Abstract The tectonic framework of NW Himalaya is different from that of the central Himalaya with respect to the position of the Main Central Thrust and Higher Himalayan Crystalline and the Lesser and Sub Himalayan structures. The former is characterized by thick-skinned tectonics, whereas the thin-skinned model explains the tectonic evolution of the central Himalaya. The boundary between the two segments of Himalaya is recognized along the Ropar–Manali lineament fault zone. The normal convergence rate within the Himalaya decreases from c. 18 mm a −1 in the central to c. 15 mm a −1 in the NW segments. In the last 800 years of historical accounts of large earthquakes of magnitude M w ≥ 7, there are seven earthquakes clustered in the central Himalaya, whereas three reported earthquakes are widely separated in the NW Himalaya. The earthquakes in central Himalaya are inferred as occurring over the plate boundary fault, the Main Himalayan Thrust. The wedge thrust earthquakes in NW Himalaya originate over the faults on the hanging wall of the Main Himalayan Thrust. Palaeoseismic evidence recorded on the Himalayan front suggests the occurrence of giant earthquakes in the central Himalaya. The lack of such an event reported in the NW Himalaya may be due to oblique convergence.

ABSTRACT Spatial distributions of widespread igneous arc rocks and high-pressure–low-temperature (HP/LT) metamafic rocks, combined with U-Pb maximum ages of deposition from detrital zircon and petrofacies of Jurassic–Miocene clastic sedimentary rocks, constrain the geologic development of the northern and central Californian accretionary margin: (1) Before ca. 175 Ma, transpressive plate subduction initiated construction of a magmatic arc astride the Klamath-Sierran crustal margin. (2) Paleo-Pacific oceanic-plate rocks were recrystallized under HP/LT conditions in an east-dipping subduction zone beneath the arc at ca. 170–155 Ma. Stored at depth, these HP/LT metamafic blocks returned surfaceward mainly during mid- and Late Cretaceous time as olistoliths and tectonic fragments entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) By ca. 165 Ma and continuing to at least ca. 150 Ma, erosion of the volcanic arc supplied upper-crustal debris to the Mariposa-Galice and Myrtle arc-margin strata. (4) By ca. 140 Ma, the Klamath salient had moved ~80–100 km westward relative to the Sierran arc, initiating a new, outboard convergent plate junction, and trapping old oceanic crust on the south as the Great Valley Ophiolite. (5) Following end-of-Jurassic development of a new Farallon–North American east-dipping plate junction, terrigenous debris began to accumulate as the seaward Franciscan trench complex and landward Great Valley Group plus Hornbrook forearc clastic rocks. (6) Voluminous deposition and accretion of Franciscan Eastern and Central belt and Great Valley Group detritus occurred during vigorous Sierran igneous activity attending rapid, nearly orthogonal plate subduction starting at ca. 125 Ma. (7) Although minor traces of Grenville-age detrital zircon occur in other sandstones studied in this report, they are absent from post–120 Ma Franciscan strata. (8) Sierra Nevada magmatism ceased by ca. 85 Ma, signaling transition to subhorizontal eastward underflow attending Laramide orogeny farther inland. (9) Exposed Paleogene Franciscan Coastal belt sandstone accreted in a tectonic realm unaffected by HP/LT recrystallization. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain clasts derived chiefly from the Sierran and Klamath ranges. Detritus from the Sierra Nevada ± Idaho batholiths is present in some Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstone.

The Cordillera of western North America occupies the central 5000 km of the circum-Pacific orogenic belt, which extends for 25,000 km along a great-circle path from Taiwan to the Antarctic Peninsula. The North American Cordillera is anomalous because dextral transform faults along its western flank have supplanted subduction zones, the hallmark of circum-Pacific tectonism, along much of the Cordilleran continental margin since mid-Cenozoic time. The linear continuity of the Cordilleran orogen terminates on the north in the Arctic region and on the south in the Mesoamerican region at sinistral transform faults of Mesozoic and Cenozoic age, respectively. The Cordilleran margin of Laurentia was formed initially by rift breakup of the supercontinent Rodinia followed by development of the Neoproterozoic to early Paleozoic Cordilleran miogeocline along a passive continental margin, but it was modified in California and Mexico by Permian to Triassic transform truncation of Paleozoic tectonic trends. Late Paleozoic and Mesozoic accretion of oceanic island arcs and subduction complexes expanded the width of the Cordilleran orogen both before and after Triassic initiation of ancestral circum-Pacific subduction beneath the Cordilleran margin. Mesozoic to Cenozoic extensions and counterparts of Cordilleran accreted terranes extend southward into the Caribbean Antilles and northern South America. The development of successive forearc and retroforeland basins accompanied the progress of Cordilleran orogenesis over time, and coeval Mesozoic to Cenozoic batholith belts reflect continuing plate consumption at subduction zones along the continental margin. The assembly of subduction complexes along the Cordilleran continental margin continued into Cenozoic time, but dextral strike slip along the Pacific flank of the Cordilleran orogen displaced elongate coastal segments of the orogen northward during Cenozoic time. In the United States and Mexico, Laramide breakup of the Cordilleran foreland during shallow slab subduction and crustal extension within the Basin and Range taphrogen also expanded the width of the Cordilleran orogen during Cenozoic time.

Along Pacific convergent margins of the Americas, high-standing relief on the subducting oceanic plate “collides” with continental slopes and subducts. Features common to many collisions are uplift of the continental margin, accelerated seafloor erosion, accelerated basal subduction erosion, a flat slab, and a lack of active volcanism. Each collision along America’s margins has exceptions to a single explanation. Subduction of an ~600 km segment of the Yakutat terrane is associated with >5000-m-high coastal mountains. The terrane may currently be adding its unsubducted mass to the continent by a seaward jump of the deformation front and could be a model for docking of terranes in the past. Cocos Ridge subduction is associated with >3000-m-high mountains, but its shallow subduction zone is not followed by a flat slab. The entry point of the Nazca and Juan Fernandez Ridges into the subduction zone has migrated southward along the South American margin and the adjacent coast without unusually high mountains. The Nazca Ridge and Juan Fernandez Ridges are not actively spreading but the Chile Rise collision is a triple junction. These collisions form barriers to trench sediment transport and separate accreting from eroding segments of the frontal prism. They also occur at the separation of a flat slab from a steeply dipping one. At a smaller scale, the subduction of seamounts and lesser ridges causes temporary surface uplift as long as they remain attached to the subducting plate. Off Costa Rica, these features remain attached beneath the continental shelf. They illustrate, at a small scale, the processes of collision.

In southwestern Mexico, Late Cretaceous to Early Tertiary deformation has been generally associated with the Laramide orogeny of the Cordillera. Several alternative models consider the deformation to result from the accretion of the Guerrero terrane, formed by the Zihuatanejo, Arcelia, and Teloloapan intraoceanic island arcs, to the continental margin of the North American plate. Here, we present a detailed geologic and structural study and new 40 Ar/ 39 Ar and U-Pb ages for a broad region in the central-eastern part of the Guerrero terrane that allow the accretion model to be tested. In the Huetamo–Ciudad Altamirano part of the region, an almost complete Cretaceous-Paleogene succession records the transition from an early Cretaceous shallow-marine environment to continental conditions that began in Santonian times, followed by the development of a major continental Eocene magmatic arc. Folding of the marine and transitional successions signifies a shortening episode between the late Cenomanian and the Santonian, and a subsequent, out-of-sequence, coaxial refolding event in Maastrichtian-Paleocene time amplified the previous structures. A major left-lateral shear zone postdates the contractional deformation, and it passively controlled the geographic distribution of Eocene silicic volcanism. Minor transcurrent faulting followed. Our results indicate that the Huetamo–Ciudad Altamirano region, which has been considered part of the Zihuatanejo subterrane, was in proximity to a continent during most of the Mesozoic. We found continental recycled material at various stratigraphic levels of the Huetamo Cretaceous succession and Grenvillian inherited ages in zircons from the ca. 120 Ma Placeres del Oro pluton. More importantly, detrital zircon ages from the pre-Cretaceous basement of the Huetamo succession (Tzitzio metaflysch) and the pre–Early Jurassic basement of the Arcelia subterrane (Tejupilco suite) yield very similar Late Permian and Ordovician age peaks. These ages are typical of the Acatlán complex, onto which the Guerrero terrane has been proposed to have been accreted in the Late Cretaceous. Similarly, Paleozoic and Precambrian ages are reported in detrital zircons from the volcano-sedimentary successions of the Zihuatanejo, Arcelia, and Teloloapan subterranes. Models considering this part of the Guerrero terrane as having formed by intraoceanic island arcs separated by one or more subduction zones cannot explain the ubiquitous presence of older continental material in the Mesozoic succession. We favor a model in which most of the Guerrero terrane consisted of autochthonous or parautochthonous units deposited on the thinned continental margin of the North American plate and where the Mesozoic magmatic and sedimentary record is explained in the framework of an enduring west-facing migrating arc and related extensional backarc and forearc basins. The results presented here exclude the accretion of allochthonous terranes as the cause for Laramide deformation and require an alternative driving force to explain the generation of the Late Cretaceous–early Tertiary shortening and shearing on the southern margin of the North American plate.

The volcanic basement of the Ecuadorian Western Cordillera (Pallatanga Formation and San Juan unit) is made up of mafic and ultramafic rocks that once formed an oceanic plateau. Radiometric ages from these rocks overlap with a hornblende 40 Ar/ 39 Ar plateau age of 88 ± 1.6 Ma obtained for oceanic plateau basement rocks of the Piñon Formation in coastal Ecuador, and with ca. 92–88 Ma ages reported for oceanic plateau sequences in the Caribbean and western Colombia. These results suggest that the oceanic plateau rocks of the Western Cordillera and flat forearc in Ecuador are derived from the Late Cretaceous Caribbean-Colombia oceanic plateau. Intraoceanic island-arc sequences (Rio Cala Group) overlie the plateau in the Western Cordillera and yield crystallization ages that range between ca. 85 and 72 Ma. The geochemistry and radiometric ages of island-arc lavas from the Rio Cala Group, combined with the age range and geochemistry of their turbiditic, volcaniclastic products, indicate that the arc was initiated by westward subduction beneath the Caribbean Plateau. They are coeval with island-arc rocks of coastal Ecuador (Las Orquideas, San Lorenzo, and Cayo Formations) and Colombia (Ricaurte Arc). These island-arc units may be related to the Late Cretaceous Great Arc of the Caribbean. Paleomagnetic analyses of volcanic rocks of the Piñon and San Lorenzo Formations of the southern external forearc show that they erupted at equatorial or low southern latitudes. The initial collision between South America and the Caribbean-Colombia oceanic plateau caused rock uplift and exhumation (>1 km/m.y.) within the continental margin during the Late Cretaceous (ca. 75–65 Ma). Magmatism associated with the Campanian–early Maastrichtian Rio Cala Arc ceased during the Maastrichtian because the collision event blocked the subduction zone below the oceanic plateau. Paleomagnetic data from basement and sedimentary cover rocks in the coastal forearc reveal 20°–50° of clockwise rotation during the Campanian, which was synchronous with the collision of the oceanic plateau and arc sequence with South America. East-dipping subduction beneath the accreted oceanic plateau formed the latest Maastrichtian to early Paleogene (ca. 60 Ma) Silante volcanic arc, which was deposited in a terrestrial environment. Subsequently, Paleocene to Eocene volcanic rocks of the Macuchi unit were deposited, and these probably represent a continuation of the Silante arc. This submarine volcanism was coeval with the deposition of siliciclastic rocks of the Angamarca Group, which were mainly derived from the emerging Eastern Cordillera.

Abstract Accretionary orogens form at convergent plate boundaries and include the supra-subduction zone forearc, magmatic arc and backarc components. They can be broken into retreating and advancing types, based on their kinematic framework and resulting geological character. Accretionary systems have been active throughout Earth history, extending back until at least 3.2 Ga, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. Accretionary orogens have been responsible for major growth of the continental lithosphere, through the addition of juvenile magmatic products, but are also major sites of consumption and reworking of continental crust through time. The aim of this volume is to provide a better understanding of accretionary processes and their role in the formation and evolution of the continental crust. Fourteen papers deal with general aspects of accretion and metamorphism and discuss examples of accretionary orogens and crustal growth through Earth history, from the Archaean to the Cenozoic.

The Indonesian region includes several volcanic island arcs that are highly active at the present day, and also contains a record of Cenozoic volcanic activity owing to subduction of oceanic lithosphere at the margins of SE Asia. As a result of long-term subduction, there is a high regional heat flow, and a weak crust and lithosphere, as identified in other subduction zone backarcs. The stratigraphic record in the Indonesian region reflects a complex tectonic history, including collisions, changing plate boundaries, subduction polarity reversals, elimination of volcanic arcs, and extension. The arcs have not behaved as often portrayed in many arc models. They mark subduction but were not continuously active, and it is possible to have subduction without magmatism. Subduction hinge retreat was accompanied by significant arc volcanism, whereas periods of hinge advance were marked by reduction or cessation of volcanic activity. Growth of the region occurred in an episodic way, by the addition of ophiolites and continental slivers, and as a result of arc magmatism. In Indonesia, relatively small amounts of material were accreted from the downgoing plate during subduction, but there is also little evidence for subduction erosion. During collision the arc region may fail, resulting in thrusting, and the weakest point is the position of the active volcanic arc itself. Volcanic arcs shift position suddenly, and arcs can disappear during collision by overthrusting. Arcs are geologically ephemeral features and may have very short histories in comparison with most well-known older orogenic belts. The stratigraphic record of the basins within arc regions is complex. Because of a weak lithosphere the character of sedimentary basins may be unusual, and basins are commonly very deep and subside rapidly. There is a high sediment flux. The volcanic arc itself influences the stratigraphic record and basin development. The load imposed by the volcanic arc causes flexure and provides accommodation space. The volcanic arc thus can form the basin and supply most of its sediment. Tropical processes influence the mineralogy and apparent maturity of the sediment, especially volcanogenic material. A complex stratigraphy will result from the waxing and waning of volcanic activity.

The NW corner of the Pacific Ocean is a place of unique Tertiary tectonism, which provides one of the clearest examples of arc-arc collision. Voluminous Cretaceous rhyolitic-granitic magmatism along the continental margin continues into the Paleogene. In contrast, Miocene island arc volcanism follows Eocene boninitic magmatism in the Izu-Mariana Arc, in association with the opening of backarc basins, including those in the Philippine and Japan Seas. The triple junction between the Eurasian, Philippine Sea, and Pacific plates arrived in the area south of Tokyo during the Miocene, just as the Japan Sea was opening. After the beginning of Philippine Sea plate subduction to the north, the Izu Island Arc began to collide obliquely with the Honshu Arc. As a result, this unique tectonic setting in the NW Pacific has produced a miniature Alpine-type orogenic belt (Tanzawa) in the collisional center, whereas in the eastern part of the Izu Arc sediment has been actively accreting in that forearc. Such settings have resulted in systematic accretionary prism formation from the early Miocene in the Boso-Miura peninsular area to the present in the Sagami Trough area. We modeled the tectonics by a simple sandbox experiment. Systematic fault and fracture patterns of the oblique subduction type are predicted to occur during arc-arc collision.

The Moesian Platform is a crustal block within southern Europe, located beyond the southwestern margin of the East European craton. Along this margin lie terranes that were accreted to Baltica as part of Far Eastern Avalonia during Late Ordovician–Early Devonian time and terranes that already formed part of Cambrian Baltica, displaced as proximal terranes together with Far East Avalonian terranes. The tectonic history and crustal affinity of the Moesian Platform, however, remain poorly understood. A review of available tectonostratigraphic, paleontological, and geochronologic data suggests that the Moesian Platform comprises four distinct terranes, two with Baltican and two with Avalonian affinities. A fifth terrane, North Dobrogea, lies between the Moesian Platform and the East European craton and records Variscan (Carboniferous) accretion. This accretionary record leads to the paradox that the youngest accreted crust (North Dobrogea) lies closest to the craton, whereas the earlier accreted crust and crust derived from the craton itself are now located more externally. A review of terranes along the southwestern margin of the East European craton, between the North Sea and the Black Sea, suggests that a dextral strike-slip dominated the southwestern Baltican margin during Late Ordovician–Early Devonian accretion of Far Eastern Avalonia, much as is the case in western North America today. Variscan indentation of the Bohemian Massif led to escape-displacement of some Caledonian terranes, and strike-slip displacement during the Mesozoic opening of Mediterranean-style oceanic basins led to the current juxtaposition of Moesian terranes, inverted with respect to their accretionary history.

The eastern flank of the Appalachian orogen is composed of extensive Neoproterozoic–early Paleozoic crustal blocks that originated in a peri-Gondwanan setting. Three of these blocks record the evolution of Neoproterozoic magmatic-arc systems, including Carolinia in the southern Appalachians and Ganderia and Avalonia in the northern Appalachians. Relationships among these three crustal blocks are important for understanding both the accretionary history of the orogen and the evolution of the Iapetus and Rheic Oceans, first-order geographic features of the Paleozoic globe. Traditionally, Carolinia and Avalonia have been considered to represent a single microcontinental magmatic arc that accreted to Laurentia in the middle to late Paleozoic. The early lithotectonic history (ca. 680–570 Ma) of the two blocks is obscure; however, their latest Neoproterozoic-Paleozoic histories are distinct. This disparity is manifest in the first-order features of (1) timing and style of magmatic-arc cessation and (2) the nature of their Paleozoic lithotectonic records. Magmatic arc activity ceased in Avalonia in the late Neoproterozoic (ca. 570 Ma), succeeded by extension-related magmatism and sedimentation that was transitional into a robust latest Neoproterozoic–Silurian platformal clastic sedimentary sequence. This platform was tectonically unperturbed until the Late Silurian–Early Devonian. In contrast, Carolinia records late Neo-proterozoic tectonothermal events coeval with arc magmatism, which extended into the Cambrian; a relatively thin Middle Cambrian shallow-marine clastic sequence is preserved unconformably atop the Carolinia arc sequences. Subsequently, Carolinia experienced widespread Late Ordovician–Silurian deformation and metamorphism. However, we note striking similarities between Carolinia and Ganderia; specifically, in Ganderia, like Carolinia, late Neoproterozoic tectonism was accompanied by arc magmatism that extended into the Cambrian. Ganderian arc rocks are capped unconformably by a Middle Cambrian to Early Ordovician clastic sequence, and they were tectonized in the Late Ordovician–Silurian, similar to relations in Carolinia. Independent studies indicate that the Late Ordovician–Silurian tectonism in both blocks was related to their accretion to Laurentia. Thus, Carolinia and Ganderia show parallel development of first-order lithotectonic characteristics for two endpoints in their global strain path, i.e., their Gondwanan source region and their accretion to Laurentia. Consequently, we posit that Carolinia appears to be more closely affiliated with Ganderia than with Avalonia. The recognition of this linkage between Appalachian peri-Gondwanan realm crustal blocks in light of paleomagnetic and isotopic data leads to a unified model for the accretion of these blocks to the eastern margin of Laurentia.