Discovery of an Eo-Meso-Neoarchean Terrane in the East Greenland Caledonides

tThis study investigates basement gneisses from the Niggli Spids thrust sheet of the East Greenland Caledonides in an attempt to place them into the broader context of the Archean–Paleoproterozoicarchitecture of the Greenland shield. Our combined whole-rock geochemical and U–Pb zircon geochronol-ogy results from Gåseland reveal an Archean terrane defined by TTG magmatic events at 3607 Ma and3070–2980 Ma followed by metamorphism and high-K granite intrusion at 2790–2677 Ma. These results identify a relatively pristine Archean terrane with previously unknown Eoarchean rocks that holds poten-tial for future investigations into the early evolution of continental crust, and adds to a growing body of data characterizing the Archean–Paleoproterozoic architecture of East Greenland

Detrital zircon geochronology and evolution of the Nacimiento block late Mesozoic forearc basin, central California coast

Forearc basins are first-order products of convergent-margin tectonics, and their sedimentary deposits offer unique perspectives on coeval evolution of adjacent arcs and subduction complexes. New detrital zircon U-Pb geochronologic data from 23 sandstones and 11 individual conglomerate clasts sampled from forearc basin strata of the Nacimiento block, an enigmatic stretch of the Cordilleran forearc exposed along the central California coast, place constraints on models for forearc deformation during evolution of the archetypical Cordilleran Mesozoic margin. Deposition and provenance of the Nacimiento forearc developed in three stages: (1) Late Jurassic– Valanginian deposition of lower Nacimiento forearc strata with zircon derived from the Jurassic–Early Cretaceous arc mixed with zircon recycled from Neoproterozoic– Paleozoic and Mesozoic sedimentary sources typical of the continental interior; (2) erosion or depositional hiatus from ca. 135 to 110 Ma; and (3) Albian–Santonian deposition of upper Nacimiento forearc strata with zircon derived primarily from the Late Cretaceous arc, accompanied by Middle Jurassic zircon during the late Albian–Cenomanian. These data are most consistent with sedimentary source terranes and a paleogeographic origin for the Nacimiento block south of the southern San Joaquin Basin in southern California or northernmost Mexico. This interpreted paleogeographic and depositional history of the Nacimiento block has several implications for the tectonic evolution of the southern California Mesozoic margin. First, the Nacimiento forearc depositional history places new timing constraints on the Early Cretaceous unconformity found in forearc basin strata from the San Joaquin Valley to Baja California. This timing constraint suggests a model in which forearc basin accommodation space was controlled by accretionary growth of the adjacent subduction complex, and where tectonic events in the forearc and the arc were linked through sediment supply rather than through orogenic-scale wedge dynamics. Second, a paleogeographic origin for the Nacimiento forearc south of the southern San Joaquin Valley places new constraints on end-member models for the kinematic evolution of the Sur-Nacimiento fault. Although this new paleogeographic reconstruction cannot distinguish between sinistral strike-slip and thrust models, it requires revision of existing sinistral-slip models for the Sur-Nacimiento fault, and it highlights unresolved problems with the thrust model

Size and Exhumation Rate of Ultrahigh-Pressure Terranes Linked to Orogenic Stage

A growing set of data indicates a stark contrast between the evolution of two types of ultrahigh-pressure (UHP) terranes: large terranes that evolved slowly (over 10–30 Myr), and small terranes that formed and were exhumed on timescales of \u3c 10 Myr. Here we compare the characteristics – area, thickness, formation rate, exhumation rate, age, and tectonic setting – of these two endmember types of UHP terrane worldwide. We suggest that the two UHP terrane types may form during different orogenic stages because of variations in the buoyancy and traction forces due to different proportions of subducting crust and mantle lithosphere or to different rates of subduction. The initial stages of continent collision involve the subduction of thin continental crust or microcontinents, and thus tectonic forces are dominated by the density of the oceanic slab; subduction rates are rapid and subduction angles are initially steep. However, as collision matures, thicker and larger pieces of continental material are subducted, and the positive buoyancy of the down-going slab becomes more prominent; subduction angles become gentle and convergence slows. Assessing the validity of this hypothesis is critical to understanding the physical and chemical evolution of Earth\u27s crust and mantle. Included here is the post-print copy of this article. The final publication is available via ScienceDirect at http://www.sciencedirect.com/science/article/pii/S0012821X1100756

Almacık Complex-an exhumed lower to middle crust in northwest Anatolia

The Almacık Complex is a tectonic unit of high-grade metamorphic rocks in the Intra-Pontide Suture Zone in northwest Turkey. It consists mainly of amphibolite, metaultramafic rock, and gneiss, which are intruded by numerous pre-, syn-, and posttectonic felsic veins. The Almacık Complex is variously interpreted as a Cretaceous or Neoproterozoic ophiolite, or a Permian mafic-ultramafic complex representing the middle to lower crust of the Sakarya Zone. Herein, new petrological and geochronological data were presented from the Almacık Complex. Two-pyroxene geothermometry in the metawebsterites indicated that the Almacık Complex has undergone upper amphibolite-facies metamorphism at 750 ± 30 °C and 8 ± 4 kbar. U-Pb zircon and Ar-Ar ages from 11 samples indicate the presence of late Neoproterozoic, Permian, and Jurassic thermal events. Most of the Almacık Complex consists of late Neoproterozoic amphibolites and gneisses, representing the basement of the İstanbul Zone. This basement was intruded by voluminous mafic magma during the Late Permian. The basement and the Permian ultramafic-mafic rocks subsequently underwent upper amphibolite-facies metamorphism during the Jurassic, possibly at the base of a magmatic arc

High-Temperature Deformation During Continental-Margin Subduction & Exhumation: The Ultrahigh-Pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction

Age of eclogite-facies metamorphism and exhumation in northwestern Bhutan

Across northern Bhutan, cryptic south-directed thrusts place granulite-facies rocks of the Greater Himalayan Sequence (GHS) over amphibolite-facies rocks belonging to the same unit. Within this northern high-T region, rare garnet-bearing mafic rocks are interpreted as retrogressed eclogite, based on textural information that includes the presence of symplectite after omphacite. Previous studies have suggested that eclogite-facies metamorphism peaked at ~16–14 Ma, with subsequent rapid exhumation at initially high temperatures. We report new zircon U/Th–Pb analyses from three granulitized eclogites collected in the Jomolhari region of northwest Bhutan, about 40 km SW of the previous locality and separated from it by a klippen of structurally higher Tethyan Sedimentary Sequence rocks. All contain grt (partially replaced by hbl + plag ± opx symplectites) and cpx–plag symplectite after former omphacite. Zircon cores and rims were analyzed by split stream LA–ICPMS at the UC Santa Barbara geochronology facility. Two samples yield a relatively tight cluster of lower intercept and concordant dates from 18–17 Ma in one sample, and ~20–16 Ma in the other. A few rim analyses in the second sample yield dates as young as 15–13 Ma. The third sample yields older concordant to slightly discordant zircon cores ranging from 28–22 Ma, with no younger ages. All these data are consistent with previously reported 36–18 Ma U/Th–Pb monazite ages from pelites in this region. These new zircon dates indicate that the GHS in the Jomolhari Massif resided at high temperatures in the mid- to lower orogenic crust for a prolonged period prior to c. 18 Ma. The Jomolhari domain was then rapidly exhumed between 18–15 Ma, potentially 2–3 Ma earlier than similar rocks in northern Bhutan (~40 km northeast from Jomolhari). Whether this difference represents either a coherent tectonic unit (during prograde–peak conditions) across north and northwest Bhutan that exhumed diachronously, or separate evolution within units following different kinematic paths, is currently being investigated

Late Paleocene – Middle Eocene magmatic flare-up in western Anatolia

A 3000-km long magmatic belt of predominantly Eocene age extends from Anatolia into Iran representing a major magmatic flare-up. We present new zircon U-Pb, Ar/Ar mica and apatite fission-track ages for this magmatism from northwestern Turkey, and review its geochemistry and geodynamic setting. The new age data show that magmatism started at the Late Paleocene (58 Ma) during the final stages of continental collision and continued into the early Middle Eocene (45 Ma) with most of the magmatism taking place in the Early-Middle Eocene (54 to 45 Ma). The Late Paleocene-Middle Eocene magmatism is separated from Late Cretaceous and Oligo-Miocene magmatic flare-ups by periods of magmatic quiescence. The Late Paleocene-Middle Eocene magmatism consists of plutonic and volcanic belts. The plutonic belt cuts across and post-dates the İzmir-Ankara suture. The plutonic rocks are mainly middle- to high-K calc-alkaline I-type granodiorite and granite, and the volcanic rocks are middle- to high-K calc-alkaline basalt, basaltic andesite and andesite. Geochemically, all the rocks are similar to those found in subduction-related environments. Crustal thicknesses calculated based on geochemistry suggest a thickened crust (60–70 km) at 58 to 54 Ma, and a relatively thin crust (ca. 40 km) at 54 to 45 Ma, which match with uplift and erosion during the Late Paleocene, and marine sedimentation during the Early-Middle Eocene in northwest Anatolia, respectively. The Late Paleocene-Middle Eocene magmatism is tentatively assigned to subduction of the southern branch of the Neo-Tethys

Palaeocene faulting in SE Sweden from U–Pb dating of slickenfibre calcite

Estimating the timing of faulting is crucial to modelling tectonics, palaeoseismicity, landscape evolution and fault mechanics. Four slickenfibre calcite samples from a conjugate strike-slip fault set in a platformal limestone, SE Sweden, were dated using U–Pb. Three of the samples yielded an average age of 64.8 ± 6.5 Ma, while the fourth yielded a marginally younger age of 54.7 ± 5.5 Ma. Precipitation of the fibres is interpreted as syn-deformational. Age uncertainty and dispersion reflect incorporation of common Pb and tiny host-rock components into the dated calcite and/or possible fault reactivation through ca. 55 Ma. We infer from crystal characteristics, stable isotopes (δ18O and δ13C) and rare-earth elements that fibres formed in an environment rich in deep-seated fluids, at temperatures of 40–200°C, with shear stresses exceeding 10 MPa and at a maximum burial depth of c. 4 km. This Palaeocene faulting may reflect far-field stresses from shortening in the Alps

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland, USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (∼1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics