Manfred R. Brix

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.

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.

Miocene high-pressure metamorphic rocks of Crete, Greece: rapid exhumation by buoyant escape

Abstract The pre-Neogene thrust sheets of Crete, Greece, accreted during Oligocene and Early Miocene time, can be divided into two main groups juxtaposed by a Miocene extensional detachment. Oligo-Miocene high pressure-low temperature (HP-LT) metamorphic rocks crop out in the lower plate to the detachment, and rocks that show no evidence of Tertiary metamorphism in the upper plate. Detailed pressure, temperature and structural information from the HP-LT metamorphic rocks combined with new fission-track data from the upper plate reveal that the lower plate rocks were subducted, then pervasively deformed and metamorphosed at their maximum depth of burial (30–35 km, 300–400°C) between c. 24 and 19 Ma and then exhumed to less than 10 km depth at rates >4 km Ma−1 before c. 17 Ma. Microstructural studies reveal that during exhumation the lower plate acted as a coherent block, with deformation and retrograde metamorphism localized along the extensional detachment. The rocks of the upper plate can be shown to have remained in the upper 4–7 km of the crust since at least Eocene time. After cessation of movement along the main extensional detachment, both the upper and lower plates were subjected to brittle extension and increased erosion initiated at c. 16–17 Ma. These boundary conditions imply continuous subduction retreat since at least Eocene time along the Aegean segment of the Hellenic convergent plate boundary. A tectonic model is presented where exhumation is driven by positive buoyancy of the subducted continental crust following lithospheric delamination. We propose that the subducted microcontinent was exhumed by a process of ‘oblique buoyant escape’ and entered the space created by the retreating subducting oceanic slab.

Crustal history of Margarita Island (Venezuela) in detail: Constraint on the Caribbean plate-tectonic scenario

The pressure-temperature-time-deformation evolution for the crust of Margarita Island (Venezuela) has been established to allow comparison with current plate-tectonic models for the Caribbean region. On Margarita, the 12 recognizable stages of development can be summarized in terms of the following evolving tectonic settings: Protolith evolution as Aptian-Albian or older oceanic crust, as well as continental crust with Paleozoic basement (stages 1 and 2); accretion and high-pressure metamorphism (500–600 °C, 10–14 kbar) as the Margarita Complex in the deep level of a fore arc at 100–90 Ma (stage 3); ascent, cooling, and emplacement into the intermediate crustal level of a volcanic arc at 90–80 Ma (stage 4); transform plate-margin setting at a comparable level at 80–50 Ma (stage 5); second episode of rapid uplift and cooling (stages 6 and 7); and shallow crustal level close to transform plate margin from 50 Ma to present (stages 8 to 12). This complex sequence is in excellent agreement with plate-tectonic scenarios that require a Pacific origin for the Caribbean plate and eastward migration of the Margarita Complex and its correlatives along northern South America since the Cretaceous.

Apatite Fission-Track Thermochronology of the Uppermost Tectonic Unit of Crete, Greece: Implications for the Post-Eocene Tectonic Evolution of the Hellenic Subduction System

Apatite fission-track thermochronology is applied to 31 samples from various components of the uppermost tectonic unit of the Island of Crete, Greece. This unit is one of several that presently lie in the hanging wall to a major extensional detachment fault that juxtaposes rocks metamorphosed at high pressure and low temperature during the Late Oligocene — Early Miocene below against rocks that lack any Oligo-Miocene metamorphism. The data reveal a phase of accelerated Middle Miocene denudation that can be linked to erosion between ca. 17 Ma and 11 Ma. The discovery of the base of a denuded apatite partial annealing zone limits the total amount of denudation during this time period to ca. 4km. This implies a mean denudation rate of ca. 650 m/m.y.

Jurassic sedimentation of the Miers Bluff Formation, Livingston Island, Antarctica: Evidence from SHRIMP U-Pb ages of detrital and plutonic zircons

Detrital zircon populations from two sandstone samples from the lower member (Johnsons Dock Member) of the Miers Bluff Formation at Hurd Peninsula have been dated by the Sensitive High Resolution Ion Microprobe (SHRIMP) U–Pb method. In one of the samples, zircons as young as early Middle Jurassic (Bajocian) age are present. In the second sample, the youngest detrital zircons are Middle Triassic in age. The detrital zircon age spectra indicate that Permian, early Palaeozoic and Meso- to Neoproterozoic zircon bearing rocks were present in the source areas of the Miers Bluff Formation. The sedimentary rocks are intruded by the Hespérides Point Intrusive diorite stock which yielded a U–Pb zircon crystallization age of 137.7 ± 1.4 Ma (Early Cretaceous, Valanginian). These results indicate that sedimentation of the Johnsons Dock Member of the Miers Bluff Formation is bracketed in time between the Bajocian and the Valanginian. The Miers Bluff Formation has been correlated with the Trinity Peninsula Group from the Antarctic Peninsula, based on sedimentological and structural similarity. Since the Trinity Peninsula Group is older than Middle Jurassic a direct chronological correlation is not supported by our new U–Pb zircon data. However, we suggest that the tectonic setting may have migrated in time with deposition of the pre-Middle Jurassic TPG on the peninsula, to Livingston Island where the maximum age for deposition of the MBF is Bajocian (about 170 Ma).

Tracing an Early Jurassic magmatic arc from South to East China Seas

Drilling has revealed suites of magnesian granite and diorite emplaced in Early Jurassic time (198-195 Ma) and an arc-related low-temperature (678 to 696 °C) magmatism in NE South China Sea. These rocks have 87Sr/86Sri (0.705494 to 0.706623) and eNdt (−0.9 to +2.2) as evidence of evolved mantle-derived magmas, coupled with enriched fluid-mobile elements Cs to K and Pb implying involvement of subduction-zone fluids. Another Early Jurassic granodiorite (zircon U-Pb 187 Ma) drilled from the SW East China Sea, is a magnesian high-K calc-alkaline, is comparably confined to a range of low-temperature (~675 °C) arc-related granite, characterized by enrichment of fluid-mobile elements and Nb-Ta depletion. Its Sr-Nd isotopes (87Sr/86Sri = 0.705200, eNdt = 1.1) suggest a product of evolved mantle-derived melts. Together with detrital igneous zircons from Paleocene sequences, these observations reveal an Early Jurassic arc-related low-temperature (600 to 740 °C) magmatism in the SW East China Sea. These arc-related granitoids, along with those from SE Taiwan, could define an Early Jurassic NE-SW-trending Dongsha-Talun-Yandang magmatic arc zone along the East Asian continental margin paired with Jurassic accretionary complexes from SW Japan, E Taiwan to the W Philippines. This arc-subduction complex assembly was associated with oblique subduction of the paleo-Pacific slab beneath Eurasia, presumably responsible for early Jurassic lithospheric extension in South China block.