Bernhard Stöckhert

Microdiamond daughter crystals precipitated from supercritical COH + silicate fluids included in garnet, Erzgebirge, Germany

Microdiamonds associated with phlogopite, quartz, paragonite, phengite, apatite, and rutile are found as regular constituents of minute polyphase inclusions in garnet of ultrahigh-pressure ( P ) metamorphic gneiss lenses within migmatites of the gneiss-eclogite unit, Erzgebirge, Germany. These aggregates are interpreted to represent original inclusions of a supercritical dense COH fluid rich in K, Na, and SiO 2 . From this fluid, diamond was precipitated as a daughter crystal due to cooling at ultrahigh- P conditions. Brittle failure of the garnet host due to overpressure during release of confining pressure is demonstrated by healed radial cracks. During further cooling, the silicate phase assemblage of the inclusions crystallized at reduced internal pressures outside the stability field of diamond, as indicated by the presence of quartz, paragonite, and plagioclase. It is proposed that the mica-dominated mineral assemblage of the inclusions formed by reaction between the fluid and the garnet host, the previously formed diamond daughter crystals being preserved metastably. These diamond-bearing inclusions provide an unequivocal record of dense supercritical COH fluids rich in alkalies and silica within subducted continental crust during ultrahigh- P metamorphism.

Thermochronology of the high-pressure metamorphic rocks of Crete, Greece: Implications for the speed of tectonic processes

New fission-track thermochronologic data from the high-pressure ( P )–low-temperature ( T ) rocks of Crete, Greece, combined with pressure, temperature, and stratigraphic constraints reveal that their subduction began between 36 and 29 Ma. Metamorphism took place in western Crete at peak conditions of 10 ± 2 kbar and 400 ± 50 °C between 24 and 19 Ma, and rapid exhumation to <10 km and <300 °C at a minimum rate of 4 km/m.y. was completed before 19 Ma. Constraints from the thermal history of the plate above the inferred extensional detachment reveal that tectonic unroofing contributed 85% to 90% of the overall exhumation of the high- P –low- T rocks of Crete. We propose that the Hellenic subduction zone has acted as a retreating plate boundary since at least the early Oligocene, and collision and extension during this time were driven by roll-back associated with slab-pull rather than by gravitational collapse as a consequence of crustal thickening. The speed of subduction and exhumation of the high- P –low- T rocks of Crete within ∼10 m.y. has important implications for other orogenic belts, showing that rocks can be subducted, metamorphosed at high pressure, and exhumed, despite slow overall plate convergence, within the uncertainties of many paleontologic and isotopic age data.

Constraints on the late thermotectonic evolution of the western Alps: EVidence for episodic rapid uplift

Forty-two new apatite and zircon fission track ages are presented for samples from the Western Alps in southern Switzerland, northern Italy, and southeastern France. Measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling, uplift, and exhumation of the Western Alps. Similar fission track zircon ages in the Penninic Gran Paradiso massif, Dent Blanche nappe, Sesia-Lanzo Zone, and Ivrea Zone indicate cooling of all four units to ∼225°C by 33 Ma. Differences in apatite ages reveal differential cooling of the four blocks between 33 Ma and the present. In the Sesia-Lanzo Zone, similarity of apatite ages regardless of elevation, together with near-volcanic confined fission track length patterns suggest rapid cooling and uplift at ∼25 Ma compared with slow cooling of other Western Alps units around 12 Ma. Uplift is thus not continuous but episodic, often over a short time interval beyond the resolution of other methods. Such episodes of uplift, as revealed here in the Sesia-Lanzo Zone, may be the rule rather than the exception.

Mesozoic‐Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes

Fission track (FT) analysis is applied to assess the Mesozoic and Cenozoic thermal and denudational history of the Patagonian Andes between 44° and 51°S and the geologic and geomorphic response of late Cenozoic subduction of the active Chile rise mid-oceanic spreading center on the overriding plate. Seventy-two FT ages from 43 samples are presented. Zircon FT ages indicate fast post intrusion cooling of Cretaceous parts of the Patagonian batholith and previously unreported Miocene magmatic rocks south of 48°S. Metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early Carboniferous and Late Permian. Apatite FT data reveal initiation of accelerated cooling and denudation at ca. 30 Ma at the western margin of southern continental South America followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca. 12–8 Ma at the position of the present-day main topographic divide. This migration is proposed to be related to either coeval eastward migration of the retroarc deformation, the effects of subduction erosion in the overriding plate at the Peru-Chile trench or less likely, shallowing of the angle of subduction. East of the divide, <3 km of denudation has occurred since the Late Cretaceous. Enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca. 28–26 Ma that triggered orographically enhanced precipitation on the west side of the Patagonian Andes allowing increased erosion by fluvial incision and mass transport processes. The actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation.

Thermobarometric data from a fossil zircon partial annealing zone in high pressure–low temperature rocks of eastern and central Crete, Greece

Abstract A fossil partial annealing zone of fission tracks in zircon is described from high pressure–low temperature (HP–LT) rocks of the Phyllite–Quartzite Unit (PQ) on the island of Crete, Greece. Correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data, and with new experimental annealing results, allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples. The zircon fission track (FT) ages of samples from the PQ across Crete range from original detrital signature through reduced to completely reset. The annealing is the result of a single heating period related to the HP–LT metamorphism with near-peak temperatures lasting for only a few million years some time between 24±1 and 20±1 Ma. In eastern Crete, where rocks have experienced temperatures of 300±50 °C and pressures of 0.8±0.3 GPa, zircon FT ages range from 414±24 to 145±10 Ma. Ages above 300 Ma occur mostly near the east coast of the island in rocks which have not been heated to above ca. 280 °C and probably represent a pre-Variscan source. Track lengths are already indicative of a substantial annealing at this temperature. Most of the zircon FT ages from eastern Crete scatter within error around the stratigraphic age. Samples with apparent zircon FT ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca. 320 °C. Towards the west, a sudden decrease to very young ages ranging from 17±2 to 18±1 Ma reflects a complete resetting at ca. 350 °C. Short tracks, however, are still observed. Throughout the central and western part of the island, ages are consistently below 22 Ma. Thermobarometric data for this area indicate maximum temperatures of 400±50 °C and pressures of 1±0.3 GPa. Only samples from western Crete, which have been exposed to 400±50 °C, show exclusively long tracks. Consequently, the high temperature limit of the zircon partial annealing zone (ZPAZ) appears to be between 350 and 400 °C. A significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0.5 GPa and at different temperatures, which fit the curves previously obtained by other authors at ambient pressure.

Low effective viscosity during high pressure metamorphism due to dissolution precipitation creep: the record of HP–LT metamorphic carbonates and siliciclastic rocks from Crete

The (micro)structural record of high pressure–low temperature (HP–LT) metamorphic rocks (T=400±50°C, P=10±2 kbar) of the Phyllite-Quartzite Unit in western Crete, Greece, is interpreted in terms of deformation mechanisms and flow stress. Phyllites were deformed at low stress by dissolution precipitation creep, governed by strongly enhanced dissolution along quartz–mica (001) phase boundaries. Quartzites and quartz veins were deformed by dislocation creep at higher flow stress. The contrasting effective viscosities caused stress concentration in the quartzites and quartz veins. Also, microstructures from aragonite marbles indicate that dissolution precipitation creep was the dominant deformation mechanism and that dislocation creep has not been activated in these rocks. Pervasive ductile deformation was restricted to HP–LT metamorphic conditions and the microstructural record of deformation at maximum depth has been preserved, with all subsequent deformation localized and confined to the brittle field. Constraints on the timing of deformation allow an estimation of strain rates. Experimentally determined flow laws for dislocation creep are used to pose upper bounds on flow stress and bulk viscosities of rocks deformed by dissolution precipitation creep. For the phyllites, a conservative estimate is about 1019 Pa s, or below, in contrast to 1020 Pa s derived for the quartzites. This compares well to the viscosity contrast of 1–2 orders of magnitude reflected by the mesoscopic structures. Since phyllites and carbonate rocks form large portions of the subducted sedimentary pile, the low flow stress during rapid deformation of these rocks at HP–LT metamorphic conditions, and the lack of deformation along the burial and exhumation path, imply very low strength of the plate boundary shear zones and negligible shear heating.

Exhumation of ultrahigh-pressure metamorphic oceanic crust from Lago di Cignana, Piemontese zone, western Alps: the structural record in metabasites

The metamorphic and deformational history of coesite-bearing, ultrahigh-pressure metamorphic oceanic crust from Lago di Cignana, Valtoumanche, western Alps, as recorded along the decompression and cooling path, has been resolved to derive constraints on the physical state of the crust during exhumation. The following history is proposed: Eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km, in the stability field of coesite. The deformation was progressively localised into shear bands. Over the first 40 km of subsequent exhumation, the rocks remained essentially unmodified. Any deformation must have been localised beyond the limits of the present outcrop. Retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500°C. Subsequently, the system size increased. Fluid infiltration, focussed along tensile fractures, is documented by the formation of veins. Vein shapes and orientations indicate high pore-fluid pressures and low differential stress. After cooling to about 350–450°C at depths of 12 to 18 km, localised ductile deformation transformed the retrogressed eclogites into greenschists. For that stage, the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress. Final cooling to below ca. 300°C took place at depths of 7 to 12 km, as indicated by the density of fluid inclusions. Later deformation in the brittle field cannot be correlated with the P-T path.

Rheology of Crustal Rocks at Ultrahigh Pressure

An improved understanding of tectonic processes in the deep levels of subduction zones and collisional belts requires information on the mechanical behavior of continental crust during ultrahigh-pressure (UHP) metamorphism. Predictions are based on the results of experimental deformation of minerals stable at ultrahigh pressure and on the anticipated effect of pressure on deformation mechanisms. Flow laws for dislocation creep of coesite and aragonite indicate that, at ultrahigh pressure, the strength of continental material remains well below a few MPa at natural strain rates. We question the high strength that has been inferred for eclogites, based on preliminary experimental data on jadeite and on natural microstructures of omphacite. The (micro)structural record of natural UHP rocks indicates that strain localization into weak shear zones, albeit not yet identified, must be common. We propose that the presence of dense fluids or hydrous melts at grain and solid phase boundaries could accomplish deformation analogous to liquid phase sintering in ceramics. The low strength of continental material at ultrahigh pressure precludes notable shear heating, thus cool geotherms and P-T paths are implied. The low strength also places upper bounds on the stress drop of seismic events in presently subducted continental crust and limits the size of coherent subducted continental slices.

PRESSURE SOLUTION IN SILICICLASTIC HP-LT METAMORPHIC ROCKS : CONSTRAINTS ON THE STATE OF STRESS IN DEEP LEVELS OF ACCRETIONARY COMPLEXES

Abstract The magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured. The only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high-pressure/low-temperature metamorphic rocks recorded during their crustal evolution. Microfabrics in HP-LT metamorphic ( T = 400 ± 50°C, P = 10 ± 2 kbar) phyllites and quartzites of the Phyllite-Quartzite Unit on the island of Crete, southern Aegean, reveal (1) that progressive deformation was by pressure solution (dissolution precipitation creep), (2) that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation, (3) that the unilaterally rational (001) mica quartz phase boundaries were sites of strongly enhanced dissolution, and (4) dislocation creep was restricted to a minor role in quartzites poor in mica. Consequently, the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial, to a depth of more than 30 km, and subsequent exhumation. Currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 MPa at temperatures around 400°C at a depth of 30 km. This implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep.

HIGH DIFFERENTIAL STRESS AND SUBLITHOSTATIC PORE FLUID PRESSURE IN THE DUCTILE REGIME - MICROSTRUCTURAL EVIDENCE FOR SHORT-TERM POST-SEISMIC CREEP IN THE SESIA ZONE, WESTERN ALPS

Abstract The microstructures developed during a late stage of inhomogeneous ductile deformation in the Sesia Zone (Western Alps, lower Aosta valley, Italy) suggest an exceptionally high flow stress. Quartz is recrystallized with a grain size down to ca. 5 μm, jadeite and omphacite are deformed by mechanical twinning, calcite reveals very high twin densities up to 400 mm−1, and garnet in mylonites deformed by cataclastic flow. Based on available paleopiezometers these microstructures indicate a differential stress on the order of 300±100 MPa. Plastic flow under these conditions requires a high effective confining pressure and is incompatible with a lithostatic pore fluid pressure, as commonly assumed for the ductile regime. The Goetze criterion predicts fully plastic flow for σ1−σ3