Stuart N. Thomson

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.

Glaciation as a destructive and constructive control on mountain building

Theoretical analysis predicts that enhanced erosion related to late Cenozoic global cooling can act as a first-order influence on the internal dynamics of mountain building, leading to a reduction in orogen width and height. The strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion, producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude, where long-term glacier mass gain equals mass loss. However, acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive. Here we present an extensive new low-temperature thermochronologic data set from the Patagonian Andes, a high-latitude active orogen with a well-documented late Cenozoic tectonic, climatic and glacial history. Data from 38° S to 49° S record a marked acceleration in erosion 7 to 5 Myr ago coeval with the onset of major Patagonian glaciation and retreat of deformation from the easternmost thrust front. The highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen, as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity. In contrast, towards higher latitudes (49° S to 56° S) a transition to older bedrock cooling ages signifies much reduced late Cenozoic erosion despite dominantly glacial conditions here since the latest Miocene. The increased height of the orogenic divide at these latitudes (well above the equilibrium line altitude) leads us to conclude that the southernmost Patagonian Andes represent the first recognized example of regional glacial protection of an active orogen from erosion, leading to constructive growth in orogen height and width.

Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 42°S and 46°S: An appraisal based on fission-track results from the transpressional intra-arc Liquiñe-Ofqui fault zone

Fission-track (FT) thermochronology has been applied to investigate the low-temperature cooling and denudation history of the Patagonian Andes along the southern part of the intra-arc transpressional Liquine-Ofqui fault zone between 42° and 46°S. The Liquine-Ofqui fault is shown to have been the focus of enhanced cooling and denudation initiated between ca. 16 and 10 Ma. Several fault blocks with different cooling histories are identified; these are separated by major oblique- or reverse-slip faults proposed to form the eastern part of a major (crustal-scale) dextral transpression zone. Local very fast rates of cooling and denudation between ca. 7 and 2 Ma were coeval with collision of the Chile Rise (an active mid-oceanic ridge) with the Peru-Chile Trench between ∼47° and 48°S. This location is close to the southern termination of the Liquine-Ofqui fault, implying that the collision of the ridge was a major force driving late Cenozoic transpression. The lack of significant cooling and denudation before ca. 16 Ma is indicative of pure strike-slip or transtensional movement along the Liquine-Ofqui fault before the collision of the ridge. Digital landscape analysis supports glacial and periglacial erosion as the main contributor to denudation since ca. 7 Ma, leading to restriction of topographic development. The combination of transpression-induced rock uplift and glacial erosion is shown to be very effective at causing localized denudation. Anomalously young FT ages along the Liquine-Ofqui fault are attributed to the existence of a late Cenozoic localized heat-flow anomaly along the fault.

Fission track analysis of the crystalline basement rocks of the calabrian arc, southern italy: evidence of oligo-miocene late-orogenic extension and erosion

Abstract Fission track analysis is used to provide low temperature and time constraints on the late-orogenic cooling and exhumational history of the crystalline basement rocks of the Calabrian Arc, southern Italy. 65 samples yielded 57 apatite fission track ages, 54 zircon fission track ages and 25 apatite-confined fission track length distributions. Interpretation of this data reveals a phase of increased cooling rates related to exhumation between approximately 35 Ma (mid-Oligocene) and 15 Ma (Middle Miocene). Evidence from the sedimentary record indicates significant Oligo-Miocene erosion related to this period of exhumation. New chronological constraints are also applied to localised late-orogenic extensional tectonism that has recently been identified within the basement rocks of the Calabrian Arc. This information is used to produce a new model of the Oligo-Miocene tectonic evolution of the Calabrian Arc. It proposes that the crystalline basement rocks were part of a critical orogenic wedge between the mid-Oligocene and the Middle Miocene.

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.

Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece

Markedly different cooling histories for the hanging- and footwall of the Vari detachment on Syros and Tinos islands, Greece, are revealed by zircon and apatite fission-track data. The Vari/Akrotiri unit in the hangingwall cooled slowly at rates of 5-15 ◦ CM yr −1 since Late Cretaceous times. Samples from the Cycladic blueschist unit in the footwall of the detachment on Tinos Island have a mean zircon fission-track age of 10.0 ± 1.0 Ma, which together with a published mean apatite fission-track age of 9.4 ± 0.5 Ma indicates rapid cooling at rates of at least ∼ 60 ◦ CM yr −1 .W e derive am inimum slip rate of ∼ 6.5 km Myr −1 and a displacement of >∼ 20 km and propose that the development of the detachment in the thermally softened magmatic arc aided fast displacement. Intra-arc extension accomplished the final ∼ 6-9 km of exhumation of the Cycladic blueschists from ∼ 60 km depth. The fast-slipping intra-arc detachments did not cause much exhumation, but were important for regional- scale extension and the formation of the Aegean Sea.

An Oligocene extrusion wedge of blueschist-facies nappes on Evia, Aegean Sea, Greece: implications for the early exhumation of high-pressure rocks

We show that the Styra Nappe of the Cycladic Blueschist Unit on Evia constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The nappe pile on Evia was assembled during D2 top-to-the-SSW-directed thrusting (in restored Oligocene coordinates), which emplaced the Styra and Ochi nappes of the Cycladic Blueschist Unit above the Almyropotamos Nappe between c. 33 and 21 Ma. Peak metamorphism of the Almyropotamos Nappe at c. 23 Ma occurred at lower metamorphic pressure, showing exhumation of the Styra Nappe during underthrusting and burial of the Almyropotamos Nappe. This exhumation was largely accomplished by the D2 top-to-the-NNE-displacing Mt. Ochi normal-sense shear zone. Normal shearing commenced at c. 33 Ma under peak high-pressure metamorphism in the Styra and Ochi nappes. Fission-track dating indicates slow cooling after D2 in the Styra Nappe. Subsequently, the former thrust contact between the Almyropotamos Nappe and the Styra Nappe was reactivated as a D3 top-to-the-ESE extensional shear zone and this extensional phase led to the formation of a number of Middle to Late Miocene graben. Our main conclusion is that there is strong evidence for an Oligocene extrusion wedge accomplishing the early exhumation of the Styra Nappe, which demonstrates the importance of extrusion wedges for the initial exhumation of the Cycladic Blueschist Unit.

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.

New time constraints for the age of metamorphism at the ancestral Pacific Gondwana margin of southern Chile (42-52°S)

Nuevas limitaciones temporales a la edad del metamorfismo, en el margen ancestral pacifico de Gondwana, sur de Chile (42-52°S). La datacion por trazas de fision (FT) fue realizado en circones detriticos previamente datados por U-Pb SHRIMP, pertenecientes a distintos complejos metamorficos del sur de Chile (42-52°S), de edad pre-Jurasico Inferior, ubicados en el margen pacifico ancestral de Gondwana. La combinacion de edades de proveniencia U-Pb en circones, ya publicados, con sus historias de enfriamiento posmetamorfico permitieron establecer las siguentes edades maximas y minimas de deposicion y metamorfismo: para el complejo metamorfico andino oriental (EAMC) 364 a 250 Ma (Devonico Superior-Permico/Triasico), para el flysch Duque de York perteneciente al complejo acrecionario Madre de Dios (MDAC) 234 a 195 Ma (Triasico Medio-Jurasico Inferior), y para el complejo metamorfico de Chonos/Chiloe (CMC) 213 a 198 Ma (Triasico Superior-Jurasico Inferior). Estos resultados implican: (1) que la deposicion, acrecion y metamorfismo de la mayor parte del EAMC tuvo lugar con anterioridad al flysch de derivacion continental en los complejos ubicados al oeste del Batolito Norpatagonico (MDAC y CMC), apoyando anteriores proposiciones sobre la existencia de dos terrenos de basamento de diferente edad; (2) que los depositos de flysch de derivacion continental en los CMC y MDAC comparten una historia equivalente de deposicion y metamorfismo, indicativa de una amplia acrecion a lo largo de esta parte del margen pacifico de Gondwana ocurrida entre el Triasico Superior y el Jurasico Inferior. Edades FT mixtas posmetamorficas en granos individuales de circon y una edad FT en apatita provenientes de la isla de Chiloe (43° S) revelan una fase cretacica de recalentamiento en estas rocas (entre 109 y 60 Ma), no documentada previamente, y de probable relacion con actividad plutonica de esta edad en otras partes del CMC