A new model is proposed for the problematic preservation of an Ordovician forearc basin, which records a complete sedimentary record of arc-continent collision during the Grampian (Taconic) orogeny in the west of Ireland. The South Mayo Trough represents an arc and forearc complex developed above a subduction zone in which the slab dipped away from the Laurentian passive margin. The collision of this arc with Laurentia caused the Middle Ordovician Grampian orogeny. However, the South Mayo Trough, in the hanging wall of this collision zone, remained a site of marine sedimentation during the entire process. Early sediments show derivation from an island-arc complex, an ophiolitic backstop, and polymetamorphic trench sediments. These are conformably overlain by marine deposits derived from a more evolved arc complex and an emerging juvenile orogen. This transition is dated as being coeval with the Grampian metamorphism of the Laurentian footwall. The problem remains as to why subsidence continued in a basin on the hanging wall. It is proposed that the suppression of the expected topography is due to the nature of the Laurentian continental margin. Geophysical and geological evidence suggests that this was a volcanic margin during Neoproterozoic rifting. It is argued that the subduction of this margin caused the formation of eclogites, which reduced its buoyancy. Simple numerical models are presented which show that this is a viable mechanism for the suppression of topography during early stages of arc-continent collision and hence for the preservation of forearcs.

This paper quantifies the flexural subsidence expected from loading by a volcanic arc. The resulting mathematical model shows that the arc width should grow with time and that the subsidence beneath the load can be estimated from the observed arc width at the surface. Application of this model to the Halmahera Arc in Indonesia shows an excellent fit to observations if a broken-plate model of flexure is assumed. The model also gives an excellent fit to data from East Java, also in Indonesia, where it is possible to forward model gravity anomalies. In particular, the depth, location, and width of the depocenter-associated gravity low are accurately reproduced, although the model does require a high density for the volcanic arc (2900 kg m −3 ). This may indicate additional buried loads due, for example, to magmatic underplating. Our main conclusion is that loads generated by the volcanic arc are sufficient to account for much, if not all, of the subsidence in basins within ∼100 km of active volcanoes at subduction plate boundaries, if the plate is broken. The basins will be asymmetrical and, close to the arc, will contain coarse volcaniclastic material, whereas deposits farther away are likely to be volcaniclastic turbidites. The density contrast between arc and underlying crust required to produce the Indonesian arc basins means that they are unlikely to form in young intraoceanic arcs but may be common in older and more mature arcs.

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.

Shallow-marine carbonate facies from volcanic-arc settings provide an important, but commonly overlooked, record of relative sea-level change, differential subsidence-uplift, paleoclimate trends, and other environmental changes. Carbonate strata are thin where volcanic eruptions are frequent and voluminous, unless shallow, bathy-metric highs persist for long periods of time and volcaniclastic sediment and erupted materials are trapped in adjacent depocenters. Carbonate platforms and reefs can attain significant thickness, however, if subsidence continues after volcanic activity ceases or the volcanic front migrates. The areal extent of shallow-marine carbonate sedimentation is likewise affected by differential tectonic subsidence, although carbonate platforms are most laterally extensive during transgressive to highstand conditions and when arc depocenters are filled with sediment. Tectonic controls on shallow-marine carbonate sedimentation in arc depocenters include (1) coseismic fault displacements and associated surface deformation; (2) long-wavelength tectonic subsidence related to dynamic mantle flow, flexure, lithospheric thinning, and thermal subsidence; and (3) large-scale plate deformation related to local conditions of subduction. Depositional controls on carbonate sedimentation in arc depocenters include (1) the frequency, volume, and style of volcanic eruptions; (2) accumulation rates for siliciclastic-volcaniclastic sediment; (3) the frequency, volume, and dispersal paths of erupted material; (4) (paleo)wind direction, which influences both carbonate facies development directly and indirectly by controlling the dispersal of volcanic ash and other pyroclastic sediment, which can bury carbonate-producing organisms; (5) the frequency and intensity of tsunami events; and (6) volcanically or seismically triggered mass-wasting events, which can erode or bury carbonate strata. Regarding platform morphologies in arc-related settings, (1) fringing reefs or barrier reef systems with lagoons may develop around volcanic edifices throughout the long-term evolution of volcanic arcs; (2) local reefs and mounds may build on intrabasinal, fault-bounded highs within underfilled forearc, intra-arc, and backarc basins; (3) isolated platforms with variable platform margin-to-basin transitions are common in “underfilled” and tectonically active depocenters; and (4) broad ramps and rimmed carbonate shelves are typically found in tectonically mature and sediment-filled depocenters.

In the Bismarck Volcanic Arc in Papua New Guinea, six fields of sediment waves were imaged with sonar. Sediment structures observed in seismic data and swath bathymetry are not unique and can result from predominantly continuous (bottom) currents, or episodic (turbidity) currents, or from deformation of sediment. Two of these wave fields overlap and appear to be of turbidity-current origin and modified by bottom currents, with one field unconformably overlying the other field. A field off the coast of Dakataua caldera displays an arcuate morphology, and a series of enclosed depressions within the field suggests creation by extensional deformation of rapidly deposited sediment. Scour features in side-scan imagery suggest turbidity-current activity, which also likely modifies the sediment waves. The wave field is isolated from hyperpycnal currents, however, suggesting that in the absence of a shelf, coastal erosion and small landslides can produce semiregular gravity-driven sediment flows that deposit in deep (>1400 m) water. In Kimbe Bay a fourth sediment-wave field also displays arcuate morphology and enclosed depressions within the field. This wave field is found within a bay >40 km from shore and also appears to have been formed by a combination of extensional deformation of sediment and energetic current activity. Two additional fields in Hixon Bay are fed by small and medium rivers (<∼450 m 3 /s mean annual discharge) draining volcanoes and mountainous regions. One small field appears within a slide scar, suggesting that the initial topography of the scar provided the conditions for early sediment-wave growth. A much larger field is best explained by repeated hyperpycnal currents originating from the Pandi River. We cored a series of upward-fining, graded sequences consistent with a turbidity-current origin. Ages from these cores and measurements of relative thickness in sub-bottom imagery of the field constrain deposition rates for the field and suggest that a large part of the Pandi River discharge must be bypassing the shelf and depositing on the sediment-wave field in deep water (>1200 m). These findings suggest that the sedimentary record in arc collision zones will be dominated by mass-wasting deposits very close to volcanoes, and by river discharge depositing in select, extent regions far from shore. Because sedimentation rates can vary by a factor of 2 between the two flanks of a sediment wave, care must be taken when comparing bed thickness across an entire sedimentary section.

Marine surveys show that the submarine Huatung Ridge extends northward to the Lichi Mélange in the southwestern Coastal Range, suggesting that formation of the Lichi Mélange is related to arcward thrusting of the forearc strata in the western part of the North Luzon Trough during active arc-continent collision off southern Taiwan. A new seismic survey along the 21° N transect across the North Luzon Trough in the incipient arc-continent collision zone further reveals that deformation of the Huatung Ridge occurred soon after sedimentation in the western forearc basin, whereas sedimentation was continuous in the eastern part of the remnant North Luzon Trough until the complete closure of the forearc basin approaching SE Taiwan. This suggests that the sequence in the Huatung Ridge can be coeval with just the lower sequence of the remnant-forearc-basin strata. Multiple lines of new evidence, including micropaleontology, clay mineralogy, and fission track analyses along the Mukeng River and its tributary key sections, are used to test this thrusting-forearc-origin hypothesis of the Lichi Mélange. In the SW Coastal Range the Lichi Mélange lies between the collision suture of Longitudinal Valley to the west and the Taiyuan remnant forearc basin to the east. A field survey indicates that the Taiyuan forearc-basin sequence and its volcanic basement were thrust westward over the Lichi Mélange along the east-dipping Tuluanshan Fault. The Lichi Mélange shows varying degrees of fragmentation of strata, mixing, and shearing. An apparently wide range of facies is present, from the weakly sheared broken formation facies, with discernible relict sedimentary structures, to the intensely sheared block-in-matrix mélange facies, with pervasively scaly foliation dipping to the SE. Sedimentological study reveals that the subangular to subrounded, fractured, matrix-supported metasandstone conglomerates in the pebbly mudstone layers are repeatedly found in the broken formation facies of the Lichi Mélange. Their composition and occurrence are identical to the deep-sea-fan conglomerate beds in the Taiyuan remnant-forearc-basin strata to the east. Benthic foraminiferal faunas are similar in the Lichi Mélange, regardless of the varying intensity of shearing and strata disruptions, and are compatible with the benthic foraminiferal fauna in the Taiyuan remnant-forearc-basin turbidites, supporting the interpretation that the protolith of the Lichi Mélange was originally deposited in the North Luzon Trough. Age determination of planktic microfossils further demonstrates that the Lichi Mélange is early Pliocene (3.5–3.7 Ma), implying that this mélange was deposited in a short time and that deformation occurred soon after its deposition. The early Pliocene age of the Lichi Mélange is coeval with just the lower part of the Taiyuan remnant forearc strata, and is much younger than the upper forearc sequence (3–1 Ma). Thus the Taiyuan coherent-forearc-basin strata (3.7–1 Ma) were deposited continuously in the remnant North Luzon Trough regardless of the deformation in its western part (the protomélange). This scenario is an analogue for the modern configuration of the Huatung Ridge–remnant North Luzon Trough off the southern Coastal Range in the active arc-continent collision zone north of lat 21° N. In addition to its kaolinite content (11–15%), the clay mineral composition of the Lichi Mélange is compatible with the Taiyuan remnant forearc turbidites. In the Coastal Range, kaolinites are found only in the volcanic rocks of the Tuluanshan Formation. This additional kaolinite in the Lichi Mélange could not have been derived from the exposed accretionary prism to the North Luzon Trough by sedimentary mass slumping, because no such volcanic rocks are now exposed in the accretionary prism west of the Coastal Range. Instead, they could have been derived from the Tuluanshan Formation when it was emplaced into the Lichi Mélange by thrusting during the last 1 Ma when the Luzon arc-forearc was accreted to form the southern Coastal Range. Thus the kaolinites of the volcanic arc rocks were redistributed into the Lichi Mélange by fluid flows along the ubiquitous geological fractures in the mélange, consistent with the field occurrences of the large, rootless, fault-bounded volcanic rocks of andesitic breccia, tuff, and agglomerates that were floating in the intensely sheared block-in-matrix mélange facies of the Lichi Mélange. Mélange is commonly considered to develop in the accretionary prism of a subduction zone. However, the Lichi Mélange in the SW Coastal Range originated from the thrust forearc strata, representing a unique forearc mélange for orogenic belts worldwide. The young age and wide distribution—especially the continuous offshore-onshore connection—of the Lichi Mélange provides a unique example for further research into active modern mélange-forming processes by forearc thrusting during progressive closure of the forearc basin in this active region of arc-continent collision.

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.

An integrated structural, stratigraphic, and sedimentological analysis of the West Crocker formation in northwest Borneo suggests that it is best interpreted as an accretionary prism. The structural geology provides clear evidence of at least two episodes of syndepositional folding and thrust faulting. A probable Eocene age, indicated by foraminiferal and palynological assemblages, differs from the generally accepted Oligocene to early Miocene age and is consistent with deposition of the West Crocker formation during a phase of tectonism at the northwest Borneo margin. Sandstones within the West Crocker formation were deposited by high-density turbidity currents that constructed relatively small, progradational lobes in a slope apron environment, and trace fossil assemblages confirm bathyal water depths of ∼1000 m or more. The composition of the sandstones, which contain abundant feldspars and lithic fragments, suggests that their provenance was the first-cycle product of an eroded orogenic belt, whereas immature textures indicate a short distance of transport.

The Izu Arc has been colliding with the Honshu Arc in central Japan since ca. 15 Ma. In order to understand the provenance changes related to this collision, we studied lower to middle Miocene sandstones in and around the collision zone by analyzing their framework composition and the chemistries of detrital clinopyroxene, garnet, and chromian spinel. Sandstone deposited in the trench and forearc basin of the Honshu Arc prior to collision includes grains of detrital garnet and chromian spinel, which originated mainly from granites and low pressure-temperature (P-T) metamorphic rocks, and forearc peridotite, respectively, parts of the Honshu Arc. The forearc and trench-fill sandstones differ in terms of their framework composition; sedimentary lithics are more abundant in the forearc sandstone than in the trench. The two groups of sediments were supplied from different parts of the Honshu Arc. The lower part of the clastic sequence deposited within the Izu Arc is composed mainly of volcaniclastic rocks and yields detrital clinopyroxenes that originated from the Izu Arc. In contrast, the upper part is similar to the lower Miocene trench-fill deposits in terms of its framework composition and the chemistry of detrital garnet and chromian spinel. This reflects a change in provenance triggered by the initial contact of the Izu Arc and the trench between the Eurasian and Philippine Sea plates. The lower part of the middle Miocene trench-fill that was deposited following initial contact is also similar to the lower Miocene trench-fill. The upper part, however, resembles lower Miocene sedimentary rocks of the forearc basin. This suggests that the transport path was changed by collision. During the initial stages of collision between the Honshu and Izu Arcs, the Honshu Arc was preferentially uplifted, and therefore supplied most of the detritus to the collision zone.

The stratigraphic record of volcanic arcs provides insights into their eruptive history, the formation of associated basins, and the character of the deep crust beneath them. Indian Ocean lithosphere was subducted continuously beneath Java from ca. 45 Ma, resulting in formation of a volcanic arc, although volcanic activity was not continuous for all of this period. The lower Cenozoic stratigraphic record on land in East Java provides an excellent opportunity to examine the complete eruptive history of a young, well-preserved volcanic arc from initiation to termination. The Southern Mountains Arc in Java was active from the middle Eocene (ca. 45 Ma) to the early Miocene (ca. 20 Ma), and its activity included significant acidic volcanism that was overlooked in previous studies of the area. In particular, quartz sandstones, previously considered to be terrigenous clastic sedimentary rocks derived from continental crust, are now known to be of volcanic origin. These deposits form part of the fill of the Kendeng Basin, a deep flexural basin that formed in the backarc area, north of the arc. Dating of zircons in the arc rocks indicates that the acidic character of the volcanism can be related to contamination of magmas by a fragment of Archean to Cambrian continental crust that lay beneath the arc. Activity in the Southern Mountains Arc ended in the early Miocene (ca. 20 Ma) with a phase of intense eruptions, including the Semilir event, which distributed ash over a wide area. Following the cessation of the early Cenozoic arc volcanism, there followed a period of volcanic quiescence. Subsequently arc volcanism resumed in the late Miocene (ca. 12–10 Ma) in the modern Sunda Arc, the axis of which lies 50 km north of the older arc.

The Andaman Islands are part of the Andaman-Nicobar Ridge, an accretionary complex that forms part of the outer-arc ridge of the Sunda subduction zone. The Tertiary rocks exposed on the Andaman Islands preserve a record of the tectonic evolution of the surrounding region, including the evolution and closure of the Tethys Ocean. Some of the Paleogene sediments on Andaman may represent an offscraped part of the early Bengal Fan. Through field and petrographic observations, and use of a number of isotopic tracers, new age and provenance constraints are placed on the key Paleogene formations exposed on South Andaman. A paucity of biostratigraphic data poorly define sediment depositional ages. Constraints on timing of deposition obtained by dating detrital minerals for the Mithakhari Group indicate sedimentation after 60 Ma, possibly younger than 40 Ma. A better constraint is obtained for the Andaman Flysch Formation, which was deposited between 30 and 20 Ma, based on Ar-Ar ages of the youngest detrital muscovites at ca. 30 Ma and thermal history modeling of apatite fission-track and U-Th/He data. The latter record sediment burial and inversion (uplift) at ca. 20 Ma. In terms of sediment sources the Mithakhari Group shows a predominantly arc-derived composition, with a very subordinate contribution from the continental margin to the east of the arc. The Oligocene Andaman Flysch at Corbyn's Cove is dominated by recycled orogenic sources, but it also contains a subordinate arc-derived contribution. It is likely that the sources of the Andaman Flysch included rocks from Myanmar affected by India-Asia collision. Any contribution of material from the nascent Himalayas must have been minor. Nd isotope data discount any major input from cratonic Greater India sources.

The Ilan Basin of northern Taiwan forms the western limit of the Okinawa Trough, where the trough meets the compressional ranges of central Taiwan. Apatite fission-track ages of 1.2 ± 0.5 Ma and 3.5 ± 0.5 Ma, measured north and south of the basin, respectively, indicate faster exhumation rates in the Hsüehshan Range to the north (>1.6 mm/yr) than in the Backbone Range to the south (0.7 mm/yr). Reconstructed subsidence rates along the northern basin margin are also faster than in the south (6–7 compared with 3–5 mm/yr). Global positioning system (GPS) and active seismological data indicate motion of the southern basin margin to the east and southeast. We propose that the Ilan Basin is being formed as a result of extension of northern Taiwan, largely controlled by a major southeast-dipping fault, modeled at ∼30° dip, and mapped as a continuation of the Lishan Fault, a major thrust structure in the Central Ranges. Flexural rigidity of the lithosphere under the basin is low, with elastic thickness ∼3 km. A southwest-migrating collision between the Luzon Arc and southern China, accompanied by subduction polarity reversal in the Ryukyu Trench, has allowed crustal blocks that were previously held in compression between the Eurasian and Philippine Sea plates to move trenchward as they reach the northern end of the collision zone. Subduction polarity reversal permits rapid extension and formation of the Ilan Basin and presumably, at least, the western Okinawa Trough, as a direct consequence of arc-continent collision, not because of independent trench rollback forces. This conceptual model suggests that migrating arc-continent collision causes the rapid formation of deep marginal basins that are then filled by detritus from the adjacent orogen, and that these should be common features in the geologic record.

The Guerrero Composite Terrane of western Mexico is the second largest terrane in North America. Mostly characterized by submarine volcanism and formed by five terranes, the Guerrero records vast and complex subduction-related processes influenced by major translation and rifting. It is composed of the Teloloapan, Guanajuato, Arcelia, Tahue, and Zihuatanejo Terranes. The Teloloapan Terrane is made up of Lower Cretaceous island-arc (IA) andesitic to basaltic submarine lava flows, interbedded with limestone and shallow-marine volcaniclastic rocks. The Guanajuato and Arcelia Terranes are characterized by Lower Cretaceous supra-subduction ophiolite successions formed by deep-marine volcanic and sedimentary rocks with mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc basalt (IAB) signatures. These two terranes are placed between the continent and the more evolved arc assemblages of the Zihuatanejo Terrane. The Tahue Terrane is composed of Paleozoic accreted arc and eugeoclinal sedimentary rocks, Triassic rift-related metaigneous rocks, and overlain unconformably by pillow basalts, limestone, and volcaniclastic rocks. The Zihuatanejo Terrane was formed by Triassic ocean-flank to ocean-floor assemblages accreted in Early Jurassic time (subduction complexes). The subduction complexes are overlain by Middle Jurassic–evolved volcanic arc rocks, which are in turn unconformably overlain by Early and Late Cretaceous subaerial and marine arc-related volcano-sedimentary assemblages. Mesozoic stratigraphy at the paleocontinental margin of Mexico (Oaxaquia and Mixteca Terranes) is formed by Triassic submarine fan turbidites accreted during Early Jurassic time; Middle Jurassic–evolved volcanic arc rocks are unconformably covered by a Late Jurassic to Cretaceous calcareous platform. Six stages in the tectonic evolution are proposed on the basis of the stratigraphic and deformational events recorded in western Mexico: (1) A passive or rifting margin developed along the western margin of continental Mexico throughout the Triassic. A thick siliciclastic turbiditic succession of the Potosi Submarine Fan was accumulated on the paleo-continental shelf-slope and extended to the west in a marginal oceanic basin. (2) Subduction began in the Early Jurassic, and the turbidites of the Potosi Fan with slivers of the oceanic crust were accreted, forming a wide subduction prism. (3) Exhumation of the accretionary prism and development of a Middle Jurassic continental arc onto the paleo-continental margin (Oaxaquia and Mixteca Terrane) took place, and also in the Zihuatanejo Terrane. (4) Intra-arc strike-slip faulting and rifting of the Middle Jurassic continental arc took place along with migration of the subduction toward the west and development of a calcareous platform in Oaxaquia and the Mixteca Terrane (continental Mexico). (5) Drifting of the previously accreted Tahue and Zihuatanejo Terranes formed a series of marginal arc-backarc systems, or one continuously drifting arc with intra-arc and backarc basins during Early to middle Cretaceous time. (6) Deformation of the arc assemblages, and development of Santonian to Maastrichtian foreland and other basins, date the final amalgamation of the Guerrero Composite Terrane with the continental margin.

The Appalachian-Caledonian orogen records a complex history of the closure of the Cambrian-Ordovician Iapetus Ocean. The Dunnage Zone of Newfoundland preserves evidence of an Ordovician arc-arc collision between the Red Indian Lake Arc, which forms part of the peri-Laurentian Annieopsquotch accretionary tract (ca. 480–460 Ma), and the peri-Gondwanan Victoria Arc (ca. 473–453 Ma). Despite the similarity in age, the coeval arc systems can be differentiated on the basis of the contrasts that are apparent across the suture zone, the Red Indian Line. These contrasts include structural and tectonic history, stratigraphy, basement characteristics, radiogenic lead in mineral deposits, and fauna. The arc-arc collision is considered in terms of modern analogues (Molucca and Solomon Seas) in the southwest Pacific, and the timing is constrained by stratigraphic relations in the two arc systems. The Victoria Arc occupied a lower-plate setting during the collision and underwent subsidence during the collision, similar to the Australian active margin and Halmahera arcs in the southwest Pacific. The timing of the subsidence is constrained by three new ages of volcanic rocks in the Victoria Arc (457 ± 2; 456.8 ± 3.1; 457 ± 3.6 Ma) that immediately predate or are coeval with deposition of the Caradoc black shale. In contrast the Red Indian Lake Arc contains a sub-Silurian unconformity and a distinct lack of Caradoc black shale, suggesting uplift during the collision. The emergent peri-Laurentian terranes provided detritus into the newly created basin above the Victoria Arc. The evidence of this basin is preserved in the Badger Group, which stratigraphically overlies the peri-Gondwanan Victoria Arc but incorporated peri-Laurentian detritus. Thus the Badger Group forms a successor basin(s) over the Red Indian Line. Following the collision, subduction stepped back into an outboard basin, the Exploits-Tetagouche backarc, closing the Iapetus Ocean along the Dog Bay Line in the Silurian. Correlative tracts in the Northern Appalachians and British Caledonides support the Ordovician arc-arc collision; however, the evidence is less obvious than in Newfoundland.

The Catalina Schist underlies the inner southern California borderland of southwestern North America. On Santa Catalina Island, amphibolite facies rocks that recrystallized and partially melted at ca. 115 Ma and at 40 km depth occur atop an inverted metamorphic stack that juxtaposes progressively lower grade, high-pressure/temperature (PT) rocks across low-angle faults. This inverted metamorphic sequence has been regarded as having formed within a newly initiated subduction zone. However, subduction initiation at ca. 115 Ma has been difficult to reconcile with regional geologic relationships, because the Catalina Schist formed well after emplacement of the adjacent Peninsular Ranges batholith had begun in earnest. New detrital zircon U-Pb age results indicate that the Catalina Schist accreted over a ∼20 m.y. interval. The amphibolite unit metasediments formed from latest Neocomian to early Aptian (122–115 Ma) craton-enriched detritus derived mainly from the pre-Cretaceous wall rocks and Early Cretaceous volcanic cover of the Peninsular Ranges batholith. In contrast, lawsonite-blueschist and lower grade rocks derived from Cenomanian sediments dominated by this batholith's plutonic and volcanic detritus were accreted between 97 and 95 Ma. Seismic data and geologic relationships indicate that the Catalina Schist structurally underlies the western margin of the northern Peninsular Ranges batholith. We propose that construction of the Catalina Schist complex involved underthrusting of the Early Cretaceous forearc rocks to a subcrustal position beneath the western Peninsular Ranges batholith. The heat for amphibolite facies metamorphism and anatexis observed within the Catalina Schist was supplied by the western part of the batholith while subduction was continuous along the margin. Progressive subduction erosion ultimately juxtaposed the high-grade Catalina Schist with lower grade blueschists accreted above the subduction zone by 95 Ma. This coincided with an eastern relocation of arc magmatism and emplacement of the ca. 95 Ma La Posta tonalite-trondjhemite-granodiorite suite of the eastern Peninsular Ranges batholith. Final assembly of the Catalina Schist marked the initial stage of the Late Cretaceous–early Tertiary craton-ward shift of arc magmatism and deformation of southwestern North America that culminated in the Laramide orogeny.