1.2 mm a−1.

Neotectonic faulting and forearc sliver motion along the Atirro-Rio Sucio fault system, Costa Rica, Central America

The Atirro–Rio Sucio fault system forms a major northwest-trending strike-slip fault zone in east-central Costa Rica. We examined the kinematics and temporal evolution of this fault system through geomorphic, structural, and seismologic analysis. This 150-km-long strike-slip fault zone traverses the northern flank of the paleovolcanic Cordillera de Talamanca and extends northwestward into the active Cordillera Volcanica Central. Historical seismicity includes frequent minor swarms and occasional moderate-magnitude (M 5.0–6.5) damaging earthquakes. Field geomorphic evidence, fault kinematic data, and earthquake focal mechanisms are consistent in showing dextral slip along the mapped traces of northwest-striking faults. Continuity with other transcurrent faults in northwest Costa Rica indicates that the Atirro–Rio Sucio fault system may form the southeastern end of a regional network of northwest-trending dextral faults that accommodate margin-parallel displacement of the Central American forearc sliver. The Atirro–Rio Sucio fault system originates within the Central Costa Rica Deformed Belt inboard of the indenting Cocos Ridge. We infer that ridge collision drives lateral escape of crustal fragments northwestward along an array of dextral Central Costa Rica Deformed Belt faults including the major structures of the Atirro–Rio Sucio fault system. This zone of arc-parallel extrusion thus represents the root of the Central American forearc sliver. Consistent with recent geodynamic models, we propose that northwestward sliver escape along the Atirro–Rio Sucio faults is driven by rigid indentation of the aseismic Cocos Ridge into southern Costa Rica.

Crustal structure of the Antarctic Peninsula sector of the Gondwana margin around Anvers Island from geophysical data

We investigated the crustal structure of the northern Antarctic Peninsula (AP) — a composite magmatic arc terrane at the Pacific margin of Gondwana. It was formed in Mesozoic–Cenozoic by collision/accretion of allochthonous terranes to the Gondwana margin during subduction and progressive ridge collision of former Phoenix Plate under the AP. The AP is distinguished by belts of high-amplitude, gravity and magnetic anomalies, inferred to indicate the presence of plutons of mafic composition comprising the huge AP batholith. For a better understanding of the lithosphere structure and tectonic processes of the AP continental margin we performed 2D gravity and magnetic modeling along line III-III, which crosses the margin south of Anvers Island. Our model incorporates data on seismic refraction (DSS-13) and reflection (line 8545-03) profiles and physical property data.Our modeling shows that the crust of the AP margin near Anvers Island is of continental type and comprises two crustal domains, which were welded together during mid-Cretaceous collision. On the east there is a thick (~40km), high-velocity/density continental crust of the Gondwana margin, which occupies the AP inner shelf and mainland, and Larsen Ice Shelf. The space between the Gondwanian block and oceanic crust of the Bellingshausen Sea is occupied by thinner (~26km) crust of lower densities of supposed accreted terrane, characterized by gravity anomalies caused probably by accretional tectonics as a result of Cretaceous subduction. Two belts of magnetic anomalies forming the PMA (Pacific Margin Anomaly) probably belong to different crustal domains: the western belt of the PMA relates to accretional block, while the eastern belt of the PMA corresponds to the Gondwana margin block. This may represent an evidence that the PMA relates to a tectono-magmatic event occurred after collision of Gondwana margin and accreted terrane (in mid-Cretaceous) and had been imprinted on already formed continental margin.

Geomorphic and exhumational response of the Central American Volcanic Arc to Cocos Ridge subduction

The timing of collision of the Cocos Ridge at the Middle America Trench remains one of the outstanding questions in the tectonic evolution of the Central American convergent margin. New analyses of the tectonic geomorphology of the Cordillera de Talamanca, the extinct volcanic arc inboard of the Cocos Ridge, coupled with low temperature thermochronometry data, provide insight into the cooling and erosional history of the arc from late Miocene to present. We identify a low‐relief surface at high elevation along the northeastern flanks of the range, which represents a relict erosional landscape cut across shallow plutonic rocks of the arc edifice. Longitudinal profiles of rivers on this surface are isolated from steep downstream sections by prominent knickzones that are interpreted to reflect a migrating wave of transient incision generated during differential rock uplift of the range. Reconstruction of pre‐incision profiles suggests that rock uplift during the growth of the Cordillera de Talamanca is no greater than ∼2 km. This inference is corroborated by results from our apatite (U‐Th)/He and apatite fission track analyses along an elevation transect on Mt. Chirripó, the highest mountain in the Cordillera de Talamanca. Low‐temperature cooling ages overlap significantly with published high‐temperature40Ar/39Ar ages; the combined results imply that rapid cooling in the late Miocene was related to secular cooling of a shallow pluton, rather than exhumation. Our results imply that rapid incision along downstream channel segments, differential rock uplift, and growth of the Talamanca as a bivergent orogenic wedge associated with the onset of Cocos Ridge subduction are relatively young characteristics of the range. A review of previously published radiometric ages and revised plate reconstructions for the late Miocene further suggest that the cessation of arc volcanism in both the Cordillera de Talamanca and the Cordillera Central of western Panama was coeval with the initiation of oblique subduction of the Nazca plate during the late Miocene. Thus, we conclude that the cessation of arc volcanism in the late Miocene (>5–8 Ma) and the onset of Cocos Ridge collision (<3 Ma) are separate events that reflect recent changes in the configuration of the plate boundary system.

Early abyssal- and late SSZ-type vestiges of the Rheic oceanic mantle in the Variscan basement of the Sakarya Zone, NE Turkey: Implications for the sense of subduction and opening of the Paleotethys

The boundary between the Gondwana and Laurussia, or a terrane separated from it, is traced by a suture recording the closure of the Rheic ocean in the Pulur, Beyçam and Kurtoğlu areas of the eastern Sakarya Zone of Turkey. Pulur lherzolite has a lower Cr# [100Cr/(Cr+Al)=5−37] of the spinel and slightly enriched flat medium-heavy rare earth element (M-HREE) whole-rock pattern typical for those of the fertile abyssal-type lherzolite. By contrast, the Beyçam harzburgite displays the mineralogical and geochemical features of a moderately depleted residue at a supra subduction zone (SSZ). Some of spinels from the Beyçam peridotites show TiO2 contents (0.15–0.48wt.%) identical to a later interaction with a melt of island-arc tholeiite (IAT) composition. Meta-basalt from the Beyçam area is the representative of such a melt and displays the whole-rock M-HREE pattern of a complementary product of the Beyçam harzburgite.Data presented in this study support the primarily northward directed closure of the Rheic ocean, but suggest that the Variscan basement in the eastern part of the Sakarya Zone consists of mantle peridotites formed in a south-vergent SSZ. This dual polarity of subduction in the Rheic ocean, at least in the second half of its closure, is also favored by the geological structure and lithological diversity of the Variscan rocks. These are: (1) spatial distribution of the peridotites, e.g., SSZ-type harzburgites to the north and abyssal-type lherzolites to the south, (2) lack of high-pressure metamorphic rocks, (3) decrease in the degree of Variscan metamorphism toward the north, and (4) emplacement of Variscan syn-collisional magmatism into the low grade metamorphic rocks to the north.Paleotethys was the other ocean of the region created in the Late Paleozoic. Northward subduction of the Rheic ocean from Early Devonian to Early Carboniferous appears to be the most plausible mechanism responsible for the opening of Paleotethys. In this scenario the Paleotethys is envisaged as a back-arc basin opened to the north in the wake of separated terranes from Laurussia. Onset of trench-Rheic mid-ocean ridge collision in the Early Devonian along the northern margin of Laurussia has been proposed to account for the initiation of SSZ-type subduction to the south. The short duration of this subduction appears insufficient to open a fully developed back-arc basin to the south because the elimination of Rheic mid-ocean ridge to the north accelerated the oceanic closure.

Fallamiento neotectónico de la península de Nicoya y su relación con el escape tectónico del antearco centroamericano

Se ha determinado un arreglo de fallas neotectonicas dentro de la peninsula de Nicoya. En la parte central y NW de la peninsula predominan bloques limitados por fallas de desplazamiento de rumbo dextral con rumbo N-S, dentro de los cuales se reconocen fallas sinestrales de rumbo ENE, oblicuas sinestrales-inversas con rumbo E-W e in- versas de rumbo NW. Las anteriores relaciones espaciales entre los diversos tipos de fallas se explican por traslacion al N de este sector de la peninsula y la transrotacion que ocurre dentro de los bloques. Las fallas cartografiadas en la zona S de la peninsula se han interpretado como sinestrales y dextrales con una relevancia aparentemente similar, ademas se han definido fallas inversas subordinadas. Movimientos translacionales al N y menos probable al E se infieren en la misma. El arreglo de fallas neotectonicas de la peninsula de Nicoya es asociado con la colision de la cordillera del Coco bajo el margen convergente del centro y S de Costa Rica, el cual origina el escape tectonico del bloque del Antearco Centroamericano hacia el NW, paralelo a la fosa Mesoamericana.

Arc‐parallel shear deformation and escape flow in the mantle wedge of the Central America subduction zone: Evidence from P wave anisotropy

The P wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of Central America covering offshore and onshore parts of southern Nicaragua and northern and central Costa Rica. Our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half‐year intervals between 2002 and 2006. The P wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic P wave velocity distribution. In the upper crust beneath Costa Rica, the seismic anisotropy is of the order of 2%–5% and locally strongly variable in orientation corresponding to the complicated tectonic structure. In contrast, the upper mantle beneath both study regions shows stronger anisotropy of the order of 5%–10% and a more coherent orientation of the axis of fast P wave velocity that varies systematically on regional scales: The upper mantle of the incoming oceanic plate is characterized by a trench‐normal orientation of the fast P wave velocity axis presumably corresponding to mineral alignment (lattice preferred orientation LPO) in transport direction. This pattern is corroded in the uppermost part of the subduction zone, possibly by the influence of bending‐related trench‐parallel faults and serpentinization that can overprint or annihilate the LPO anisotropy. The upper mantle of the overriding plate and mantle wedge shows a clear trench‐parallel orientation of the fast P wave velocity axis. The anisotropy is stronger in Nicaragua than in southern central Costa Rica (factor of 2 orders of magnitude). The development of a stronger anisotropy in the Nicaragua could be driven by a change in the stress regime from compressional in southern Costa Rica to transpressional in Nicaragua corresponding to a change from near‐orthogonal subduction in the SE to oblique subduction and slab retreat in the NW. From a comparison with S/SKS wave, GPS, and geochemical observations follows that the most likely explanation for the observed pattern of P wave anisotropy in the mantle wedge is LPO caused by a trench‐parallel shear deformation and/or NW oriented escape flow originating in the compressional zone near the Cocos Ridge collision area.

The steeply subducting edge of the Cocos Ridge: Evidence from receiver functions beneath the northern Talamanca Range, south‐central Costa Rica

The deep structure of the south‐central Costa Rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible. We present a receiver function study along a transect of broadband seismometers through the northern flank of the Cordillera de Talamanca (south Costa Rica). Below Moho depths, the receiver functions image a dipping positive conversion signal. This is interpreted as the subducting Cocos Plate slab, compatible with the conversions in the individual receiver functions. In finite difference modeling, a dipping signal such as the one imaged can only be reproduced by a steeply (80°) dipping structure present at least until a depth of about 70–100 km; below this depth, the length of the slab cannot be determined because of possible scattering effects. The proposed position of the slab agrees with previous results from local seismicity, local earthquake tomography, and active seismic studies, while extending the slab location to greater depths and steeper dip angle. Along the trench, no marked change is observed in the receiver functions, suggesting that the steeply dipping slab continues until the northern flank of the Cordillera de Talamanca, in the transition region between the incoming seamount segment and Cocos Ridge. Considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision, the southern Costa Rican subduction zone may at present be undergoing a reconfiguration of subduction style, where the transition to shallow underthrusting may be underway but still incomplete.

The tectonic regime along the Andes: Present‐day and Mesozoic regimes

AbstractThe analyses of the main parameters controlling the present Chile‐type and Marianas‐type tectonic settings developed along the eastern Pacific region show four different tectonic regimes: (1) a nearly neutral regime in the Oregon subduction zone; (2) major extensional regimes as the Nicaragua subduction zone developed in continental crust; (3) a Marianas setting in the Sandwich subduction zone with ocean floored back‐arc basin with a unique west‐dipping subduction zone and (4) the classic and dominant Chile‐type under compression. The magmatic, structural and sedimentary behaviours of these four settings are discussed to understand the past tectonic regimes in the Mesozoic Andes based on their present geological and tectonic characteristics. The evaluation of the different parameters that governed the past and present tectonic regimes indicates that absolute motion of the upper plate relative to the hotspot frame and the consequent trench roll‐back velocity are the first order parameters that control the deformation. Locally, the influences of the trench fill, linked to the dominant climate in the forearc, and the age of the subducted oceanic crust, have secondary roles. Ridge collisions of seismic and seismic oceanic ridges as well as fracture zone collisions have also a local outcome, and may produce an increase in coupling that reinforces compressional deformation. Local strain variations in the past and present Andes are not related with changes in the relative convergence rate, which is less important than the absolute motion relative to the Pacific hotspot frame, or changes in the thermal state of the upper plate. Changes in the slab dip, mainly those linked to steepening subduction zones, produce significant variations in the thermal state, that are important to generate extreme deformation in the foreland. Copyright © 2009 John Wiley & Sons, Ltd.

Paired metamorphic belts revisited

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high d T/d P type and the other low d T/d P type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high d T/d P metamorphic belt is juxtaposed against an outboard low d T/d P metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate d T/d P environment in the subduction zone and a high d T/d P environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high d T/d P and low d T/d P metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate d T/d P and the upper plate will register penecontemporaneous high d T/d P metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

GPS-derived coupling estimates for the Central America subduction zone and volcanic arc faults: El Salvador, Honduras and Nicaragua

We invert GPS velocities from 32 sites in El Salvador, Honduras and Nicaragua to estimate the rate of long-term forearc motion and distributions of interseismic coupling across the Middle America subduction zone offshore from these countries and faults in the Salvadoran and Nicaraguan volcanic arcs. A 3-D finite element model is used to approximate the geometries of the subduction interface and strike-slip faults in the volcanic arc and determine the elastic response to coupling across these faults. The GPS velocities are best fit by a model in which the forearc moves 14–16 mm yr−1 and has coupling of 85–100 per cent across faults in the volcanic arc, in agreement with the high level of historic and recent earthquake activity in the volcanic arc. Our velocity inversion indicates that coupling across the potentially seismogenic areas of the subduction interface is remarkably weak, averaging no more than 3 per cent of the plate convergence rate and with only two poorly resolved patches where coupling might be higher along the 550-km-long segment we modelled. Our geodetic evidence for weak subduction coupling disagrees with a seismically derived coupling estimate of 60 ± 10 per cent from a published analysis of earthquake damage back to 1690, but agrees with three other seismologic studies that infer weak subduction coupling from 20th century earthquakes. Most large historical earthquakes offshore from El Salvador and western Nicaragua may therefore have been intraslab normal faulting events similar to the Mw 7.3 1982 and Mw 7.7 2001 earthquakes offshore from El Salvador. Alternatively, the degree of coupling might vary with time. The evidence for weak coupling indirectly supports a recently published hypothesis that much of the Middle American forearc is escaping to the west or northwest away from the Cocos Ridge collision zone in Costa Rica. Such a hypothesis is particularly attractive for El Salvador, where there is little or no convergence obliquity to drive the observed trench-parallel forearc motion.

Geochemistry of ferromanganese sediment formed by a ridge collision in the late Cretaceous southwestern Japan: Geochemical analogy to a modern hydrothermal sediment at a mid-ocean ridge

Geochemistry of ferromanganese sediment formed by a ridge collision in the late Cretaceous southwestern Japan: Geochemical analogy to a modern hydrothermal sediment at a mid-ocean ridge

Fore‐arc motion and Cocos Ridge collision in Central America

We present the first regional surface velocity field for Central America, showing crustal response to interaction of the Cocos and Caribbean plates. Elastic half‐space models for interseismic strain accumulation on the dipping subduction plate boundary fit the GPS data well and show strain accumulation offshore and beneath the Nicoya and Osa peninsulas in Costa Rica but not in Nicaragua. Since large subduction zone earthquakes occur in Nicaragua, we suggest that interseismic locking in Nicaragua and some other parts of Central America occurs but is mainly shallow, <20 km depth, too far offshore to be detected by our on‐land GPS measurements. Our data also show significant trench‐parallel motion for most of the region, generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates. However, trench‐parallel motion is also observed in central Costa Rica, where plate convergence is normal to the trench, and in the Nicaraguan fore arc, where trench‐parallel motion is fast, up to 9 mm a−1, but mechanical coupling is low. A finite element model of collision (as opposed to subduction) involving the aseismic Cocos Ridge also fits the GPS surface velocity field, most significantly reproducing the pattern of trench‐parallel motion. We infer that buoyant, thickened CNS‐2‐Cocos Ridge crust resists normal subduction and instead acts as an indenter to the Caribbean plate, driving crustal shortening in southern Costa Rica and contributing to trench‐parallel fore‐arc motion in Costa Rica and perhaps Nicaragua as a type of tectonic escape.

Origin of the regional stress field along the Liquine-Ofqui Fault Zone (LOFZ), Southern Chilean Andes by means of FE simulation

The Liquine-Ofqui Fault Zone (LOFZ) of southern Chilean Andes is one of the largest active strike-slip fault zones. There is an ongoing debate regarding the origin of the stress field along the LOFZ due to its complex geometry. This paper represents a study of the origins of the LOFZ regional stress field. Stress fields are calculated by finite element (FE) analysis. The two possible stress origins, i.e., oblique plate convergence and ridge collision/indenter tectonics of Chile ridge against Peru-Chile trench, have been emphasized in the present study. Three types of boundary conditions for the three particular models have been applied to calculate stress fields. Models are assumed to be elastic and plane stress condition. Modeling results are presented in terms of four parameters, i. e., orientation of maximum horizontal stress (sigma (H max)), displacement vector, strain distribution, and maximum shear stress (tau (max)) contour line within the model. The results of the first model with oblique plate convergence show inconsistency between the geometric shape of the LOFZ and the distribution of the four parameters. Although more realistic results are obtained from the second model with normal ridge collision, there are few coincident in the LOFZ geometry and regional stress field. The third model with normal and oblique ridge collision is reasonable in understanding the origin of stress field and geometrical condition in the lithosphere of the LOFZ.

Enigmatic, highly active left-lateral shear zone in southwest Japan explained by aseismic ridge collision

Global positioning system (GPS) site velocities and earthquake focal mechanisms reveal an active left-lateral shear zone cutting across Kyushu in southwest Japan. Surprisingly, no active faults have been identified in association with this zone of rapid contemporary deformation. To explain the existence of this shear zone, we propose a model comprising subduction of an aseismic ridge (Kyushu-Palau Ridge) at the southwest end of the Nankai Trough. Because of rapid (~40 mm/yr) along-strike migration of the ridge, we suggest that the ridge subduction point (and resulting left-lateral shear zone) is never in one place long enough to enable the development of a through-going fault zone that can be identified at the ground surface, reconciling the mismatch between the GPS, seismological, and geological data in this region. Our conceptual model is supported by numerical modeling results. We also suggest that the along-strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the Kyushu forearc documented in paleomagnetic studies, as is found in many other western Pacific subduction margins.

Collision versus sliver transport in the hanging wall at the Middle America subduction zone: Constraints from background seismicity in central Costa Rica

Earthquake focal mechanism solutions for 135 small‐magnitude events are inverted for best fitting partial strain rate tensors that characterize contemporary strain in five areas that span the western margin of the Panama microplate in central Costa Rica. The results indicate the predominance of subhorizontal maximum stretching subparallel to the Middle America Trench (MAT) and provide constraints on the role of Cocos ridge collision at the MAT. The trajectory of maximum stretching changes ∼25°–45° over several tens of kilometers from the Central Costa Rica Deformed Belt (CCRDB) where it is nearly E–W to the area inboard of the Cocos ridge where it is NW–SE. This change suggests that background seismogenic deformation reflects the transition from the trailing edge of a fore‐arc sliver to an area of the upper plate affected by ridge collision. This diffuse deformation may be localized, in part, on conjugate strike‐slip faults of the CCRDB.

Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt, Inner Mongolia of China: Implications for continental growth

We present a detailed, new time scale for an orogenic cycle (oceanic accretion–subduction–collision) that provides significant insights into Paleozoic continental growth processes in the southeastern segment of the long-lived Central Asian Orogenic Belt (CAOB). The most prominent tectonic feature in Inner Mongolia is the association of paired orogens. A southern orogen forms a typical arc-trench complex, in which a supra-subduction zone ophiolite records successive phases during its life cycle: birth (ca. 497–477 Ma), when the ocean floor of the ophiolite was formed; (2) youth (ca. 473–470 Ma), characterized by mantle wedge magmatism; (3) shortly after maturity (ca. 461–450 Ma), high-Mg adakite and adakite were produced by slab melting and subsequent interaction of the melt with the mantle wedge; (4) death, caused by subduction of a ridge crest (ca. 451–434 Ma) and by ridge collision with the ophiolite (ca. 428–423 Ma). The evolution of the magmatic arc exhibits three major coherent phases: arc volcanism (ca. 488–444 Ma); adakite plutonism (ca. 448–438 Ma) and collision (ca. 419–415 Ma) of the arc with a passive continental margin. The northern orogen, a product of ridge-trench interaction, evolved progressively from coeval generation of near-trench plutons (ca. 498–461 Ma) and juvenile arc crust (ca. 484–469 Ma), to ridge subduction (ca. 440–434 Ma), microcontinent accretion (ca. 430–420 Ma), and finally to forearc formation. The paired orogens followed a consistent progression from ocean floor subduction/arc formation (ca. 500–438 Ma), ridge subduction (ca. 451–434 Ma) to microcontinent accretion/collision (ca. 430–415 Ma); ridge subduction records the turning point that transformed oceanic lithosphere into continental crust. The recognition of this orogenic cycle followed by Permian–early Triassic terminal collision of the CAOB provides compelling evidence for episodic continental growth.

Invited commentary

The Fontan circulation has undergone important technical changes throughout the years. The right atrium to pulmonary artery (PA) connection has made place for the total cavopulmonary connection (TCPC), first of the lateral tunnel type and thereafter of the extracardiac conduit type. As the Fontan circulation has no right-sided ventricular driving force, all situations that cause energy loss should be avoided. The flow path must be streamlined as much as possible. Collision of caval vein flows, sharp bends, ridges, and expansions result in energy losses. A T-junction of the lateral tunnel or the extracardiac conduit and the PA is inferior in terms of energy preservation than a junction that is more curved toward one PA and thus more streamlined. In vitro models and computational fluid dynamics (CFD) studies have supported and stimulated the ongoing surgical improvements of the Fontan circulation. Several groups (Bove, de Leval and coworkers, Yoganathan and coworkers) have been working in this field for many years. They have demonstrated that an extracardiac TCPC with the inferior vena cava flow more directed toward the left PA is theoretically the optimal situation with the least loss of kinetic energy. In the article by Soerensen and colleagues [1Soerensen D.D. Pekkan K. de Zélicourt D. et al.Introduction of a new optimized total cavopulmonary connection.Ann Thorac Surg. 2007; 83: 2182-2190Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar], further improvements of the cavopulmonary connections are presented. An innovative model of bifurcated caval veins connecting equally to both PAs is even more streamlined than the now best available technique, which is an extracardiac TCPC with offsetting of the vena cava to PA connections. The authors have demonstrated with the use of in vitro studies and CFD calculations that their prototype has lower energy losses while an even distribution of inferior caval flow to both PAs (hepatic factor) is preserved. The advantages of this model become even more evident when higher exercise conditions are mimicked by increasing the flows. Although the bifurcated vena cava model may be superior in terms of energy preservation, the road toward clinical implication is long and full of obstacles. Will it be easy to perform this procedure in a young child? Will the risk of thrombosis be increased? What will happen with growth? These are just some of the questions that immediately arise. However, as the theoretical advantages are evident, these obstacles should be considered as challenges for further study. Introduction of a New Optimized Total Cavopulmonary ConnectionThe Annals of Thoracic SurgeryVol. 83Issue 6PreviewSeveral variations of the total cavopulmonary connection (TCPC) have been investigated for favorable fluid mechanics and flow distribution. This study presents a hemodynamically optimized TCPC configuration code-named “OptiFlo.” Featuring bifurcated vena cava (superior venacava to inferior vena cava SVC/IVC), it was designed to lower the fluid mechanical power losses in the connection and to ensure proper hepatic blood perfusion to both lungs. Full-Text PDF

Continental response to active ridge subduction

Apatite fission track ages from a ∼2000 m elevation transect from the Patagonian fold and thrust belt (47.5°S) allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction. Accelerated cooling started at 17 Ma, predating the onset of ridge collision (14–10 Ma), and was followed by reheating between 10 and 6 Ma. Thermal modeling favors reheating on the order of 60°C at ∼28°C/Ma due to east‐migration of a slab window after the ridge‐trench collision. Final rapid cooling since 4 Ma of ∼18°C/Ma (geothermal gradient of 14°C/km) correlates with the presence of an orographic barrier and >1 km rock uplift in this region between 17.1 and 6.3 Ma. Increased precipitation and erosion since 4 Ma caused asymmetric exhumation, with 3–4 km on the leeside. Repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the Chile Triple Junction.