162 research outputs found
Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins
The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P-T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5-3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10-20Myr (after 400-600km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P-T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable li
Quartz microstructures in nature and experiment — evidence of rapid plastic deformation and subsequent annealing
Quartz microstructures produced in
short-term deformation and annealing
experiments are compared with those in
naturally deformed vein quartz in cores
from the Long Valley Exploratory Well
(Long Valley Caldera, California). The
experiments are designed to simulate
i) co-seismic deformation of quartz in
the uppermost plastosphere and
ii) annealing during post-seismic
stress relaxation.
The experiments are performed in a
modified Griggs type solid medium apparatus.
Natural polycrystalline quartz
samples (grain size on the order of millimetres)
are deformed at a temperature
of 400°C, a confining pressure of 2GPa,
and strain rates of ca. 10−4 s−1. The
differential stress reaches 2–4GPa and
the irreversible axial shortening is typically
a few percent. In some experiments
the samples have subsequently
been annealed for ca. 14–15 h at elevated
temperatures of 800–1000°C and
low stresses (quasi-hydrostatic or nonhydrostatic
conditions)...conferenc
Numerical simulations of an ocean/continent convergent system: influence of subduction geometry and mantle wedge hydration on crustal recycling
The effects of the hydration mechanism on continental crust recycling are
analyzed through a 2D finite element thermo-mechanical model. Oceanic slab
dehydration and consequent mantle wedge hydration are implemented using a
dynamic method. Hydration is accomplished by lawsonite and serpentine
breakdown; topography is treated as a free surface. Subduction rates of 1, 3,
5, 7.5 and 10 cm/y, slab angles of 30o, 45o and 60o and a mantle rheology
represented by dry dunite and dry olivine flow laws, have been taken into
account during successive numerical experiments. Model predictions pointed out
that a direct relationship exists between mantle rheology and the amount of
recycled crustal material: the larger the viscosity contrast between hydrated
and dry mantle, the larger the percentage of recycled material into the mantle
wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic
evolution of recycled material is influenced by subduction style. TPmax,
generally representative of eclogite facies conditions, is sensitive to changes
in slab dip. A direct relationship between subduction rate and exhumation rate
results for different slab dips that does not depend on the used mantle flow
law. Thermal regimes predicted by different numerical models are compared to PT
paths followed by continental crustal slices involved in ancient and recent
subduction zones, making ablative subduction a suitable pre-collisional
mechanism for burial and exhumation of continental crust.Comment: 10 figures, 3 table
Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?
Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He, 40Ar/39Ar hornblende, and U-Pb zircon ages from the granites of Tinos Island in the Aegean Sea, Greece, suggest, together with published ZFT data, that there are three extensional detachments on Tinos. The Tinos granites crosscut the Tinos detachment. Cooling of the granites was controlled by the Livadi detachment, which occurs structurally above the Tinos detachment. Our U-Pb zircon age is 14.6 ± 0.2 Ma and two 40Ar/39Ar hornblende ages are 14.4 ± 0.4 and 13.7 ± 0.4 Ma. ZFT and AFT ages go from 14.4 ± 1.2 to 12.2 ± 1.0 Ma and 12.8 ± 2.4 to 11.9 ± 2.0 Ma. (U-Th)/He ages are from 10.4 ± 0.2 to 9.9 ± 0.2 Ma (zircon) and 11.9 ± 0.5 to 10.0 ± 0.3 Ma (apatite). All ages decrease northeastward in the direction of hanging wall transport on the Livadi detachment and age-distance relationships yield a slip rate of 2.6 (+3.3 / −1.0) km Ma−1. This rate is smaller than a published slip rate of 6.5 km Ma−1 for the Vari detachment, which is another detachment structurally above the Tinos detachment. Because of the different rates and because published ZFT ages from the footwall of the Vari detachment are ∼10 Ma, we propose that the Vari detachment has to be distinguished from the older Livadi detachment. We discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the Vari detachment cut down into the footwall of the Livadi detachment successively exhuming deeper structural units. The thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation
Integration of natural data within a numerical model of ablative subduction: A possible interpretation for the Alpine dynamics of the Austroalpine crust
A numerical modelling approach is used to validate the physical and ge-
ological reliability of the ablative subduction mechanism during Alpine con-
vergence in order to interpret the tectonic and metamorphic evolution of an
inner portion of the Alpine belt: the Austroalpine Domain. The model pre-
dictions and the natural data for the Austroalpine of the Western Alps agree
very well in terms of P-T peak conditions, relative chronology of peak and
exhumation events, P-T-t paths, thermal gradients and the tectonic evolu- tion
of the continental rocks. These findings suggest that a pre-collisional
evolution of this domain, with the burial of the continental rocks (induced by
ablative subduction of the overriding Adria plate) and their exhumation (driven
by an upwelling flow generated in a hydrated mantle wedge) could be a valid
mechanism that reproduces the actual tectono-metamorphic config- uration of
this part of the Alps. There is less agreement between the model predictions
and the natural data for the Austroalpine of the Central-Eastern Alps. Based on
the natural data available in the literature, a critical discus- sion of the
other proposed mechanisms is presented, and additional geological factors that
should be considered within the numerical model are suggested to improve the
fitting to the numerical results; these factors include varia- tions in the
continental and/or oceanic thickness, variation of the subduction rate and/or
slab dip, the initial thermal state of the passive margin, the oc- currence of
continental collision and an oblique convergence.Comment: 11 Figures and 3 Tabe
Structural, petrological and chemical analysis of syn-kinematic migmatites: insights from the Western Gneiss Region, Norway.
International audienceEvidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnets that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (1) as zoned segregations found in high-pressure (HP) (ultra) mafic bodies, (2) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss, and (3) as undeformed pegmatites cutting the main Caledonian structures. Segregations post-date the eclogite facies foliation and predate the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post-kinematic and were formed at the end of the deformation history. Geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca-poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K-poor compositions corresponds to segregation hosted in (ultra) mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward-buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800°C implies the presence of an H2O-rich fluid phase at great depth to facilitate H2O-present partial melting
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