Ridge Collision Research Articles

Zircon sensitive high mass‐resolution ion microprobe U–Pb and fission‐track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications

Abstract The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass‐resolution ion microprobe U–Pb and fission‐track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid‐oceanic Ridge must have been emplaced during the 6 Ma event.

Spatial and temporal evolution of the subducting Pacific plate structure along the western Pacific margin

Tomographic images of the subducting Pacific plate beneath the Izu‐Bonin‐Mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc. Recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western Pacific island arcs, but reasons for the variation in geometry and morphology are still poorly understood. By comparing high‐resolution tomographic images of the western Pacific and updated paleogeographic reconstructions, we are able to link the spatiotemporal evolution of the subducting Pacific plate back to the mid‐Miocene. We have reconstructed tectonic motions along the Kurile‐Japan‐Izu‐Bonin‐Mariana arc system and the Ryukyu arc to provide an independent, additional interpretation of the subducting Pacific plate using the most current plate motion data. We then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images. The new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the Ogasawara Plateau with the trench in the mid‐Miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary. The combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes.

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.

Reply to comment by R. Hesse on “Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision”

Reply to comment by R. Hesse on “Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision”

Comment on “Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision” by P. D. Clift et al.

Comment on “Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision” by P. D. Clift et al.

Seismic ridge subduction and topography: Foreland deformation in the Patagonian Andes

The Patagonian Andes recorded several episodes of active ridge subduction in the last 80 million years. An analysis of the spatial and temporal relation between the present segment of collision of the Chile ridge and the digital topography of the foreland shows a correlation with the beginning of deformation and uplift in the inner sector of the Patagonia fold and thrust belt. Several magmatic episodes related to the collision such as near trench magmatism, adakite emplacement, OIB plateau basalts in the retroarc, and the arc volcanic gap, are associated with the uplift and deformation of the Patagonian Cordillera. Based on these correlations, a collision of the Aluk (or Phoenix)-Farallon ridge during Paleogene times south of 43°30′ is identified. Changes in magmatic patterns, molasses deposits, deformation and uplift of the Patagonian Cordillera constrain the region affected by the collision. Similar evidence implies a third period of collision in the Late Cretaceous, based on the occurrence of adakitic rocks, arc magmatic gap, and deformation along the southern Patagonian Andes. This earliest hypothesized collision would require the existence of a new oceanic microplate between the Pacific and the Aluk plates during Late Cretaceous times. Present rapid isostatic rebound related to the continental ice cap retreat in the Patagonian Andes is restricted to the region south of Chile triple junction (46°30′S). The uplift rate here is more than two times more rapid than normal isostatic rebounds recorded in the Northern Hemisphere, and requires an abnormally hot mantle with low viscosity. This abnormal mantle may be a consequence of several episodes of ridge collision and development of asthenospheric windows that are inconsistent with periods of cold flat-slab subduction proposed by some authors to explain the arc volcanic gaps.

An 8–10 Ma tectonic event on the Cocos Plate offshore Costa Rica: Result of Cocos Ridge collision?

Upper oceanic crust within the Cocos Plate offshore northwestern Costa Rica is dominated by sill intrusions of various ages, recognized regionally as smooth, high amplitude seismic reflections at the base of the sedimentary sequence and locally by dolerites and microgabbros recovered by drilling. Earlier interpretation of seafloor magnetic anomalies documented a set of spreading ridge jumps at 19.5 and 14.5 Ma, continuing to the present. In addition to these events, we present evidence for a widespread tectonic event in the period 8–10 Ma displayed in reflection seismic data recorded during the Ticoflux 1 and 2 experiments and dated by seismic correlation to ODP Site 1039. The 8–10 Ma events may have been triggered by collision of the Cocos Ridge at this time and facilitated by widespread sill intrusion associated with the Galapagos Hot Spot.

Basalts from the Efate Island Group, central section of the Vanuatu arc, SW Pacific: geochemistry and petrogenesis

Basalts of the Efate Island Group (<0.7 Ma) occur in the central part of the Vanuatu island arc between the ‘normal’ southern section of the arc and the central part of the arc affected by collision with the Eocene D'Entrecasteaux Ridge. These basalts form small cones constructed upon a volcaniclastic apron principally composed of trachydacitic breccias. Temporal and compositional features allow identification of three basalt groups. Subtle compositional differences exist between two of these suites, whereas the third shows pronounced enrichment in K and Rb, with few differences for other elements. These variations are taken to record slight compositional variations in the ambient mantle wedge composition, with the high-K, high-Rb suite deriving from localised zones relatively enriched in phlogopite. Compared with other Vanuatu arc basalts, the Efate basalts show appreciable enrichment in Sr and P 2O 5. The Pb isotopic characteristics of the Efate basalts are commensurate with the transitional position of this island within the Vanuatu arc, between the southern arc lavas and those erupted in the region affected by D'Entrecasteaux Ridge collision. However, 87Sr/ 86Sr values (0.70387–0.70424) are slightly higher and 143Nd/ 144Nd values slightly lower than predicted by the spatial change of these values along the arc. These features are taken to indicate derivation of the Efate basalts from a local, unusual mantle source. Two contrasting models are considered to account for the unusual features of the source mantle of these basalts. Model 1 suggests that the high Sr and P 2O 5 characteristics of the peridotite source of the Efate basalts were produced by subduction of an intraplate volcano similar to ORSTOM seamount, currently subducting immediately northwest of Efaté. Recent studies have demonstrated massive P 2O 5 and Sr enrichments in highly altered formerly glassy seamount lavas and hyaloclastites. Model 2 proposes that the ambient strongly depleted mantle wedge has been locally modified by carbonatite metasomatism. A well-defined tear or gap in the subducted slab beneath the Efate sector of the Vanuatu arc may have provided appropriate conditions for production of an ephemeral carbonatite partial melt from asthenosphere convecting into the hole in the slab. This melt invaded the mantle wedge, stamping enrichment of Sr and P 2O 5 on a mantle wedge peridotite that had already been metasomatised by fluids derived from sediments on the subducting plate.

Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision

Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift (700–850 m) likely due to preferential basal erosion under the center of Lima Basin. Long‐term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km myr−1. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km myr−1) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4–11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long‐term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust.

The Palaeo-Asian ocean in the Neoproterozoic and early Palaeozoic: new geochronologic data and palaeotectonic reconstructions

The Central Asian fold belt (Urals–Mongolian belt in the Russian literature), which incorporates rock units derived from a variety of geodynamic settings, evolved the Palaeo-Asian Ocean (PAO) in the Neoproterozoic and Palaeozoic. We present new geological and isotopic data for ophiolites and metamorphic complexes of the Eastern Sayan Range of southern Siberia (Dunzhugur complex), the Polar Urals (Enganepe Range), and the Dariv and Khantaishir Ranges of western Mongolia. We also review existing geological, geochronological and palaeomagnetic data for rock assemblages indicative of past tectonic settings within that ocean. Based on these data, we suggest geodynamic reconstructions for the principal time-slices that reflect turning points in the history of this ocean at ∼1000–650, ∼650–510, and 510–450 Ma. The age of the Circum-Siberian ophiolites is critical to constrain the timing of break-up of the large Mesoproterozoic Eurasian continent into Baltica and Siberia, and this break-up could not have occurred later than about 1100 Ma. The main events in the formation of volcanic arcs and marginal basins in the PAO were at 1000–1010, 830, 740–700, 670–640, 570, 540, and 500–490 Ma, whereas the main phases of accretion and ridge collision in the PAO were at around 800, 570, and 470 Ma. We also reconstruct the spatial positions of palaeobasins and continents at about 740 and 640 Ma.

Plio–Pleistocene basalts from the Meseta del Lago Buenos Aires, Argentina: evidence for asthenosphere–lithosphere interactions during slab window magmatism

Plio–Pleistocene (3.4–0.125 Ma) post-plateau magmatism in the Meseta del Lago Buenos Aires (MLBA; 46.7°S) in southern Patagonia is linked with the formation of asthenospheric slab windows due to ridge collision along the Andean margin ∼6 Ma ago. MLBA post-plateau lavas are highly alkaline (43–49% SiO 2; 5–8% Na 2O+K 2O), relatively primitive (6–10% MgO) mafic volcanics that have strong OIB-like geochemical signatures. Their relatively enriched Sr–Nd isotope ratios ( 87Sr/ 86Sr=0.7041–0.7049; 143Nd/ 144Nd=0.51264–0.51279), low 206Pb/ 204Pb (18.13–18.45), steep REE patterns (La/Yb=11–54), and low LILE/LREE and LILE/HFSE ratios (Ba/La<15, La/Ta<15, Ba/Ta<180; Sr/La=15–22; Th/La<0.13; Ce/Pb>15) are distinctive from most other Neogene Patagonian slab window lavas. These data are interpreted to indicate contamination of OIB-like asthenosphere-derived slab window magmas with an EM1-type component derived from the Patagonian continental lithospheric mantle (CLM). The EM1-type signature in Patagonian slab window lavas are geographically associated with the Deseado Massif and indicate important regional differences in lithospheric mantle chemistry beneath southern Patagonia. We propose that hot, upwelling subslab asthenosphere in slab window tectonic settings can cause significant thermo-mechanical erosion and thinning of the continental lithospheric mantle and, thus, may be an important process in slab window magma petrogenesis.

Late Cenozoic transpressional ductile deformation north of the Nazca–South America–Antarctica triple junction

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes. Two east–west structural transects were mapped at Puyuhuapi and Aysén, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysén (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it. New 40Ar– 39Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysén transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation. The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.

Balance of tectonically accreted and subducted sediment at the Chile Triple Junction

An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. Mass balance estimations to characterize temporal and spatial variations in the frontal accretion, or underplating and subduction of sediments since the Late Miocene, were made using seismic and drill-hole data. At 200 km north of the triple junction, almost 80% of the sediment on the downgoing Nazca plate are subducted. Sediment subduction rate decreases towards the triple junction because of a low in sedimentation rates as the flank of the spreading ridge approaches the trench. At the triple junction, the forearc is almost completely destroyed by spreading ridge collision and subduction erosion. Less than 12% of the available sedimentary input is accreted. South of the triple junction, where the spreading ridge passed 6 Ma ago, a large fraction (>60%) of the sediment on the incoming Antarctic plate has been scraped off and was frontally accreted to the Chile forearc. Spreading ridge subduction leaves a distinctive geological fingerprint, and has a large impact on the mass balance of the subduction zone. However, the high rates of change in the process may make this fingerprint hard to detect in fossil convergent orogens. In the ridge collision zone the sediment supplied to the trench, and the amount of sediment subducted, show strong and distinctive variations on a 1- to 5-million-year time scale. On a 10-million-year time scale, sediment subduction to the Earth's mantle is reduced by spreading ridge collision, caused by the need of the overriding forearc to regain a low angle of taper by frontal accretion.

Ridge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm

Mantle wedge–derived arc volcanism ceased in southern Costa Rica after ca. 8 Ma because of subduction of the aseismic Cocos Ridge beneath the Central American arc and the subsequent opening of a slab window. Geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5.8 and 2 Ma identify a source derived from the Galapagos plume. The presence of this source is explained by an influx of Pacific upper mantle into the Caribbean mantle wedge through a slab window, where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted Cocos Ridge. By using geochemical and isotopic signatures, we trace this upper mantle flow beneath Central America from southern Costa Rica northward at a rate of 40 mm/yr.

Timing and origin of deformation along the Patagonian fold and thrust belt

The Andean orogeny in the Patagonian Cordillera of southern South America reflects the consequences of the Mesozoic and Cenozoic subduction of an oceanic plate beneath the South American continental margin. The geological evolution of the region has been influenced by the Eocene collision and subduction of the Farallon–Aluk Ridge and the Miocene–Recent subduction of the Chile Ridge. Another aspect of plate interaction during this period was two intervals of rapid plate convergence, one at 50–42 Ma, and the other at 25–10 Ma, between the South American and the oceanic plates. It has been proposed that the collision of the Chile Ridge with the trench was responsible for the development, at least in part, of the Patagonian fold and thrust belt. This belt extends for more than 1000 km along the eastern foothills of the southern Andes between 46° and 54° S along the southwestern rim of the Austral Basin. The interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during Middle and Late Miocene times. The main Tertiary contractional events in the Patagonian fold and thrust belt took place during latest Cretaceous–Palaeocene–Eocene and during Miocene times. Although the timing of deformation is still poorly constrained, the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments. Most if not all of the contractional tectonism pre-dated the latest episodes of ridge collision. Collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years. Contrary to the suggestion of a relationship between ridge subduction and compression, the main result of this collision has been fast uplift and extensional tectonism. The initiation of the Patagonian fold and thrust belt in latest Cretaceous or early Tertiary times coincided with a fundamental change in the tectonic evolution of the Austral Basin. Throughout the Cretaceous most of this basin subsided as a broad backarc continental shelf. Only in latest Cretaceous times, and coinciding with the initiation of the fold and thrust belt, the basin underwent a transition to a retro-arc foreland basin. This change to an asymmetrically subsiding foreland basin, with an associated foreland fold and thrust belt, was related to uplift of the Andean orogenic belt in the west.

Contrasting nature of deformation along an intra-arc shear zone, the Liquiñe–Ofqui fault zone, southern Chilean Andes

The Liquiñe–Ofquifault zone (LOFZ) cuts the Patagonian batholith and the modern volcanic arc of southern Chile for ca 1000 km. The rock fabric along three transects of the LOFZ combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike. In the Liquiñe transect (39°S), a 1 km wide, northeast-striking subvertical mylonitic zone shows north-plunging stretching lineations. This mylonitic zone has been juxtaposed by high-angle reverse faulting against a nearly undeformed Miocene granitoid. Metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement. Deformation pre-dates a 100±2 Ma undeformed porphyritic dike (hornblende 40Ar– 39Ar mean age). In the Reloncavı́ transect (41–42°S), deformation in Cretaceous and Miocene plutons is predominantly brittle. Kinematic analysis of two fault populations yields compressional and dextral strike-slip stress regimes, interpreted as late Miocene in age. In the Hornopirén transect (42–43°S), a 4 km wide mylonitic zone, developed in plutons and wallrocks, shows subhorizontal stretching lineations and dextral displacement. A single fault population overprinting the mylonites supports a dextral strike-slip stress regime. Available U–Pb, K–Ar and 40Ar– 39Ar dates on deformed Cenozoic plutons and wallrock range from 9 to 13 Ma on hornblende and from 6 to 3 Ma on biotite. Microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies, similar to the biotite Ar closure temperature. Subparallel magmatic and solid-state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons. Dextral-oblique subduction of the Nazca plate beneath western South America has driven long-term intra-arc deformation at the southern Andes plate boundary zone; ridge collision, in turn, has favored dextral displacement at the leading edge of the continent since the Pliocene

Proto-Avalonia: A 1.2–1.0 Ga tectonothermal event and constraints for the evolution of Rodinia

The Neoproterozoic evolution of Avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of Rodinia. Similar Sm-Nd isotopic signatures for different periods of arc activity suggest that Avalonian basement, or proto-Avalonia, was generated in a series of primitive oceanic island arcs between 1.2 and 1.0 Ga. Because this interval coincides with the amalgamation of Rodinia, proto-Avalonia is inferred to have been located in a Panthalassa-like peri-Rodinian ocean. An early (760–660 Ma) phase of Avalonian arc activity is attributed to renewed subduction in the peri-Rodinian ocean following the breakup of Rodinia, which caused the accretion of Avalonian terranes to the Gondwanan margin by ca. 650 Ma. Further subduction along the margin occurred outboard of these terranes and resulted in the onset of main-phase Avalonian volcanism at 630 Ma. The diachronous cessation of arc magmatism is attributed to ridge-trench collision and the generation of a continental transform. The geodynamic link between Avalonia and Rodinia is analogous to that between the Mesozoic dispersal of Pangea and the tectonothermal evolution of western North America. This event also resulted in the accretion of outboard terranes and in arc-related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the San Andreas transform. The model implies that the Neoproterozoic evolution of Avalonia and other peri-Gondwanan terranes provide important constraints on the tectonic history of a large portion of the Rodinian continental margin.

Strain partitioning in the zone of transition from Nankai subduction to Izu–Bonin collision (Central Japan): implications for an extensional tear within the subducting slab

The Izu–Bonin ridge collision with Central Japan is a type example of collision of two island arcs in a subduction context. Based on the analysis of active faults, paleomagnetic data and recent seismicity in the northern tip of the Izu-Bonin arc and the adjacent oceanic plate, we propose a tectonic schema of the Izu collision system and its transition to the Nankai subduction to the west. The dense GPS network deployed in Central Japan allows us to constrain the kinematics of the different blocks identified. This new kinematic model demonstrates the essential role of strain partitioning there. As the northward component of the Izu–Bonin arc motion is locked in the collision zone, 30% of the convergence between the Philippine Sea plate and Central Japan is accommodated within the Philippine Sea slab, along the Zenisu-East Izu deformation system. We show that this pattern of deformation is related to an extensional tear of the Philippine Sea slab propagating southward along the volcanic arc. We discuss the implications of this kinematic model on the seismogenic potential of the area and the dynamics of the Central Japan convergence zone.

Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M w≥7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 – from 6°N to 2.5°N with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 – from 2.5°N to 1°S showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 – from 1°S to 2°S with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 – south of 2°S with flat subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local `flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench–ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision.

Tectonic controls on sedimentation and diagenesis in the Tonga Trench and forearc, southwest Pacific

Sedimentation in the Tonga forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front by mass flows and turbidity currents onto the adjacent Tonga Platform, the shallowest, flattest part of the forearc region. The greatest sediment thicknesses accumulate in debris aprons close to the volcanic front. Collision of seamounts, notably the Capricorn Seamount and the Louisville Ridge, with the forearc radically shortens and steepens the adjacent modern trench slope, allowing sediment to be redeposited deep into the trench. Rotation, usually arcward, of existing basins on the midslope during collision generates angular unconformities, while synchronous uplift of the outer forearc high results in canyon development and downcutting along the eastern edge of the Tonga Platform. Collision also reactivated major across-strike fault zones on the forearc; the zones are subsequently exploited by canyons depositing sediment into the trench. Collapse and renewed extension of the forearc in the wake of collision result in the development of small perched basins, measuring approximately 5 km by 15 km in the midslope area. This morphology is especially developed at 18°30′S to 20°S, implying a 2–3 m.y. interval for their formation following Louisville Ridge collision. Trenchward of these depocenters sedimentation is slow, resulting in manganese crust formation and localized mass wasting along fault scarps. Over longer periods of time (>5 m.y.) tectonic erosion reestablishes a wide, gently sloping forearc into which canyons incise the shallow Tonga Platform by headwall erosion and collapse.