Retreating Subduction Zone Research Articles

Mantle Metasomatism at the Edge of a Retreating Subduction Zone: Late Neogene Lamprophyres from the Island of Kos, Greece

Mantle Metasomatism at the Edge of a Retreating Subduction Zone: Late Neogene Lamprophyres from the Island of Kos, Greece

Detrital zircons as palaeodrainage indicators: insights into southeastern Gondwana from Permian basins in eastern Australia

AbstractRadioisotope geochronology of detrital grains coupled with quantitative classification of grain morphology can provide valuable insight into the history of sediment transportation and recycling. Here we present ca. 750 new concordant U‐Pb ages from detrital zircon grains from a relatively understudied Permian sedimentary succession in the New England Orogen (eastern Australia), coupled with values of abrasion that provides a proxy for the relative source‐to‐sink distance. We show that cumulative proportion curves for age groups that correspond to plausible source regions provide insights into the palaeodrainage, even if the basin stratigraphy is relatively poorly constrained. This approach is particularly suitable for investigating complex depositional systems that received inflow from different provenance, such as back‐arc and intra‐cratonic basins. Using the example from eastern Australia, our results show that during the Early Permian, a large regional fluvial system transported detritus from continental Gondwana across the landscape of the former active continental margin (New England Orogen) and the simultaneously developing East Australian Rift System. In addition, a local drainage system mobilised detritus within the New England Orogen. Our new constraints for the Early Permian palaeogeography support the idea that the Lower Permian successions of the southern New England Orogen were deposited in a back‐arc region that was likely linked to a retreating subduction zone.

High‐pressure metamorphism in the southern New England Orogen: Implications for long‐lived accretionary orogenesis in eastern Australia

AbstractNew geochemical, metamorphic, and isotopic data are presented from high‐pressure metamorphic rocks in the southern New England Orogen (eastern Australia). Conventional and optimal thermobarometry are augmented by U‐Pb zircon and 40Ar/39Ar phengite dating to define pressure‐temperature‐time (P‐T‐t) histories for the rocks. The P‐T‐t histories are compared with competing geodynamic models for the Tasmanides, which can be summarized as (i) a retreating orogen model, the Tasmanides formed above a continuous, west dipping, and eastward retreating subduction zone, and (ii) a punctuated orogen model, the Tasmanides formed by several arc accretion, subduction flip, and/or transference events. Whereas both scenarios are potentially supported by the new data, an overlap between the timing of metamorphic recrystallization and key stages of Tasmanides evolution favors a relationship between a single, long‐lived subduction zone and the formation, exhumation, and exposure of the high‐pressure rocks. By comparison with the retreating orogen model, the following links with the P‐T‐t histories emerge: (i) exhumation and underplating of oceanic eclogite during the Delamerian Orogeny, (ii) recrystallization of underplated and exhuming high‐pressure rocks at amphibolite facies conditions coeval with a period of rollback, and (iii) selective recrystallization of high‐pressure rocks at blueschist facies conditions, reflecting metamorphism in a cooled subduction zone. The retreating orogen model can also account for the anomalous location of the Cambrian‐Ordovician high‐pressure rocks in the Devonian‐Carboniferous New England Orogen, where sequential rollback cycles detached and translated parts of the leading edge of the overriding plate to the next, younger orogenic cycle.

Assembly of the Lhasa and Qiangtang terranes in central Tibet by divergent double subduction

Integration of lithostratigraphic, magmatic, and metamorphic data from the Lhasa–Qiangtang collision zone in central Tibet (including the Bangong suture zone and adjacent regions of the Lhasa and Qiangtang terranes) indicates assembly through divergent double sided subduction. This collision zone is characterized by the absence of Early Cretaceous high-grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca. 120–110Ma. Two Jurassic–Cretaceous magmatic arcs are identified from the Caima–Duobuza–Rongma–Kangqiong–Amdo magmatic belt in the western Qiangtang Terrane and from the Along Tso–Yanhu–Daguo–Baingoin–Daru Tso magmatic belt in the northern Lhasa Terrane. These two magmatic arcs reflect northward and southward subduction of the Bangong Ocean lithosphere, respectively. Available multidisciplinary data reconcile that the Bangong Ocean may have closed during the Late Jurassic–Early Cretaceous (most likely ca. 140–130Ma) through arc–arc “soft” collision rather than continent–continent “hard” collision. Subduction zone retreat associated with convergence beneath the Lhasa Terrane may have driven its rifting and separation from the northern margin of Gondwana leading to its accretion within Asia.

Evolution of the Early to Middle Ordovician Popelogan arc in New Brunswick, Canada, and adjacent Maine, USA: Record of arc-trench migration and multiple phases of rifting

A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordovician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476–457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate–like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arc-trench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470–467 Ma) corresponds to cessation of the ca. 476–470 Ma calc-alkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467–457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessation of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.

Structure of the Texas Orocline beneath the sedimentary cover (southeast Queensland, Australia)

The New England Orogen in eastern Australia is characterised by orogenic-scale curvatures (oroclines). The largest and most prominent curvature in this system is the Texas Orocline, but its subsurface geometry is still poorly constrained. A large component of the orocline is covered by post-oroclinal sedimentary rocks, which obscure deeper sections of the orocline and make it difficult to understand how the structure is connected to other segments of the New England Orogen. Here, we present geophysical data that elucidate the structure of the Texas Orocline below the sedimentary cover. Using 2D seismic, aeromagnetic TMI (total magnetic intensity) and Bouguer gravity datasets, in combination with outcrop and well data, we identified the depth to the New England ‘basement’ and significant faults intersecting it. We also traced the strongly contorted subsurface continuation of the Peel-Yarrol Fault System, which is characterised by local gravity and magnetic anomalies associated with isolated serpentinite outcrops. Constraints on the timing of oroclinal bending were obtained from the interpretation of seismic transects, which showed that early Permian sedimentary rocks of the Bowen Basin were deposited in a subtrough that deviates from the general north–south trend of the Bowen Basin. The subtrough is oriented approximately parallel to the western limb of the Texas Orocline, thus suggesting that the orocline formed during and/or after early Permian rifting. Our analysis indicates that initial bending occurred contemporaneously with the development of the early Permian rift basins, most likely in the backarc region of a retreating subduction zone. Subsequently, phases of strike-slip and contractional deformation have further tightened the pre-existing curvatures.

Crustal attenuation as a tracer for the emplacement of the Beni Bousera ultramafic massif (Betico-Rifean belt)

The study of petrology (fieldwork, petrography, and phase diagram modeling) and structural data of the metapelitic granulites and the southern, high-temperature exposed peridotites in the Beni Bousera massif (northern Morocco), combined with results from previous regional studies of the Alboran, suggest a new emplacement mechanism for the mantle rocks in the Betico-Rifean belt. We document two key metamorphic episodes in the granulites within a temperature window of 710–830 ± 50 °C: (1) An earlier prograde high-pressure period (from 9 ± 1.0 to 12 ± 1.0 kbar) characterized by the assemblage garnet + biotite + kyanite + K-feldspar + rutile. Pressure differences of ~3 kbar are found over a continuous crustal section of ~1.5 km of exposed granulites that indicate a significant crustal attenuation during exhumation of the ultramafic rocks; and (2) a later post-kinematic low-pressure (5 ± 0.8 kbar) symplectic assemblage of cordierite + spinel + plagioclase + sillimanite. At the scale of the entire Betico-Rifean belt, two main contacts are observed as mirror images in both sides of the Alboran Sea: (1) the long axis of the high-temperature ductile contact between granulites and peridotites occurs in the west side of the Beni Bousera and Ronda massifs, coupled with (2) the consistent high-angle, east-dipping normal fault in the east parts of the belt massifs. The integration of the petrologic results with information on the rotation of both contacts reveals ductile deformation in the lower crust related to the emplacement of the ultramafic rocks in the Betico-Rifean belt along deep-reaching normal faults. The presence of the early high-temperature contact suggests that it was originally a shallow, west-dipping detachment fault developed in a back-arc environment of the east-dipping, retreating subduction zone (current western part of the Gibraltar arc). This scenario is in concordance with the tectonic evolution in western Italy, where anticlockwise Pleistocene rotations associated with northeast-directed thrusting in the Apennines—and coeval with the southeastward motion in the Calabria-Peloritani terrane—were triggered by retreat and rollback of the Adriatic-Ionian slab toward the southeast during the northwest-directed subduction beneath the Calabrian arc.

Thick-skin-dominated orogens; from initial inversion to full accretion: an introduction

Abstract Fifty per cent of orogens have a thick-skin character, and have evolved from passive margin and intra-cratonic rift systems. One group of thick-skin provinces can be found at both pro- and retro-wedges of orogens associated with advancing subduction zones, that is, orogenic wedges whose advance vectors oppose the mantle flow. A second group can be found at pro-wedges of orogens associated with retreating subduction zones, that is, orogens whose advance vectors have the same direction as mantle flow. A third group is formed in intra-plate settings where mechanical strengthening is produced by internal shortening. Thick-skin province development is controlled by driving factors such as individual plate movement rates, overall convergence rates, orogen movement sense with respect to mantle flow, and pro-wedge v. retro-wedge location. These driving factors are themselves constrained by numerous internal and external factors. This introductory chapter focusses primarily on least-deformed case areas in order to understand the role of different factors in controlling the evolution of thick-skin tectonic provinces from the initial inversion stage to full accretion stage.

Foredeep palaeobathymetry and subsidence trends during advancing then retreating subduction: the Northern Apennine case (Oligocene‐Miocene, Italy)

AbstractThe Northern Apennines provide an example of long‐term deep‐water sedimentation in an underfilled pro‐foreland basin first linked to an advancing orogenic wedge and then to a retreating subduction zone during slab rollback. New palaeobathymetric and geohistory analyses of turbidite systems that accumulated in the foredeep during the Oligocene‐Miocene are used to unravel the basin subsidence history during this geodynamic change, and to investigate how it interplayed with sediment supply and basin tectonics in controlling foredeep filling. The results show an estimated ca. 2 km decrease in palaeowater depth at ca. 17 Ma. Moreover, a change in basin subsidence is documented during Langhian time, with an average decompacted subsidence rate, during individual depocentre life, that increased from <0.3 to 0.4–0.6 mm y−1, together with the appearance of a syndepositional backstripped subsidence bracketed between 0.1 and 0.2 mm y−1. This change prevented the basin from complete filling during late Miocene and is interpreted as the foredeep response to initial rollback of the downgoing Adriatic slab. Thus, the Northern Apennine system provides an example of a pro‐foreland basin that experienced both a slow‐ and high‐subsidence regime as a consequence of the advancing then retreating evolution of the collisional system.

The influence of a mantle plume head on the dynamics of a retreating subduction zone

The geological record and seismic tomography together contain multiple examples of mantle plumes and subducting oceanic slabs interacting, however consequences of these interactions and a mechanism for plumes to pass from the subducting to the overriding plate are uncertain. We present three-dimensional numerical simulations of a retreating subduction zone encountering a mantle plume head. We assess the impact a plume head has on the geometry of the trench and whether it can create a window within the subducted slab. In our models we consider slab/plume head interactions to be a natural consequence of plume head entrainment by plate motion combined with slab retreat. Strong slabs prevent plume heads from reaching the surface. For weak slabs, strongly buoyant plume heads cause local trench advance, slab windowing and accelerated slab retreat at the margins of the plume head. For large plume heads, this trench advance may contribute to crustal shortening in the overriding plate. Opening a slab window creates a potential conduit for transfer of plume material from the subducting plate to the overriding plate without terminating an active magmatic arc away from the point where the plume head tears the slab.

Late Eocene–Neogene sedimentary geology of the Girne (Kyrenia) Range, northern Cyprus: A case history of sedimentation related to progressive and diachronous continental collision

Sedimentary geology can shed light on the processes of continental collision. The easternmost Mediterranean provides an exceptional opportunity to study sediments deposited during progressive and diachronous continental collision, with implications for comparable settings elsewhere. Here, we focus on sediment sources, pathways and deposition of mostly deep-sea clastic sediments exposed in northern Cyprus. The sedimentary basin forms part of the northern continental margin of the Southern Neotethys ocean and is represented by a ca. 200km-long, E–W-trending, arcuate lineament, encompassing northern Cyprus and its submarine link with Turkey. The Upper Eocene–Upper Miocene basin was divided into northerly and southerly sub-basins during Mid- to Late Miocene time by an E–W-trending, inferred syn-depositional fault lineament. The sequence begins with non-marine to shallow-marine basal conglomerates (Late Eocene) that abruptly fine upwards into siliciclastic turbidites (Oligocene), followed by biogenic calciturbidites and marls (Aquitanian–Langhian). During Mid–Late Miocene the northerly sub-basin was characterised by thin- to medium-bedded calciturbidites and marls (Serravallian), overlain by thick-bedded, medium-grained lithic sandstones (Tortonian). In contrast, the southern sub-basin is dominated by siliciclastic turbidites with abundant sole structures (Burdigalian–Tortonian). Palaeocurrent evidence indicates dominantly westward sediment transport, especially during the Late Miocene. The Serravallian–Tortonian sandstones of the northern sub-basin are relatively rich in carbonates, while the southern sub-basin contains more siliciclastic and ophiolite-derived detritus indicating differences in sediment supply. In general, the Upper Eocene basal conglomerates were mainly derived from the north, from within the Girne (Kyrenia) Range and its inferred northward extension. In response to rapid subsidence (reflecting possible southward subduction zone retreat) clastic sediment sources to the north then submerged while clastic sediment was instead mainly supplied from the South Neotethyan suture zone to the east and northeast in southern Turkey. Reflecting diachronous collision, deep-water deposition persisted in the easternmost Mediterranean basin further west until this was terminated in northern Cyprus by thrusting/folding during latest Miocene–earliest Pliocene. Quaternary collision-related uplift ensued. Similar sedimentary development is to be expected in collisional settings elsewhere (e.g. Tethyan closure in the Himalayas; Iapetus closure in the Appalachians).

Structural and geochronological constraints on the Mesozoic‐Cenozoic tectonic evolution of the Longmen Shan thrust belt, eastern Tibetan Plateau

The Longmen Shan thrust belt in the eastern margin of the Tibetan Plateau underwent deformation of D1 associated with the Late Triassic collision between the North and South China Blocks, followed by deformation of D2 and D3 related to the eastward growth of the Tibetan Plateau. The D1 is marked by moderately tight folds (f1), spaced cleavage (S1), mineral lineation (L1), multistage reactivated faults, and top‐to‐the south thrusting. This deformation initiated before 237 Ma and ended at 208 Ma. The 193–159 Ma D2 deformation is top‐to‐the‐southeast directed and normal ductile in nature and was associated with the formation of half‐graben basins. The Mesozoic deformation of the Longmen Shan belt was related to the subduction of the Paleo‐Tethys oceanic lithosphere, followed by the South China Block underneath the Songpan‐Ganze Terrane. The D1 was produced in the pro‐side of the overriding Songpan‐Ganze plate, most probably driven by slab corner flow above the retreating South China Block. The D2 and extensional Songpan‐Ganze turbidite basin were produced by roll‐back and migration of the overriding plate above the retreating subduction zone. This model for the deformation from D1 to D2 is similar to a Mediterranean‐style model. The D3 is characterized by SE‐ward thrusting with stepwise migration of rapid denudation and dextral strike‐slip faulting during the Late Cretaceous to present. Therefore, the Longmen Shan thrust belt involved Mesozoic compressional and extensional events that were overprinted by the Late Cretaceous to Cenozoic compressional event.

Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Syn-orogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell'Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27 Ma obtained at the Monti dell'Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell'Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13–10 Ma rocks were already at shallow crustal levels (< 9 km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at ~ 6–4 Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6–4 Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional “out-of-sequence” or “re-activation” event occurring in the internal sector of Tuscany already affected by a regional extensional regime.

Geodynamical models of lithospheric deformation, rotation and extension of the Pannonian Basin of Central Europe

The two major crustal blocks of the Pannonian basin, Alcapa (Alpine–Carpathian–Pannonian) and Tisza, underwent a complex process of rotation and extension of variable magnitude during the Tertiary. The northward push of the Adriatic Block initiated the eastward displacement and rotation of both the Alcapa and Tisza blocks. We have constructed geodynamical models of the rotation and extension of the two Pannonian blocks. We analyse this process in terms of the competing influences of a NE push by the Adriatic block, a NE pull from a retreating subduction zone on the eastern Carpathians, and the internal buoyancy forces arising from crustal thickness variations. Deformation of the blocks included substantial strike-slip movements, together with shortening and possible extension across the Mid-Hungarian Line, which now separates the two domains. We use an idealised thin viscous sheet model of the continental lithosphere in which deformation is described by a nonlinear viscous constitutive relationship. Our approach is based on the finite element method, and we consider several distinct models of initial geometry, boundary conditions, and constitutive parameters. Rotation and deformation vary across both blocks, with anticlockwise rotation occurring in the Alcapa plate, and clockwise rotation in the Tisza block. The opposite rotations of the two plates led to NW–SE convergence in the space between the two Intra-Carpathian terranes. Subsequently both domains underwent extension dominantly in the NE–SW direction. Extension and internal rotation can be represented using a thin sheet model in which deformation is driven by an east- or northeast-directed outward traction of order 1 to 3 MPa. Internal buoyancy forces produced by thick crust from a prior phase of Alpine-style convergence appear to be a necessary element in the force balance that drives the extension process.

Bimodal volcanism as evidence for Paleozoic extensional accretionary tectonism in the southern Appalachians

Research Article| July 01, 2010 Bimodal volcanism as evidence for Paleozoic extensional accretionary tectonism in the southern Appalachians Christopher S. Holm-Denoma; Christopher S. Holm-Denoma † 1U.S. Geological Survey, Denver, Colorado 80225, USA †E-mail: cholm-denoma@usgs.gov Search for other works by this author on: GSW Google Scholar Reshmi Das Reshmi Das § 2Department of Geological Sciences and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306, USA §Current address: Physical Research Laboratory, Ahmedabad 380 009, India Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (7-8): 1220–1234. https://doi.org/10.1130/B30051.1 Article history received: 05 Mar 2009 rev-recd: 17 Jul 2009 accepted: 27 Jul 2009 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Christopher S. Holm-Denoma, Reshmi Das; Bimodal volcanism as evidence for Paleozoic extensional accretionary tectonism in the southern Appalachians. GSA Bulletin 2010;; 122 (7-8): 1220–1234. doi: https://doi.org/10.1130/B30051.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Detailed mapping, U-Pb ages of igneous and detrital zircons, and isotopic and geochemical characterization of a bimodal volcanic sequence from the Appalachian Blue Ridge province provide constraints for the timing and tectonic setting of volcanism along the Early Paleozoic southeastern Laurentian margin. These units, the Pumpkinvine Creek Formation and related rocks, are interpreted as having been deposited along an extended Laurentian margin in a backarc setting during the mid-Ordovician, and tectonically emplaced as late as the late Paleozoic Alleghanian orogeny.Two felsite layers from the Pumpkinvine Creek Formation, a bimodal metavolcanic sequence, have ages of 466 ± 5 Ma and 461 ± 3 Ma. Geochemical analyses of metabasalts indicate compositions derived from a suprasubduction zone, and more specifically a backarc. Furthermore, εNd(460 Ma) = (+)3.3–(+)7.7 and 87Sr/86Sr(initial) = 0.7044–0.7069 indicate a juvenile source. The protoliths of the felsic lithofacies were low-potassium rhyodacite, and have εNd(460 Ma) = (−)3.2–(+)4.65 and 87Sr/86Sr(initial) = 0.709–0.722, suggesting incorporation of an evolved crustal component, which we interpret as Laurentian continental lithosphere. Furthermore, detrital zircons derived from within the Pumpkinvine Creek Formation yielded a majority of ages between 1017 and 1190 Ma, indicating that the source is likely a component of the rifted eastern Laurentian margin.A backarc origin for the Pumpkinvine Creek Formation indicates that the geodynamic setting of the southeastern margin of Laurentia included a retreating subduction zone resulting in extensional accretionary tectonism. These results have implications for the evolution of the southeastern Laurentian margin during a time traditionally linked to collision of an exotic Taconic arc with Laurentia. Additionally, we suggest that there are significant along-orogen disparities that may result from fundamental tectonic boundaries. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Age and tectonic framework of structurally controlled Palaeoproterozoic gold mineralization in the Häme belt of southern Finland

A number of gold occurrences in the Palaeoproterozoic Svecofennian domain of southern Finland show a clear structural control on mineralization and are classified as orogenic gold. Samples of pre-, syn- and late- to post-mineralization rocks taken from two gold prospects in the Somero-Tammela gold zone of the Häme belt have been studied to gain absolute ages of mineralization by using SIMS U–Pb of zircon and titanite, and Ar–Ar single grain step heating of hornblende and biotite. Gold mineralization in the Somero-Tammela gold zone is closely related to quartz veins and controlled by WSW–ENE to SW–NE trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of Svecofennian orogenic evolution. Zircons from a felsic volcanic host rock give an age of 1881 ± 3 Ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization. Inherited zircons in a gold-bearing quartz vein with the youngest zircon rim of 1832 ± 15 Ma give a maximum age of quartz vein emplacement. A 1786 ± 7 Ma mean 207Pb/ 206Pb age of a pegmatitic quartz-feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss, whereas an 1815 ± 4 Ma zircon age is probably closer to the crystallization age of the dyke. This age overlaps with 1805.4 ± 5.4 Ma 39Ar– 40Ar age of hornblende from a quartz vein. 1.78 and 1.76 Ga 39Ar– 40Ar biotite cooling ages mark the minimum age for mineralization. In summary, the age of mineralization can be estimated at ca. 1.82–1.79 Ga, which is in good agreement with the age of dextral shearing in southern Finland. The controlling structures, WSW–ENE to SW–NE dextral shear zones and NW–SE oriented faults, formed during approximately WNW–ESE oriented sub-horizontal shortening during crossing ductile–brittle boundary conditions. The steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures. This was supported by prolonged dextral oblique contraction enabling ± continuous or episodic slip along the faults and shear zones. The late Svecofennian tectonic episode could be described as an extension–contraction cycle in a convergent plate margin setting above a retreating subduction zone. Oblique extension (1.86–1.83 Ga) caused by slab rollback was followed by a rapid switch to oblique contraction at ca. 1.82 Ga induced by arrival of positively buoyant crustal fragments (e.g. oceanic ridge, oceanic plateau or continental sliver) at the subduction zone. The contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization. The termination of the Svecofennian orogenic evolution in Finland after 1.79 Ga might be caused by lock-up of the subduction zone and slab break-off. The striking NW–SE alignment of orogenic-type gold occurrences in the Svecofennian domain of southern Finland suggests that the late Svecofennian 1.82–1.79 Ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected.

Isotopic evidence for rapid continental growth in an extensional accretionary orogen: The Tasmanides, eastern Australia

The trace element signature of Earth's continental crust resembles that of arc lavas, but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra-subduction zone magmatism. We explore the role of coupled arc–back-arc accretionary processes in crust generation by considering the tectonic context of whole rock Nd isotope and zircon Hf–O isotope data from igneous rocks of the Australian Tasmanides (515–230 Ma), which is thought to have evolved by the repeated opening and closure of sediment-filled back-arc basins behind a long-lived subduction zone. The significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective. Granitic rocks in this area define striking secular ε Nd– ε Hf–δ 18O trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity. These trends reveal that juvenile magmatic input was enhanced during extensional, back-arc rifting episodes that immediately followed crustal thickening, suggesting a relationship between slab rollback and continental growth. Interaction between juvenile magma and sedimentary units deposited during a preceding back-arc rifting cycle was integral to the formation of stable continental material. This highlights the importance of back-arc environments for both the generation and differentiation of continental crust. The juvenile component within the Tasmanide igneous rocks increased from the Cambrian to the Triassic, consistent with a diminished input from craton-derived metasedimentary material as the subduction zone migrated outboard of the Gondwana margin. Subduction zone retreat formed large tracts of new crust in eastern Australia at comparable rates to crust generation at modern island arcs, providing a mechanism for rapid continental growth at convergent margins. Using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation.

The transition from subduction arc to slab tearing: Evidence from Capraia Island, northern Tyrrhenian Sea

The magmatic island of Capraia in the northern Tyrrhenian Sea records late stages in the evolution of a retreating subduction zone following continental accretion. Subduction rollback in the northern Tyrrhenian Sea was accompanied by an eastward migrating magmatic arc that coincided with Capraia at not, vert, similar7.7 Ma. This was followed by segmentation of the subducting slab through eastward propagating tear faults. We present high-precision, step-heating 40Ar/39Ar data that provide robust constraints for the timing of magmatism in Capraia, indicating that the island was subjected to two distinct magmatic episodes. The bulk of the island, characterised by typical calcalkaline, subduction-related volcanic rocks, formed during a relatively short period between 7.7 and 7.2 Ma. This major magmatic event was followed by a period of quiescence between 7.1 and 4.8 Ma. Magmatic activity resumed at 4.8 Ma, with the eruption of shoshonites from a monogenic centre in the southern part of the island. We interpret the latter as a magmatic event triggered by the formation of a tear fault within the subduction slab. The example provides a unique insight into the destruction of a subduction system during the interlinked process of subduction rollback, slab segmentation, and the development of slab tear faults.

Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U‐Th)/He and fission‐track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally overlying, very low grade Macigno Formation. Stratigraphic, metamorphic, and thermochronologic data indicate that the Apuane rocks were structurally buried to 15–30 km and ∼400°C at about 20 Ma. Exhumation to 240°C and 9 km depth (below sea level) occurred at 10–13 Ma. By 5 Ma the Apuane rocks were exhumed to 70°C and ∼2 km. The Macigno and associated Tuscan nappe were also structurally buried and the Macigno reached its maximum depth of 7 km at ∼15 to 20 Ma. Stratigraphic evidence indicates that the Apennine wedge was submarine at this time. Thus we infer that initial exhumation of the Apuane was coeval with tectonic thickening higher in the wedge, as indicated by synchronous structural burial of the Tuscan nappe. From 6 to 4 Ma, thinning at shallow depth is indicated by continued differential exhumation between the Apuane and the Tuscan nappe at high rates. After 4 Ma, differential exhumation ceased and the Apuane and the Tuscan nappe were exhumed at similar rates (∼0.8 km/Ma), which we attribute to erosion of the Apennines, following their emergence above sea level.

Dynamic topography above retreating subduction zones

Dynamic topography provides a measure of stresses within the Earth's interior. Dense slabs induce an upper mantle flow that deflects the surface of the Earth downward above them. By combining a simple theoretical (Stokeslet) model of subduction, gravity modeling, and seismic tomography, I show that a significant fraction (as much as 2000 m) of the topographic variations observed above the Scotia, Mariana, and Hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction.