Ultrahigh-pressure (UHP) metamorphic terranes reflect subduction of continental crust to depths of 90–140 km in Phanerozoic contractional orogens. Rocks are intensely overprinted by lower pressure mineral assemblages; traces of relict UHP phases are preserved only under kinetically inhibiting circumstances. Most UHP complexes present in the upper crust are thin, imbricate sheets consisting chiefly of felsic units ± serpentinites; dense mafic and peridotitic rocks make up less than 10% of each exhumed subduction complex. Roundtrip prograde–retrograde P–T paths are completed in 10–20 Myr, and rates of ascent to mid-crustal levels approximate descent velocities. Late-stage domical uplifts typify many UHP complexes.
Sialic crust may be deeply subducted, reflecting profound underflow of an oceanic plate prior to collisional suturing. Exhumation involves decompression through the P–T stability fields of lower pressure metamorphic facies. Scattered UHP relics are retained in strong, refractory, watertight host minerals (e.g., zircon, pyroxene, garnet) typified by low rates of intracrystalline diffusion. Isolation of such inclusions from the recrystallizing rock matrix impedes back reaction. Thin-aspect ratio, ductile-deformed nappes are formed in the subduction zone; heat is conducted away from UHP complexes as they rise along the subduction channel. The low aggregate density of continental crust is much less than that of the mantle it displaces during underflow; its rapid ascent to mid-crustal levels is driven by buoyancy. Return to shallow levels does not require removal of the overlying mantle wedge. Late-stage underplating, structural contraction, tectonic aneurysms and/or plate shallowing convey mid-crustal UHP décollements surfaceward in domical uplifts where they are exposed by erosion. Unless these situations are mutually satisfied, UHP complexes are completely transformed to low-pressure assemblages, obliterating all evidence of profound subduction. 相似文献
We have used a coupled thermo-mechanical finite-element (FE) model of crustal deformation driven by mantle/oceanic subduction to demonstrate that the tectonic evolution of the Lachlan Fold Belt (LFB) during the Mid-Palaeozoic (Late Ordovician to Early Carboniferous) can be linked to continuous subduction along a single subduction zone. This contrasts with most models proposed to date which assume that separate subduction zones were active beneath the western, central and eastern sections of the Lachlan Orogen. We demonstrate how the existing data on the structural, volcanic and erosional evolution of the Lachlan Fold Belt can be accounted for by our model. We focus particularly on the timing of fault movement in the various sectors of the orogen. We demonstrate that the presence of the weak basal decollement on which most of the Lachlan Fold Belt is constructed effectively decouples crustal structures from those in the underlying mantle. The patterns of faulting in the upper crust appears therefore to be controlled by lateral strength contrasts inherited from previous orogenic events rather than the location of one or several subduction zones. The model also predicts that the uplift and deep exhumation of the Wagga-Omeo Metamorphic Belt (WOMB) is associated with the advection of this terrane above the subduction point and is the only tectonic event that gives us direct constraints on the location of the subduction zone. We also discuss the implications of our model for the nature of the basement underlying the present-day orogen. 相似文献
Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion. 相似文献
The Inner Arc of the Central Andes, broadly corresponding to the Eastern Cordillera, is the location of a rich Tertiary and Triassic Sn–W–(Ag-base metal) metallogenic province, commonly referred to as the Bolivian tin belt. We propose that the Tertiary metallogeny, which generated most of the tin ores, was a direct consequence of discrete “collisions” between the South American plate and the Nazca slab and sub-slab mantle, during the ongoing Andean orogeny. Evidence supporting this proposal include: (1) the coincidence of the tin province and the Inner Arc in a marked “hump” in the Andean orogen, which may represent tectonic indentation; (2) the symmetry of the tin province with respect to the Bolivian orocline, the axis of which corresponds to the direction of highest compression; (3) the relative symmetry of the magmatism and tin mineralization with respect to this axis; (4) the concurrent timing of mineralization and compressional pulses; (5) the similar host rock geochemistry and ore lead isotope data, testifying to a common crustal reservoir; and (6) the striking similarity of the igneous suites, associated with the ore deposits to those from “typical” collisional orogens. A number of studies have called upon a persistent tin anomaly to explain the metallogeny of the region. We propose, instead, that the latter is better explained by periodic compressional interaction between the Farallon/Nazca oceanic plate and the South American continent. This led to the generation of peraluminous magmas, which during fractional crystallization exsolved the fluids responsible for the voluminous Sn–W mineralization. 相似文献
Coupled thermal‐mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr?1, and a slope‐dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m?3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m?3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro‐side of the system, and is driven by a depth‐dependent pressure gradient that is facilitated by the development of a temperature‐dependent low viscosity horizon in the mid‐crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post‐convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust‐related exhumation on the flanks. Velocities in the pro‐side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro‐side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal‐sense offset at its upper boundary, reverse‐sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen. 相似文献