Renewed Subduction Research Articles

Middle Miocene renewed subduction of arabian-eurasian plates: Implications for convergent tectonic mechanisms, tectonically-induced fluid overpressures, and sediment recycling dynamics

The complex interplay between thrust wedge deformation, tectonically-induced fluid overpressures, and the complex phases of wedge-top sedimentary depositional systems in active plate convergent margins presents a formidable challenge. The offshore Makran accretionary wedge (MAW) is the world's largest, formed by the convergence of Arabian plate underneath Eurasian plate in the Early Cretaceous, followed by Middle Miocene renewed subduction. In this research, geological and geophysical constraints derived from seismic and borehole stratigraphy, seismic attributes, depth structural maps, isopach maps and 3D structural geological models reveal that Middle Miocene renewed subduction controls thrust wedge deformation. Additionally, it influences deep fluid overpressure and the deformational pattern of wedge-top sedimentary depositional systems. The seismic data analysis suggested that the basement pre-kinematic Himalayan turbidities depositional system (HTDs) have been tectonically incorporated into the accretionary wedge by Middle Miocene renewed subduction, which gave rise to complex folding and N-dipping imbricate thrusting features. These N-dipping imbricate thrust faults initiated sediment underplating and served as major structural pathways for deep fluid overpressure migration, resulting in the formation of fluid escape pipes. The fluid escape pipes exhibit conical and bifurcating geometries with inclination angles of approximately 73°, 90°, and 100°, signifying the upward migration of deep overpressured fluids. Furthermore, the Middle Miocene to Pliocene syn-kinematic piggyback and growth depositional system (PGDs) show progressive thickening (Max ∼3600 m) towards onshore MAW. It lies unconformable above the pre-kinematic HTDs and carries valuable sedimentation records, growth stratal patterns, onlap terminations, and truncations against growing structures, revealed the timing and spatial deformation of thrust faults in 3D space. In contrast, the Pleistocene to Recent post-kinematic progradational depositional system (PDs) exhibits clinoforms marked by top-lapping, on-lapping, and down-lapping reflection, which are primarily controlled by sediment recycling dynamics and sea level change since Pleistocene. It shows progressive thickening (Max ∼1800m) towards the Makran subduction zone and lies unconformably above the syn-kinematic PGDs. Abridging the analyses, the tectonic reconstruction and deformation mechanisms model shows four phases, e.g. (1) initiation of subduction and accretionary wedge formation in Eocene, (2) N-dipping imbricate thrust faults and deep fluid overpressure triggered by Middle Miocene renewed subduction, (3) development of piggyback basins and growth stratal geometries in Middle Miocene to Pliocene, (4) thrust deformation ceases and development of progradational depositional system.

Speculations on the Paleozoic legacy of Gondwana amalgamation

Using Gondwana as an example, we show how the geological record can be interrogated to detect significant changes in mantle convection patterns at critical junctures in Earth’s evolution. Evidence of major changes in mantle circulation in the aftermath of late Neoproterozoic-early Paleozoic Gondwana assembly is provided by widespread (i) plume-related magmatism around Gondwana’s periphery, (ii) ironstone deposits related to mantle plume-ocean ridge interaction and enhanced hydrothermal activity, and (iii) super-mature clastic deposits that reflect epeirogenic uplift triggered by mantle upwelling beneath Gondwana combined with deep tropical weathering.In our model, Gondwana assembled above a region of mantle downwelling in which subducted slabs between the converging Gondwanan continents descended to the core-mantle boundary. Renewed subduction along Gondwana’s periphery yielded early arc magmas. But as downwelling beneath Gondwana evolved into upwelling as a result of the ponding of subducted slabs at the base of the mantle, mantle plumes rose from the margins of the nascent upwelling to interact with the edges of Gondwana, where they penetrated the peripheral subduction zones via slab windows, tears and transform faults to generate voluminous calc-alkalic crustal melts in hydrated arc regions and A-type magmas in dry back-arc regions. The plumes also underplated oceanic lithosphere and interacted with adjacent ocean ridges, thereby enhancing hydrothermal activity and the flux of bioessential nutrients, leading to the recurrence of marine iron-rich sedimentary rocks in the geological record. At the same time, upwelling beneath a tropical to equatorial Gondwana led to epeirogenic uplift, deep weathering and erosion, resulting in the production of widespread super-mature clastic deposits. We contend that major changes in mantle convection patterns were encoded into the geological record of Gondwana assembly, influenced global-scale mantle convection patterns, and should be incorporated into geodynamic models for the assembly of Pangea.

Sequence stratigraphy of the Andaman Basin, northern Indian Ocean

Andaman Basin, a poly history basin in the northeastern Indian Ocean, preserves the sedimentary record from Late Cretaceous to Recent with several stratigraphic breaks. Studies from outcropping sedimentary succession, integrated with subsurface data from drilled wells in the offshore, have enables to work out a sequence stratigraphic framework of the sedimentary succession in the basin.The succession comprises of three first-order sequences viz., Late Cretaceous – Eocene, Oligocene and Neogene. These have been related to basin forming tectonic events related to accretionary prism formation and subduction in Paleogene, Oligocene uplift and renewed subduction in Neogene, respectively. Excellent bio-stratigraphic control and field evidence have enabled identification of unconformities of various magnitudes, which in turn has enabled mapping second-order sequence. The 3 second-order sequences within first-order Late Cretaceous–Eocene sequence are: Late Cretaceous-Early Paleocene, Late Paleocene-Middle Eocene and Late Eocene sequence. The Oligocene first-order sequence related to the major uplift and emergence of the main Andaman Island chain, contributed >2000m of flysch sediments. Three second-order sequences within the first-order Neogene sequence are Early Miocene - early Middle Miocene, Middle Miocene - Middle Pliocene and Late Pliocene - Pleistocene sequences separated by unconformities of various magnitude. Each sequence is discussed with respect to its extent, nature of sequence boundaries, sedimentary fill and sequence stratigraphic surfaces to understand the sequence stratigraphic architecture of the basin.

Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

AbstractNew U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-vergent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread magmatism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflecting a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Merrill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

Detrital Zircon U‐Pb Ages and Geochemistry of the Silurian to Permian Sedimentary Rocks in Central Inner Mongolia, China: Implications for Closure of the Paleo‐Asian Ocean

The Solonker suture zone has long been considered to mark the location of the final disappearance of the Paleo‐Asian Ocean in the eastern Central Asian Orogenic Belt (CAOB). However, the time of final suturing is still controversial with two main different proposals of late Permian to early Triassic, and late Devonian. This study reports integrated whole‐rock geochemistry and LA‐ICP‐MS zircon U‐Pb ages of sedimentary rocks from the Silurian Xuniwusu Formation, the Devonian Xilingol Complex and the Permian Zhesi Formation in the Hegenshan‐Xilinhot‐Linxi area in central Inner Mongolia, China. The depositional environment, provenance and tectonic setting of the Silurian‐Devonian and the Permian sediments are compared to constrain the tectonic evolution of the Solonker suture zone and its neighboring zones. The protoliths of the silty slates from the Xuniwusu Formation in the Baolidao zone belong to wacke and were derived from felsic igneous rocks with steady‐state weathering, poor sorting and compositional immaturity. The protoliths of metasedimentary rocks from the Xilingol Complex were wackes and litharenites and were sourced from predominantly felsic igneous rocks with variable weathering conditions and moderate sorting. The Xuniwusu Formation and Xilingol Complex samples both have two groups of detrital zircon that peak at ca. 0.9–1.0 Ga and ca. 420–440 Ma, with maximum deposition ages of late Silurian and middle Devonian age, respectively. Considering the ca. 484–383 Ma volcanic arc in the Baolidao zone, the Xuxiniwu Formation represents an oceanic trench sediment and is covered by the sedimentary rocks in the Xilingol Complex that represents a continental slope sediment in front of the arc. The middle Permian Zhesi Formation metasandstones were derived from predominantly felsic igneous rocks and are texturally immature with very low degrees of rounding and sorting, indicating short transport and rapid burial. The Zhesi Formation in the Hegenshan zone has a main zircon age peak of 302 Ma and a subordinate peak of 423 Ma and was deposited in a back‐arc basin with an early marine transgression during extension and a late marine regression during contraction. The formation also crops out locally in the Baolidao zone with a main zircon age peak of 467 Ma and a minor peak of 359 Ma, and suggests it formed as a marine transgression sedimentary sequence in a restricted extensional basin and followed by a marine regressive event. Two obvious zircon age peaks of 444 Ma and 280 Ma in the Solonker zone and 435 Ma and 274 Ma in Ondor Sum are retrieved from the Zhesi Formation. This suggests as a result of the gradual closure of the Paleo‐Asian Ocean a narrow ocean sedimentary environment with marine regressive sedimentary sequences occupied the Solonker and Ondor Sum zones during the middle Permian. A restricted ocean is suggested by the Permian strata in the Bainaimiao zone. Early Paleozoic subduction until ca. 381 Ma and renewed subduction during ca. 310–254 Ma accompanied by the opening and closure of a back‐arc basin during ca. 298–269 Ma occurred in the northern accretionary zone. In contrast, the southern accretionary zone documented early Paleozoic subduction until ca. 400 Ma and a renewed subduction during ca. 298–246 Ma. The final closure of the Paleo‐Asian ocean therefore lasted at least until the early Triassic and ended with the formation of the Solonker suture zone.

Tectonics and geodynamics of the eastern Tethys and northern Gondwana since the Jurassic

Southeast Asia experienced a complex tectonic and geodynamic history related to the subduction of the eastern Tethyan ocean basins, resulting from the long-term convergence between the Indo-Australian, Eurasian, and Pacific plates since Pangea breakup. The complex collage of continental and island arc terranes can be reconstructed into an estimated ancient arrangement using plate tectonic reconstruction approaches based on a synthesis of continental and marine geological and geophysical data. We use the open-source and cross-platform software GPlates (www.gplates.org) to refine the evolution of the eastern Neo-Tethys since the latest Jurassic rifting episodes along northern Gondwana. We apply the resulting plate motions to drive numerical models of mantle flow in order to predict the evolving mantle structure. New Guinea’s northward motion over subducted slabs, related to the Sepik back-arc basin and the Maramuni subduction system, resulted in long-term flooding of the margin since ~20 Ma, despite falling long-term global sea levels. The Sundaland continental promontory experienced dynamic uplift in the latest Cretaceous to Eocene times due to the accretion of the Woyla Arc at ~80 Ma, leading to slab breakoff and a temporary interruption of subduction. However, renewed subduction along the Sunda margin resulted in renewed dynamic subsidence from ~30 Ma, which was amplified by regional basin rifting events. In addition, the sinking Sunda slab likely triggered a mantle slab avalanche, resulting in a counterintuitive combination of contemporaneous basin inversion and strong dynamic subsidence from ~15 Ma. The evolution of the eastern Tethyan oceanic gateway provides an important framework for understanding the role of plate tectonics in controlling long-term oceanic circulation and climate, as well as shedding light on the complex interplay between deep Earth and surface processes in driving basin formation and evolution. These results provide new avenues for reconciling stratigraphic and tectonic processes, as well as contributing new approaches for basin analysis and hydrocarbon exploration.

1.39 Ga mafic dyke swarm in southwestern Yilgarn Craton marks Nuna to Rodinia transition in the West Australian Craton

The Archean Yilgarn Craton in Western Australia hosts at least five generations of mafic dykes ranging from Archean to Neoproterozoic in age, including the craton-wide ca. 2408 Ma Widgiemooltha and the 1210 Ma Marnda Moorn Large Igneous Provinces (LIP), the 1888 Ma Boonadgin dykes in the southwest and the 1075 Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NNW-trending mafic dyke swarm, here named the Biberkine dyke swarm, in the southwestern Yilgarn Craton dated at 1390 ± 3 Ma by ID-TIMS U-Pb geochronology of baddeleyite. The regional extent of the dyke swarm is uncertain but aeromagnetic data suggest that the dykes are part of a linear swarm several hundred kilometers long, truncated by the Mesoproterozoic Albany-Fraser Orogen to the south. Geochemical data indicate that the dykes have tholeiitic compositions with a significant contribution from metasomatically enriched subcontinental lithospheric mantle and/or lower continental crust. Paleogeographic reconstructions suggest that a prolonged tectonic quiescence in the Yilgarn Craton from ca. 1600 Ma was interrupted by renewed subduction along the southern and southeastern margin at ca. 1400 Ma, reflecting a transition from Nuna to Rodinia configuration. The 1390 Ma Biberkine dykes may be a direct consequence of this transition and mark the change from a passive to active tectonic setting, which culminated in the Albany-Fraser Orogeny at ca. 1330 Ma. The Biberkine dykes are coeval with a number of other mafic dyke swarms worldwide and provide an important target for paleomagnetic studies.

The origin of strata within the inner accretionary prism of Nankai Trough: Evidence from clay mineral assemblages along the NanTroSEIZE transect

AbstractOne of the more prominent architectural elements of the Nankai subduction margin, offshore southwest Japan, is an out‐of‐sequence thrust fault (megasplay) that separates the inner accretionary prism from the outer prism. The inner prism (hanging wall of the megasplay) is dominated by mudstone, which is enigmatic when the sedimentary facies is compared to coeval deposits in the Shikoku Basin (i.e. inputs from the subducting Philippine Sea plate) and to coarser‐grained turbidite sequences from the Quaternary trench wedge. Clay mineral assemblages amplify the mismatches of sedimentary facies. Mudstones from the inner prism are uniformly depleted in smectite, with average bulk values of 23–24 wt%, whereas the Shikoku Basin deposits show progressive decreases in proportions of smectite over time, from averages of 46–48 wt% at 10 Ma to 17–21 wt% at 1 Ma. Plate‐boundary reconstructions for the Philippine Sea region provide one solution to the conundrum. Between 15 Ma and 10 Ma, the Pacific plate subducted near the NanTroSEIZE transect, and a trench‐trench‐trench triple junction migrated to the northeast. Accretion during that period involved sediments that had been deposited on the Pacific plate. Motion of the Philippine Sea plate changed from 10 Ma to 6 Ma, resulting in sinistral slip along the proto‐Nankai Trough. Sediments accreted during that period probably had been deposited near the triple junction, with a hybrid detrital provenance. Renewed subduction of the Philippine Sea plate at 6 Ma led to reorganization of watersheds near the Izu–Honshu collision zone and gradual incision of large submarine canyons on both sides of the colliding Izu arc. Accreted Pliocene mudstones share more of an affinity to the triple junction paleoenvironment than they do to Shikoku Basin. These differences between subducting Shikoku Basin strata and accreted Pacific plate sediments have important implications for interpretations of frictional properties, structural architecture, and diagenetic fluid production.

Uplift, rupture, and rollback of the Farallon slab reflected in volcanic perturbations along the Yellowstone adakite hot spot track

AbstractField, geochemical, and geochronological data show that the southern segment of the ancestral Cascades arc advanced into the Oregon back‐arc region from 30 to 20 Ma. We attribute this event to thermal uplift of the Farallon slab by the Yellowstone mantle plume, with heat diffusion, decompression, and the release of volatiles promoting high‐K calc‐alkaline volcanism throughout the back‐arc region. The greatest degree of heating is expressed at the surface by a broad ENE‐trending zone of adakites and related rocks generated by melting of oceanic crust from the Farallon slab. A hiatus in eruptive activity began at ca. 22–20 Ma but ended abruptly at 16.7 Ma with renewed volcanism from slab rupture occurring in two separate regions. The eastern rupture resulted in the extrusion of Steens Basalt during the ascent and melting of a dry mantle (plume) source contaminated with depleted mantle. The contemporaneous western rupture resulted in renewed subduction, melting of a wet mantle source, and the rejuvenation of high‐K calc‐alkaline volcanism near the Nevada‐California border at 16.7 Ma. Here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7.8 km/Ma. Today, the uplifted slab is largely missing beneath the Oregon back‐arc region, replaced instead by a seismic hole that is bound on the south by the adakite hot spot track. We attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17–15 Ma and possible foundering and sinking of slab segments from 16 to 10 Ma.

Large fluctuations of shallow seas in low-lying Southeast Asia driven by mantle flow

The Sundaland continental promontory, as the core of Southeast Asia, is one of the lowest lying continental regions, with half of the continental area presently inundated by a shallow sea. The role of mantle convection in driving long-wavelength topography and vertical motion of the lithosphere in this region has often been ignored when interpreting regional stratigraphy, including a widespread Late Cretaceous-Eocene unconformity, despite a consensus that Southeast Asia is presently situated over a large-amplitude, dynamic topography low resulting from long-term post-Pangea subduction. We use forward numerical models to link mantle flow with surface tectonics, and compare predicted trends of dynamic topography with eustasy and regional paleogeography to determine the influence of mantle convection on regional basin histories. A Late Cretaceous collision of Gondwana-derived terranes with Sundaland choked the active margin, leading to slab breakoff and a ∼10-15 Myr-long subduction hiatus. Slab breakoff likely resulted in several hundred meters of dynamic uplift and emergence of Sundaland between ∼80 and 60 Ma, and may explain the absence of a Late Cretaceous-Eocene sedimentary record. Renewed subduction from ∼60 Ma reinitiated dynamic subsidence of Sundaland, leading to submergence from ∼40 Ma despite falling long-term global sea levels. Our results highlight a complete ‘down-up-down' dynamic topography cycle experienced by Sundaland, with transient dynamic topography manifesting as a major regional unconformity in sedimentary basins.

The earliest low and high δ18O caldera-forming eruptions of the Yellowstone plume: implications for the 30–40 Ma Oregon calderas and speculations on plume-triggered delaminations

We present new isotopic and trace element data for four eruptive centers in Oregon: Wildcat Mountain (40 Ma), Crooked River (32–28 Ma), Tower Mountain (32 Ma), and Mohawk River (32 Ma). The first three calderas are located too far east to be sourced through renewed subduction of the Farallon slab following accretion of the Yellowstone-produced Siletzia terrane at ~50 Ma. Basalts of the three eastern eruptive centers yield high Nb/Yb and Th/Yb ratios, indicating an enriched sublithospheric mantle source, while Mohawk River yields trace element and isotopic (δ18O and εHf) values that correlate with its location above a subduction zone. The voluminous rhyolitic tuffs and lavas of Crooked River (41 x 27 km) have δ18Ozircon values that include seven low δ18Ozircon units (1.8–4.5 ‰), one high δ18Ozircon unit (7.4–8.8 ‰), and two units with heterogeneous zircons (2.0–9.0 ‰), similar to younger Yellowstone-Snake River Plain rhyolites. In order to produce these low δ18O values, a large heat source, widespread hydrothermal circulation, and repeated remelting are all required. In contrast, Wildcat Mountain and Tower Mountain rocks yield high δ18Ozircon values (6.4–7.9 ‰) and normal to low εHfi values (5.2–12.6), indicating crustal melting of high-δ18O supracrustal rocks. We propose that these calderas were produced by the first appearance of the Yellowstone plume east of the Cascadia subduction zone, which is supported by plate reconstructions that put the Yellowstone plume under Crooked River at 32–28 Ma. Given the eastern location of these calderas along the suture of the accreted Siletzia terrane and North America, we suggest that the Yellowstone hotspot is directly responsible for magmatism at Crooked River, and for plume-assisted delamination of portions of the edge of the Blue Mountains that produced the Tower Mountain magmas, while the older Wildcat Mountain magmas are related to suture zone instabilities that were created following accretion of the Siletzia terrane.

Sequence stratigraphy of Paleocene–Eocene successions in selected oil wells, Kirkuk area, northern Iraq

The Middle Paleocene-Eocene Megasequence AP10 was deposited during a period of renewed subduction and volcanic arc activity associated with the final closure of the Neo-Tethys . It is subdivided into two sequences according to the regional unconformity at the base of the Middle-Eocene : the Paleocene-Early Eocene, in which the Aaliji and Sinjar Formations were deposited in the studied area and Middle_Late Eocene sequences, in which the Jaddala Formation was deposited in the studied area. The distinguishable facies throughout Aaliji and Jaddala Formations deposits indicated to deep marine environments that lack benthonic foraminifera in contrast with the high percentage of planktonic foraminifera, especially Morozovella and Acarinina reflect the deep marine environment with tropical to subtropical climate condition. The Sinjar Formation characterizes by abundance of the Nummulite and bioclastic-lime wackestone subfacies indicate shallow inner shelf of carbonate shelf margin environments, followed by shoaling upward sequence, with chance to aerial exposure. The systematic investigation of planktonic foraminifera enabled us to identify thirty–four species and subspecies in the studied section, the stratigraphic distribution of these planktonic foraminiferal assemblages permit the recognition of seven biozones, two biozones (P4 and P5) have been established in Aaliji Formation (Late Paleocene) and five biozones (P9-P13 ) in Jaddala Formation (Early-Middle Eocene), where the contact between the two formations is unconformable that represents an interval of about 4 My, recorded by disappearance of three biozones P6, P7 and P8. Sequence stratigraphy analysis, as calibrated by sedimentary facies and biostratigraphy, delineated two sequences in Paleocene deposits that together interpret as 3rd order sequence, and 2nd order sequence in Eocene deposits. The two sequences shows good correlation of the northern Arabian Plate with other part of the platform during Paleocene and Eocene.

Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole

Apparent polar wander (APW) is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large-scale upwellings in the mantle. We investigate plate motions during the last 410 million years in a reference frame where Africa is fixed. Indeed, Africa has remained a central plate from which most continents diverged since the break-up of Pangea. The exact amount of subduction is unknown prior to 120Ma. We propose an approach, based on one hand on the study of the past subduction volcanism to locate ancient subduction activity, and on the other hand microplate motion history in the Tethyan area derived from geology and paleomagnetism. The peri-Pacific subductions seem to be a quasi-permanent feature of the Earth's history at least since the Paleozoic, with however localized interruptions. The “Tethyan” subductions have a complex history with successive collisions of continental blocs (Hercynian, Indo-Sinian, Alpine and Himalayan) and episodical rebirth of E–W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 280Ma. Due to conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis is computed in a mantle reference frame where the Africa plate is fixed, and compared to the apparent polar wander (APW) observed by paleomagnetism since 280Ma. We find that a major trend of both paleomagnetic and computed APW are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps (around 230Ma, 190Ma and 140–110Ma). We find that cusps result from earlier major geodynamic events: the 230Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental collision (340–300Ma) and to renewed subduction zone West of Laurentia, whereas the 190Ma cusp results from the Indo-Sinian collision (270–230Ma) and the subsequent end of the Neo-Tethys ocean subduction.

Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: Evidence for a widespread 1786 Ma overprint based on xenotime geochronology

Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the rifted margin and subsequent foreland basin of the Penokean orogen. These strata show evidence for multiple regional metamorphic events previously attributed entirely to the Penokean orogeny (1875–1835 Ma). Metasandstones from the Marquette Range Supergroup and the Animikie, Mille Lacs, and North Range Groups were sampled at multiple localities across Minnesota, Wisconsin and Michigan for metamorphic xenotime suitable for in situ SHRIMP U–Pb geochronology. All samples are from the northern Penokean foreland basin where the metamorphic grade is greenschist to sub-greenschist and the strata are virtually undeformed. Xenotime U–Pb ages in these samples have a bimodal population with means of 1786 ± 4 Ma ( n = 32) and 1861 ± 10 Ma ( n = 9). Xenotime of both ages are contained in metasandstones from the basal Chocolay Group in Michigan and Wisconsin and the Mille Lacs Group and North Range Groups in Minnesota. The older age records a regional low-temperature thermal event that is slightly older than the overlying Menominee Group in Michigan and the Animikie Group in Minnesota and Ontario. This 1861 Ma event coincides with regional uplift that led to the formation of the unconformity between the Menominee Group and the overlying Baraga Group in Michigan; hence xenotime growth must have occurred at shallow burial depths. Younger units from the Menominee and Baraga Groups in Michigan and the Animikie Group in Minnesota, record only the 1786 Ma event. A dominant 1800–1790 Ma metamorphic monazite population that overprints Penokean-interval monazite has been documented within amphibolite- to granulite-facies rocks immediately north of the Niagara Fault Zone within the vicinity of gneiss domes and granitic plutons. In contrast, the 1786 Ma xenotime ages are from low-grade, virtually undeformed rocks 50–150 km from the high-grade zones and thus do not appear to reflect a local thermal imprint. Rather, the geographic extent of the 1786 Ma xenotime growth event suggests that it reflects a basin-wide, subtle thermal pulse. It is proposed that the xenotime ages record widespread subtle heating triggered by renewed subduction along the orogen due to Yavapai-interval convergence. The 1800–1700 Ma Yavapai terrane forms an accretionary belt throughout the central and southwestern U.S. and truncates the southern part of the Penokean orogen in central Wisconsin and southeastern Minnesota, about 200 km south of the sample sites. Alternatively, an 1800–1765 Ma interval of gravitational collapse of overthickened crust of the Penokean orogen immediately north of the Niagara Fault Zone may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in xenotime growth.

Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the fore-arc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks with arc tholeiitic or (more rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.

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.

Deformational history of the central Brooks Range, Alaska: Results from fission‐track and 40Ar/39Ar analyses

The Brooks Range is the northernmost orogenic belt in Alaska. From south to north it consists of a thin belt of oceanic basalt and chert, followed by two belts of high‐pressure/low‐temperature metamorphic rocks (the schist and central belts), a basement‐cored anticlinorium (the Doonerak Window), a fold and thrust belt, and a foreland basin (the Colville Basin). We have used apatite and zircon fission‐track (FT) and 40Ar/39Ar white mica analyses of a N‐S transect of the central Brooks Range to study the cooling history. These data are used with a balanced cross section to constrain the timing of deformational events in northern Alaska. The oldest cooling ages from this study were clusters of ∼185 Ma and ∼135 Ma zircon FT ages from the fold and thrust belt. Based on the similarity of the 185 Ma ages with the age of crystallization of the western Brooks Range ophiolites [Wirth et al., 1993], we suggest that the Brooks Range orogeny in northern Alaska had begun prior to this time. Cooling rates in the fold and thrust belt were very slow from 185 to 135 Ma, suggesting little or no deformation was occurring. Apatite and zircon FT analyses from the fold and thrust belt indicate that a substantial amount of cooling (caused by uplift and erosion) occurred from 135 to 95 Ma. We suggest that an increase in the rate of cooling at ∼135 Ma was caused by contractional deformation associated with renewed subduction with the opening of the Canada Basin. The 40Ar/39Ar white mica cooling ages of 130 to 120 Ma from the schist belt rocks of the southern Brooks Range indicate that peak metamorphism must have occurred prior to 130 Ma. An 40Ar/39Ar muscovite age of 113 Ma from the southernmost Brooks Range is interpreted to be the age of movement on normal faults juxtaposing lower‐grade phyllite belt rocks against schist belt rocks. At the same time, rapid deposition of Brooks Range‐derived sedimentary sequences began to the north, in the Colville foreland basin, and to the south, in the Yukon‐Koyukuk basin. Unstrained high‐level granitic plutons, which intruded the Yukon‐Koyukuk basin and Ruby terrane from 107 to 95 Ma, indicate the end of collision with the Yukon‐Koyukuk arc. By 95 Ma, cooling rates in the Brooks Range had slowed and little or no deformation was occurring. At 60 Ma, a major episode of rapid cooling occurred throughout northern Alaska, when the active plate margin in Alaska was more than 500 km to the south. In the Colville Basin, cooling was caused by thrust faulting and folding, and hence erosion, within the Albian‐age and younger sedimentary rocks. Within the interior of the Brooks Range, a large‐scale basement‐involved anticlinorium (the Doonerak Window) became active at this time. At 25 Ma, another episode of rapid cooling occurred within the Doonerak Window region. Both of these events may be related to shallow subduction of the Kula plate from the southern Alaskan plate margin.

Arc-related gold-copper mineralization, basement domes and crustal extension

In western Pacific magmatic arcs, many of the largest low-sulphidation epithermal gold vein deposits are situated on basement antiforms which often include ophiolite. Analogy with metamorphic core complexes, some of which also host low-sulphidation gold, implies that the antiforms result from elevation of pre-existing nappe stacks in response to subduction-induced extensional stress and crustal extension on low-angle detachment faults. Porphyry copper (-gold) deposits in the western Pacific are mostly situated away from antiform axes, in shallow stocks, with their tops at stratigraphic levels similar to these of the deeper parts of low-sulphidation epithermal veins. We suggest that epithermal systems develop where tension fractures on antiforms permit descent of meteoric fluids to depths of at least several km; here, they mix with ascending deep fluid, perhaps heated by magmatic sills rather than by the shallow stocks required for generation of porphyry-type deposits. Basement antiforms without associated magmatic arc rocks lack associated gold but, in some cases, host mercury and antimony mineralization. The association of gold with ophiolite appears to be related to the structure of the underlying crust; most ophiolite nappes themselves are too high-level to have formed a source of gold. The overall field relationships suggest that porphyry-type and low-sulphidation epithermal gold mineralization take place at specific stages in the evolution of a magmatic arc; with continued extension some arcs undergo intra-arc rifting, “back-arc” spreading and eventually renewed subduction.

Stratigraphic Constraints on the Development and Timing of Arc-Continent Collision in Northern Papua New Guinea

ABSTRACT The Finisterre Mountains of northern Papua New Guinea are the site of an active, southeast-propagating collision between the Finisterre terrane and the Australian continental margin. The clastic sedimentary sequence on the southern flank of the range records the history of the collision. The age of initial collision can be deduced from the second of two provenance shifts that occur in the sequence, as revealed by a study of 69 dated sandstones. The first shift, at about 16-18 Ma, was from volcanolithic sediments to mixed-provenance sediments rich in quartz and metasedimentary lithic fragments. These latter sediments were probably derived from the orogenic belt active along the Australian continental margin at that time. At 3.0-3.7 Ma the volcanic source was rejuvenated. Initial collis on at this time uplifted the Finisterre terrane, causing the provenance shift. Here we break the stratigraphic sequence into three units: the Sarawaget beds, the Erap Structural Complex, and the Leron Formation. The Sarawaget beds are Middle Eocene to Early Miocene volcanolithic sandstones that were deposited as clastic aprons un the flanks of the Finisterre volcanic are. Volcanic activity ceased in the Early Miocene. The Erap Complex is divided into two clastic subunits, the Sukurum and Nariawang units, and one basaltic subunit, the Gorambampan unit. The Sukurum unit consists of turbidites deposited south of the extinct Finisterre Arc from the latest Early Miocene through the Early Pliocene. Much of the sediment in the unit was derived from low- to medium-grade metasedimentary units on the active northern margin of the Australian continent. Renewed subduction i the Late Miocene or Pliocene occurred along the New Britain Trench, just south of the remnant Finisterre Arc, closing an eastward-widening small ocean basin. The Sukurum unit probably represents an accretionary prism that formed at this subduction zone. Scattered seamounts that were scraped off the downgoing plate are preserved as the Gorambampan unit. Initial collision of the Finisterre terrane with the continental margin at 3.0-3.7 Ma uplifted the Paleogene volcanic rocks, which were rapidly eroded and deposited as the volcanolithic turbidites of the Nariawang unit. Southeastward propagation of the collision tip caused incorporation of the Nariawang unit into the accretionary wedge. Rapid uplift raised the terrane foreland above sea level sometime after 1.3 Mo. The fluvial, lacustrine and marginal-marine deposits of the Leron Formation accumulated in a terrane foreland basin that developed south of the collision zone. Further propagation of the thrust front toward the foreland incorporated the Leron Formation into the fold-and-thrust belt. Rapid uplift of the Finisterre Range and voluminous volcanolithic sedimentation continue to the present. In the Finisterre Range, arc-continent collision is marked by a provenance shift from metasedimentary and quartzose detritus to volcanic detritus. In another modern arc-continent collision on Taiwan, collision has caused a shift from volcanic to metasedimentary detritus. We attribute this discrepancy to the presence of the Finisterre terrane, an extinct arc, in the forearc region of the Finisterre arc-continent collision and the absence of a similar terrane in the forearc area on Taiwan. The very different evolution of sandstone provenance in the active Taiwan and Finisterre collision zones shows that local factors can profoundly influence the provenance histories of collision zones. No one model of sediment provenance evolution can characterize all collision zones, whether modern or ncient.

REE, Th, Hf, Ta in Bamble gabbros (southern Norway) and their amphibolitized equivalents: implications for gabbro tectonic setting

The Bamble gabbros (“hyperites”) are a group of partly metamorphosed Proterozoic basic intrusions in the Bamble Sector of southern Norway. They are discordant to structures formed during the Kongsbergian (1.54 Ga) high-grade metamorphism, yet are themselves partially amphibolitized. In some localities, transitions from gabbro to amphibolite occur over only a few metres. A previous study of closely spaced gabbro-amphibolite sample pairs revealed that amphibolitization was accompanied by increases in K, Fe 3+, P, H 2O, Rb, Cl±Zr and decreases in Ca, Fe 2+, S, Zn ± Si ± Co. New data for REE, Th, Hf, Ta in the same samples indicate that the only possible metamorphic modification of these elements is a barely significant increase in La in one locality. Eu/Eu ∗ ratios are preserved. This adds confidence to the use of REE as petrogenetic indicators in amphibolitized basic rocks. The resistance of Sm/Nd and Lu/Hf ratios to the amphibolitization may indicate that these isotopic systems could be used to gain geochronological and petrogenetic information from the gabbros and their amphibolitized equivalents. The combined chemical data fingerprint the gabbros as similar to present-day destructive plate-margin-related basic rocks. This is surprising as it has previously been widely supposed that they are an expression of the tensional alkaline magmatism that was widespread in the North Atlantic province at ∼ 1.25 Ga. It thus seems more likely that the Bamble gabbros are related to renewed subduction after the ∼ 1.25 Ga anorogenic period. The geochemistry of these widespread gabbros must be accounted for in future Proterozoic Scandinavian tectonic reconstructions.