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Offshore observations at modern ocean-margin subduction zones (OMSZs) reveal that bodies of accreted material are commonly volumetrically small or missing, that crustal thinning and subsidence (3-5 km) has occurred, and that most trench axes lie close (5-30 km) to the seaward tapering edge of coastal basement rock. Onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front. These observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes, respectively, of sediment subduction and subduction erosion. Subduction erosion truncates the margin (migrates the trench inboard) at ~2.5 km/Ma. Onshore observations at ancient crust-suturing subduction zones (CSSZs) imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates. During a protracted period of suturing (20-50 million years), a 100-200 km wide (or wider) band of the seaward edge of each plate can be removed subductively. Truncation of the upper plate is effected by subduction erosion, and that of the lower plate by the necking and break-off of its subducted edge. The average linear rate of crustal loss for each plate is estimated at ~1.5 km/Ma, or ~3 km/Ma combined. Because significant crustal loss occurs before and during tectonic fusing of colliding crustal blocks, structures and rock bodies that might be expected to record a former OMSZ and the formation of a CSSZ may be absent, unimpressively small, or preserved only as exhumed masses of once deeply subducted material. Des observations au large a des zones modernes de subduction de marge oceanique revelent que des amas de materiel accrete; ont frequemment une petite taille volumetrique ou sont manquants, qu'il s'est produit un amincissement de la croute et de la subsidence (3-5 km) et que la plupart des axes des tranchees reposent a proximite (5-30 km) de la bordure, a eepaisseur deecroissante, du socle rocheux de la cote. La cartographie du continent signale souvent des terranes manquants ou seulement etroits, d'anciennes roches d'avant-arc et la migration vers le continent du front magmatique de l'arc. Ces observations constituent des preuves que la subduction est accompagneee du retrait de materiel sedimentaire et de la croute de l'avant-arc submerge par des processus tectoniques apparentes, respectivement de subduction de sediments et d'erosion de la subduction. L'eerosion de la subduction tronque la bordure (fait migrer la trancheee vers le continent) a un taux d'environ 2,5 km/Ma. Les observations sur le continent a des anciennes zones de subduction de suture de la croute impliquent que les sutures de collisions sont accompagneees de subduction de sediments et d'une troncation des plaques inferieures et superieures. Au cours d'une periode prolongee de suture (20-50 Ma), une bande de 100 a 200 km (ou plus large) de la bordure oceanique de chaque plaque peut etre retiree par subduction. La troncation de la plaque superieure est causee par une erosion de subduction, c'est-a-dire de la plaque inferieure par etranglement et par rupture de la bordure subductee. Le taux moyen de perte lineeaire de la croute pour chaque plaque est estimee a ~1,5 km/Ma ou ~3 km/Ma pour les deux ensemble. Puisqu'une importante perte de la croute a lieu avant et durant la fusion tectonique de blocs de croute en collision, les structures et les amas rocheux qui pourraient enregistrer une ancienne zone de subduction de marge oceanique et la formation de zones de subduction de suture de la croute peuvent etre manquants, tres petits ou preserves uniquement en tant que masses exhumees de materiel jadis subduit a de grandes profondeurs. [Traduit par la Redaction], Introduction Continents are constructed of a collection of thick sequences of altered and unaltered mantle-derived igneous bodies, and mass wasting and sedimentary material derived from them. Volumetric growth of the [...]

Explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans. The link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence. A compilation of 2814 K-Ar and [sup.40]Ar/[sup.39]Ar age determinations from four circum-Pacific arcs indicates that periods of high volcanic output (i.e., flare-ups) have been episodic and, in some cases, synchronous. Peak periods and subsequent lulls in arc magmatism over the past 50 m.y. have occurred coeval with major fluctuations in global climate, including the Eocene-Oligocene transition, one of the most prominent global climate reorganizations in the Cenozoic. Hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum-Pacific volcanism beginning in the late Eocene, which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the Pacific Ocean. We provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted Antarctic ice sheet growth ~34 m.y. ago.

The North Pacific Rim is a tectonically active plate boundary zone, parts of which may be characterized as a laterally moving orogenic stream. Crustal blocks are transported along large-magnitude strike-slip faults in western Canada and central Alaska toward the Aleutian-Bering Sea subduction zones. Throughout much of the Cenozoic, at and west of its Alaskan nexus, the North Pacific Rim orogenic Stream (NPRS) has undergone tectonic escape. During transport, relatively rigid blocks acquired paleomagnetic rotations and fault-juxtaposed boundaries while flowing differentially through the system, from their original point of accretion and entrainment toward the free face defined by the Aleutian-Bering Sea subduction zones. Built upon classical terrane tectonics, the NPRS model provides a new framework with which to view the mobilistic nature of the western North American plate boundary zone. Keywords: Alaska, Anatolia, orocline, extrusion, escape, Denali Tintina, fault, Bering Sea. doi: 10.1130/G23799A.1

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight [sup.40]Ar/[sup.39Ar] ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 [+ or -] 0.6 Ma, which is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Three main pulses of arc-wide magmatism occurred at 38-29, 16-11, and 6-0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude. Keywords: Aleutian Arc, magma production, [sup.40]Ar/[sup.39Ar dating, arc growth.

[1] Underlying the midslope Valparaiso Terrace, the Valparaiso Basin constitutes a unique and prominent deepwater forearc basin on the central Chile margin. The basin contains a 3-3.5 km thick sediment fill of late Cenozoic age. The seismic-stratigraphic architecture of the Valparaiso Basin fill shows that margin-wide subsidence above the underthrusting Juan Fernandez Ridge, an aseismic volcanic swell crested with seamounts on the Nazca plate, characterized the basin evolution. Contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf, the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward-vergent outer forearc highs. Subsidence of the margin above the underthrusting Juan Fernandez Ridge is ascribed to processes of subduction erosion, enhanced by factors related to subduction of the ridge, that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing Nazca plate. Accelerated midslope subsidence, 55-75 km landward of the trench, coincides with the seaward limit of interplate seismicity, suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior. Since subduction of the Juan Fernandez Ridge began near 33[degrees]S ~10 Ma ago, the Chile Trench north of 33[degrees]S is inferred to have retreated ~3 km/Myr, and the midslope Valparaiso Basin subsided ~0.3-0.5 km/Myr, approximating analogous Neogene rates for the Peruvian margin. INDEX TERMS: 8105 Tectonophysics: Continental margins and sedimentary basins; 9604 Information Related to Geologic Time: Cenozoic; 9360 Information Related to Geographic Region: South America; 3025 Marine Geology and Geophysics: Marine seismics (0935); 8015 Structural Geology: Local crustal structure; KEYWORDS: forearc basin, Chile margin, ridge subduction, subduction erosion, forearc sedimentation. Citation: Laursen, J., D. W. Scholl, and R. von Huene, Neotectonic deformation of the central Chile margin: Deepwater forearc basin formation in response to hot spot ridge and seamount subduction, Tectonics, 21(5), 1038, doi:10.1029/ 2001TC901023, 2002. Received 1 June 2001; accepted 19 September 2001; published 7 September 2002.

The regional stratigraphy of eastern Kamchatka includes an exotic, Early-Late Cretaceous ophiolite and Late Cretaceous island-arc volcanic sequence. Integrating the existing geologic and geophysical data, we examine the origin, transport, emplacement, and postemplacement deformation of the island-arc terrane, which is named the Olyutorsky island arc. Results from several paleomagnetic studies consistently indicate that the island-arc terrane originated >1000 km to the south of where it is presently exposed. Although the formative paleolatitudes of the island-arc rocks approximately correspond to th location of the Izanagi-Farallon subduction zone, the age of the volcanic rocks postdates the cessation of Izanagi-Farallon convergence, thus indicating that a unnamed plate or back-arc basin existed in the northwest Pacific during Late Cretaceous time. We examine two possible models for northward transport of the island-arc terrane to Kamchatka: (1) intra-oceanic transport with the Pacific o Kula plates and (2) coastwise translation of the island-arc terrane after accretion to the Eurasian margin far to the south of Kamchatka. For both models the dominant Eocene and Miocene deformation ages observed in eastern Kamchatka are used as two possible age limits for the cessation of northward transport. Although the observed paleolatitudes from paleomagnetic data correspond best with the intra-oceanic transport model, the provenance of the Paleogene 'transport' stratigraphy indicates a near-shore sediment supply. Our preferred interpretation is that the island-arc terrane (1) accreted onto the Eurasian margin concurrent with cessation of island-arc volcanism (Maastrichtian-Danian) and (2) underwent northward coastwise translation along a major strike-slip fault zone ending by middle-late Eocene time (43-50 Ma). It is unclear whether the ophiolite was exposed during arc-continent collision or whether the ophiolite was obducted onto the island arc prior to collision. A consequence of either intra-oceanic transport or coastwise translation is that an open corrido between the western terminus of the Aleutian Arc and Kamchatka must have existe until middle to late Eocene time. Spreading within the Komandorsky Basin, subduction of seamounts, and collision of the Aleutian Arc with Kamchatka are proposed to have instigated the second Miocene phase of deformation, which uplifted and reexposed the island-arc terrane.

The collision of arc rocks is caused by the strike slip motion across the western-most segment of the Aleutian Arc, called Kamchatka Peninsula. A region of thrust faulting and severe compressional deformation in the Cape Kamchatka region indicates the occurrence of the collision. The Cape Kamchatka region is the seismic contact zone between the Aleutian Arc and Kamchatka.

The submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time. [ABSTRACT FROM AUTHOR]

This article describes the regional effects of Cenozoic subduction along the outboard margin of the Northern Cordillera (Alaska, USA, and Western Canada), and thereby acquaints the reader with several chapters of the e-book Dynamic Geology of the Northern Cordillera (Alaska, Western Canada, and Adjacent Marine Areas). This article and the e-book are written for earth-science students and teachers. The level of writing for the article and the source e-book is that of popular science magazines, and readers are encouraged to share this article with students and laypersons. The main thrust of the article is to present and describe a suite of ten regional topographic, bathymetric, and geologic maps, and two figures portraying deep-crustal sections that illustrate the regional effects of Cenozoic subduction along the outboard margin of the North American Cordillera. The regional maps and cross sections are described in a way that a teacher might describe a map to students. Cenozoic subduction along the margin of the Northern Cordillera resulted in the formation of the following: (1) underthrusting of terranes and oceanic lithosphere beneath Southern Alaska; (2) landscapes, including narrow continental shelves along Southern and Southeastern Alaska and Western Canada (the Canadian Cordillera) and continental-margin mountain ranges, including the Alaska Peninsula, Chugach Range, Saint Elias Mountains, and Cascade Mountains; (3) sedimentary basins; (4) an array of active continental strike-slip and thrust faults (inboard of subduction zones); (5) earthquake belts related to subduction of terranes and oceanic plates; (6) active volcanoes, including continental-margin arcs (the Aleutian, Wrangell, and Cascade Arcs) linked to subduction zones, and interior volcanic belts related to strikeslip faulting or to hot spots; (7) lode and placer mineral deposits related to continental margin arcs or subduction of oceanic ridges; (8) hot springs related to continental-margin arcs; (9) plate movements as recorded from GPS measurements; and (10) underthrusting of terranes and oceanic lithosphere beneath the Northern Cordillera. [ABSTRACT FROM AUTHOR]

Research investigating the implications of the collected data from seismic activities and regional geological characteristics to the degree of tectonic erosion in the region of the Nicoya Peninsula in Costa Rica is presented. Particular attention is given to usage of paragenesis data.