Exhumation Processes Research Articles

Low-temperature thermochronology constraints on the mesozoic-cenozoic exhumation of the Huangling massif in the Middle Yangtze Block, Central China

The Huangling (黄陵) massif is an important area to understand the tectonic evolution of the northern Middle Yangtze Block. Integrating previously published thermochronology data with new zircon and apatite fission track, and apatite (U-Th)/He thermochronometry, the Meso-Cenozoic exhumation history of the Huangling massif has been quantitatively studied. Based on the data and the time-temperature thermal history modelling results, the exhumation process of the Huangling massif can be divided into four stages: the slow cooling stage during 200-150 Ma; the rapid cooling event at ca. 150-80 Ma; a period of relative thermal stability during ca. 80-40 Ma, and an increase in cooling thereafter. Two rapid cooling/exhumation indicate two tectonic events in the northern Middle Yangtze Block. The rapid exhumation between ca. 150-80 Ma is likely related to a wide range Cretaceous intra-continental reactive and deformation in the eastern China. The accelerated cooling after ca. 40-30 Ma may result from a far-field effect of the India-Asia collision.

The Eisenhower Range, Transantarctic Mountains: Evaluation of qualitative interpretation concepts of thermochronological data

The Transantarctic Mountains (TAM) were one of the first regions where apatite fission track (AFT) thermochronology was applied routinely to study exhumation processes and long term landscape evolution. Pioneering publications from the region introduced or refined interpretation concepts of thermochronological data such as the break in slope in vertical age profiles as qualitative marker for the onset of accelerated rock cooling.New AFT data were compiled from vertical profiles in the Eisenhower Range, northern TAM, and compared with published data. Samples originally examined by the population technique were re-analyzed via the external detector technique. AFT ages increase from 32±2Ma at an elevation of 220m to 175±14Ma at 2380m. Geological evidence and thermal history modeling of the AFT data require Jurassic to Late Eocene reheating of the samples and an onset of cooling at ~35–30Ma. This requires the deposition of an ~3 to 3.5km thick sedimentary sequence on the granitic basement subsequent to Jurassic Ferrar magmatism at ~180Ma. The regression of paleotemperatures against sample altitudes infers a high Jurassic geothermal gradient of ~60°C/km related to rifting processes and Ferrar magmatism, and a moderate Cretaceous/Eocene geothermal gradient of ~30°C/km.Comparison of ages generated with population and external detector technique shows the importance of determining single-grain ages for each sample, even from granitic rocks of the same intrusion, and thus strongly supports previous cases made for the determination of annealing kinetics and grain-age evaluation. Age comparison additionally illustrates that samples above a break in slope record larger deviations between population and external detector ages than samples below a break in slope.We demonstrate that the position and shape of a break in slope result from various factors, such as the thermal history prior to final cooling, maximum paleotemperatures, cooling rate, and geothermal gradient. A break in slope does not straightly date the onset of final cooling and cannot substitute thermal history modeling. Therefore, earlier studies from the TAM and similar settings elsewhere need to be validated by combining thermal history modeling of thermochronological data and supplementary geological information.

The Exhumation Process of Mufushan Granite in Jiangnan Uplift Since Cenozoic: Evidence from Low‐Temperature Thermochronology

AbstractJiangnan uplift located in the collision zone between Yangtze and Cathaysia block is the key to interpret the tectonic evolution of South China block. The Cenozoic thermal history and the uplift processes of Mufushan pluton are constrained by the combination of analysis of apatite fission track and (U‐Th‐Sm)/He ages with modeling of fission track length distribution. Based on the analysis of thermochronological dating and regional tectonic setting, the authors probed into the relationship between the thermochronological evolution and the tectonic development. The exhumation of plutons arrived at about 4800 m since 80 Ma, and the uplift processes were not uniform in space, and different plutons had somewhat different uplift processes. Thermal history modeling of the AFT data shows that the Mufushan pluton experienced three episodes of cooling event, two rapid exhumation events happened in Upper Cretaceous‐Paleocene (about 80∼50 Ma) and Upper Miocene (since 10 Ma). There is a period of relative thermal stability between 50 and 10 Ma. The first stage of rapid uplifting during 80∼50 Ma in this region is coeval with extension episode of Mid‐Upper Yangtze during Himalayan which resulted in the formation of fault basin and exhumation of shoulder block; the Upper Miocene rapid cooling since 10 Ma is attributed to the westward movement and subduction of Pacific plate. The uplift and exhumation of Mufushan pluton since Late Yanshanian led to a good deposition response in basins and the cooling events of three phases just coincided with the regional tectonic settings of study area.

Preservation of subduction‐related prograde deformation in lawsonite pseudomorph‐bearing rocks

AbstractLawsonite pseudomorphs are used to identify and distinguish the kinematic records of subduction and exhumation in blueschist‐facies rocks from Syros (Cyclades; Greece). Lawsonite is a hydrous mineral that crystallizes at high‐pressure and low‐temperature conditions. During decompression, lawsonite is typically pseudomorphed by an aggregate dominated by epidote and paragonite. Such aggregates are easily deformable and if deformation occurs after the lawsonite breakdown, the pseudomorphs are difficult to distinguish from the matrix. The preservation of the lawsonite crystal shape, despite complete retrogression, indicates therefore that the host blueschist rock has not been affected by penetrative deformation during exhumation, thus providing indication of strain‐free conditions. Therefore, tracking the lawsonite growth and destabilization along the P–T path followed by the rocks during a subduction/exhumation cycle provides information about the subduction/exhumation‐related deformation. Using microstructural observations and P–T pseudosections calculated with thermocalc, it is inferred that top‐to‐the‐south sense of shear preserved in lawsonite pseudomorph‐bearing blueschists on Syros occurred during the prograde metamorphic path within the lawsonite stability field, and is therefore associated with subduction. On the contrary, the deformation with a top‐to‐the‐north sense of shear is observed in surrounding rocks, where lawsonite pseudomorphs are deformed or apparently lacking. This deformation occurred after the lawsonite breakdown during exhumation. At the regional scale, exhumation‐related deformation is heterogeneous, allowing the preservation of lawsonite pseudomorphs in significant volumes of blueschists of the central and southern Cyclades. It is argued that such successive shearing deformation events with opposite senses more likely correspond to an exhumation process driven by slab rollback, in which subduction and exhumation are not synchronous.

Petrology and geochemistry of eclogites from the Biga Peninsula, Northwest Turkey

The Biga Peninsula in northwest Turkey contains high-pressure metabasic eclogite that occur in two localities; as lenses within a 2 km long, 500 m thick quartz-phengite schist slice that is in turn found in the greenschist-facies metasedimentary rocks of the Çamlıca metamorphics, in Çamlıca area, and in two north-south elongated eclogites occurring as a tectonic slice between Kazdağ Massif and Çetmi Group, in Çetmi area. The P-T conditions under which these two exposures of eclogites formed are important to quantify the tectonic processes of subduction, exhumation and emplacement that high-pressure rocks of the Biga Peninsula record. New geochemical data suggest that both protoliths were N-type MORB basalt with high TiO2 and K2O–Na2O content and Nb/Y ratios. Most eclogite samples have tholeiitic signatures volcanic arc settings. ∑REE abundances range from 47.55 to 107.4 ppm. Europium anomalies are slightly variable (Eu/Eu* = 0.9–1.1) and trace element contents are similar to typical MORB based on tectonic discrimination diagrams. All eclogite protoliths were probably derived from depleted mantle source, modified by fluids within the subduction zone. The high-P mineral assemblage in eclogites from both regions is omphacite + garnet + glaucophane + phengite + epidote + zoisite + quartz. The inclusions in garnet are glaucophane, quartz, phengite, Ca-amphibole and rutile. P-T conditions are similar to each other and constrained at 550–700 °C and 16–24 kbar. Geochemical data and mineral chemistry indicate that the eclogites in the Biga Peninsula represent oceanic crust processed at significant depths (50–80 km) within the subduction channel and were juxtaposed with greenschist facies as a tectonic slice in the accretionary complex at higher structural levels.

Differential exhumation at eastern margin of the Tibetan Plateau, from apatite fission-track thermochronology

New apatite fission-track (AFT) ages from Mesozoic sediments in the Sichuan basin, combined with previous fission-track data, demonstrate differential uplift and exhumation across the basin. Particularly significant change in exhumation (at least ~2000m) was found across the Huaying Mts. Modeled temperature–time histories and the Boomerang plot of AFT dataset across the basin suggest rapid cooling and exhumation events during 120–80Ma and at 20–10Ma. They reflect the start of the basin-scale differential uplift and exhumation which effected the eastern growth of Tibetan Plateau. In particular, nested old-age center separated by Huaying Mts. was found in the center-to-northwest part of the Sichuan basin. A simplified one-dimensional, steady-state solution model was developed to calculate the mean exhumation rate, which is 0.05–0.2mm/yr in most parts of the basin. It suggests a slow exhumation across much of the basin. The regional pattern of AFT age, length and erosion rate supports a progressive change from the nested old-age center towards the southwest. This pattern supports the idea of a prolonged, steady-state uplift and exhumation process across the basin, controlled by cratonic basin structure. The eastern growth of the Tibetan Plateau has exerted a significant effect on the rapid exhumation of the southwestern part of the Sichuan basin, but not on all of the basin during the Late Cenozoic.

Thickening and exhumation of the Variscan roots in the Iberian Central System: Tectonothermal processes and 40Ar/39Ar ages

40Ar/39Ar dating of white mica in metapelites, thermobarometric calculations and new structural and geological mapping have been carried out in the Somosierra region (Iberian Central System, ICS) to investigate the Variscan thickening–exhumation processes and the evolution of the paleo-thermal structure in a key region of the Central Iberian Zone. The rocks underwent an intermediate P/T event (M1) during a D1 stage of crustal thickening initiated between 354 and 347Ma. This event was probably related to a first S to SE-directed emplacement of the NW Iberian allochthon, and during the initial development of a mid-crustal subhorizontal shear zone accommodating the ductile flow of thermally weakened crust parallel to the main orogenic trend. Syn-convergent extension of the upper levels occurred by 335–327Ma, when tectonic unroofing led to D2 domal uplift of the orogenic roots and to recrystallization of rocks under intermediate to low P/T conditions (M2). Individualized gneiss domes developed along the orogenic trend, whereas some post-M2 thrusts dated at c. 319Ma reflect lateral flow related to limited gravitational spreading of the domes. Younger upright to SW verging folding of the ICS dome during D3 progressive renewal of contractive deformation took place after 316Ma. Final D4 gravitational collapse occurred under low P/T conditions (M3) at c. 300Ma, accompanied by the intrusion of large volumes of anatectic granitoids.

Kinematic and thermochronological constraints on the Xincheng–Huangpi fault and Mesozoic two-phase extrusion of the Tongbai–Dabie Orogen Belt

The Tongbai–Dabie Orogenic Belt (DOB) is composed of several fault-bounded terranes with different metamorphic and deformation intensities. These tectonically stacked terranes were extruded along their boundary faults from depths of >120km during the Mesozoic. Systematic structural studies on these boundary faults can provide important information for the exhumation processes of the orogen.In this paper, we focus on the Xincheng–Huangpi fault (XHF) which is located along the southern boundary of the central DOB ultrahigh-pressure complex unit (UC) Comprehensive macro- and micro-structural, U/Pb zircon and 40Ar/39Ar geochronological data are presented here to constraint the structural chronology of the XHF. The XHF represents the lower detachment surface formed during extrusion of the UHP slab and experienced two episodes of ductile shearing. The XHF experienced early southwestward thrusting around 234Ma and was exhumed to middle crustal levels during 234–195Ma with an average exhumation rates of ca. 1.5mm/yr. The UHP rocks then exhumed to the surface by erosion. Later NW–SE trending dextral strike-slip shearing of the XHF occurred between 145 and 140Ma and was generally contemporaraneous with sinistral-oblique slip of the Xiaotian–Mozitan fault along the northern margin of the UC. Coeval dextral and sinistral-oblique shearing along the southern and northern margins of the UC would have caused southeastern lateral extrusion. This lateral extrusion has no relationship with the UHP exhumation.

Apatite fission track and (U–Th)/He ages from the Higher Himalayan Crystallines, Kaghan Valley, Pakistan: Implications for an Eocene Plateau and Oligocene to Pliocene exhumation

Apatite fission track and apatite and zircon (U–Th)/He ages were obtained from high- and ultra high-pressure rocks from the Kaghan Valley, Pakistan. Four samples from the high altitude northern parts of the valley yielded apatite fission track ages between 24.5±3.7 and 15.6±2.1Ma and apatite (U–Th)/He ages between 21.0±0.6 and 5.3±0.2Ma. These data record cooling of the formerly deeply-subducted high-grade metamorphic rocks induced by denudation and exhumation consistent with extension and back sliding along the reactivated, normal-acting Main Mantle Thrust. Overlap at around 10Ma between fission track and (U–Th)/He ages is recognised at one location (Besal) showing that fast cooling occurred due to brittle reactivation of a former thrust fault. Widespread Miocene cooling is also evident in adjacent areas to the west (Deosai Plateau, Tso Morari), most likely related to uplift and unroofing linked to continued underplating of the Indian lower crust beneath Ladakh and Kohistan in the Late Eocene to Oligocene. In the southernmost part of the study area, near Naran, two significantly younger Late Miocene to Pliocene apatite fission track ages of 7.6±2.1 to 4.0±0.5Ma suggest a spatial and temporal separation of exhumation processes. These younger ages are best explained by enhanced Late Miocene uplift and erosion driven by thrusting along the Main Boundary Thrust.

Thermochronological constraints on the multiphase exhumation history of the Ivrea-Verbano Zone of the Southern Alps

The Ivrea-Verbano Zone of the western Southern Alps (NW Italy) exposes a well-preserved tilted section across the lower continental crust, making it a key region for studying deep crustal and exhumation processes. This paper refines the cooling and exhumation history of the Ivrea-Verbano Zone using K/Ar dating of mica and illite-rich fault gouges as well as zircon fission track and (U-Th)/He thermochronology. The adjacent Strona-Ceneri Zone, Sesia-Lanzo Zone and Lower Penninic nappes are included in the study to derive a broader picture of the low-temperature history of the area.In the Strona profile of the Ivrea-Verbano unit the biotite K/Ar, zircon fission track and (U-Th)/He geochronometers show well preserved, but unusually wide partial retention zones. The youngest ages, representing the formerly deepest position, are situated along the Insubric Line.The main foliation of the Ivrea-Verbano Zone is vertical. The exhumation of the Ivrea-Verbano Zone, which section has a horizontal position on the surface now – took place in three steps. During Jurassic time the Ivrea-Verbano Zone was exhumed to a shallow to mid-crustal position by continental-scale extension. In this displacement the Pogallo Line probably played a dominant role. The studied section occupied an oblique position with a calculated angle of ca. 15 to 23° in the Jurassic. Later the Ivrea-Verbano Zone experienced a minor cooling event in the Late Eocene (~38Ma zircon fission track ages) that was probably related to thrusting and erosion. The final exhumation towards the surface took place in the mid-Miocene as documented by the ca. 14Ma zircon (U-Th)/He ages and a 12.8MaK/Ar fault gouge age. The magnitude and the high rate of final exhumation suggest orogen-parallel extension as a driving force, which is widespread in the Alps in the Lower to Middle Miocene and is most probably connected to orogenic collapse.

Ordovician klippen structures of the Sierra de Umango: New insights on Tectonic evolution of the Western Sierras Pampeanas, Argentina

Abstract The basement rock of the Pampean flat-slab (Sierras Pampeanas) in the Central Andes was uplifted and rotated in the Cenozoic era. The Western Sierras Pampeanas are characterised by meta-igneous rocks of Grenvillian Mesoproterozoic age and metasedimentary units metamorphosed in the Ordovician period. These rocks, known as the northern Cuyania composite terrane, were derived from Laurentia and accreted toward Western Gondwana during the Early Paleozoic. The Sierra de Umango is the westernmost range of the Western Sierras Pampeanas. This range is bounded by the Devonian sedimentary rocks of the Precordillera on the western side and Tertiary rocks from the Sierra de Maz and Sierra del Espinal on the eastern side and contains igneous and sedimentary rocks outcroppings from the Famatina System on the far eastern side. The Sierra de Umango evolved during a period of polyphase tectonic activity, including an Ordovician collisional event, a Devonian compressional deformation, Late Paleozoic and Mesozoic extensional faulting and sedimentation (Paganzo and Ischigualasto basins) and compressional deformation of the Andean foreland during the Cenozoic. A Nappe System and an important shear zone, La Puntilla–La Falda Shear Zone (PFSZ), characterise the Ordovician collisional event, which was related to the accretion of Cuyania Terrane to the proto-Andean margin of Gondwana. Three continuous deformational phases are recognised for this event: the D1 phase is distinguished by relics of S1 preserved as internal foliation within interkinematic staurolite porphyroblasts and likely represents the progressive metamorphic stage; the D2 phase exhibits P-T conditions close to the metamorphic peak that were recorded in an S2 transposition or a mylonitic foliation and determine the main structure of Umango; and the D3 phase is described as a set of tight to recumbent folds with S3 axial plane foliation, often related to thrust faults, indicating the retrogressive metamorphic stage. The Nappe System shows a top-to-the S/SW sense direction of movement, and the PFSZ served as a right lateral ramp in the exhumation process. This structural pattern is indicative of an oblique collision, with the Cuyania Terrane subducting under the proto-Andean margin of Gondwana in the NE direction. This continental subduction and exhumation lasted at least 30 million years, nearly the entire Ordovician period, and produced metamorphic conditions of upper amphibolite-to-granulite facies in medium- to high-pressure regimes. At least two later events deformed the earlier structures: D4 and D5 deformational phases. The D4 deformational phase corresponds to upright folding, with wavelengths of approximately 10 km and a general N–S orientation. These folds modified the S2 surface in an approximately cylindrical manner and are associated with exposed, discrete shear zones in the Silurian Guandacolinos Granite. The cylindrical pattern and subhorizontal axis of the D4 folds indicates that the S2 surface was originally flat-lying. The D4 folds are responsible for preserving the basement unit Juchi Orthogneiss synformal klippen. This deformation corresponds to the Chanica Tectonic during the interval between the Devonian and Carboniferous periods. The D5 deformational phase comprehends cuspate-lobate shaped open plunging folds with E–W high-angle axes (D5 folds) and sub-vertical spaced cleavage. The D5 folds and related spaced cleavage deformed the previous structures and could be associated with uplifting during the Andean Cycle.

Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes

Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. We present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region. In the Duktoth transect, we recognize older, approximately east-west–trending structures that are overprinted by open, northwest-trending fold systems, which we correlate to a system of northeast-trending, out-of-sequence, probably active thrusts. These younger structures overprint a fold-thrust stack that is characterized by variable structural complexity related to detachment folding along coal-bearing horizons and duplexing within Eocene strata. In the Icy Bay transect, we recognize a similar structural style, but a different kinematic history that is constrained by an angular unconformity at the base of the syntectonic Yakataga Formation. At high structural levels, near the suture, structures show a consistent northwest trend, but fold-thrust systems rotate to east-west to northeast trends in successively younger structures within the Yakataga Formation. We present balanced cross sections for each of these transects where we project the top of basement from offshore seismic data and assume a subsurface structure with duplex systems similar to, but simplified from, structures observed in the onshore transects. These sections can account for 150–200 km of shortening within the fold-thrust system, which is Our section restorations also provide a simple explanation for the observed elongate bullseye pattern of low-temperature cooling ages in the thrust belt as a consequence of exhumation above the growing duplex and/or antiformal stack. Comparison with analog model studies suggests that structural feedbacks between erosion and development of decollement horizons in coal-bearing strata led to this structural style. Although previous studies based on thermochronology suggested an active backthrust at the northern edge of the thrust belt, section restorations indicate that a backthrust is allowable but not required by available data. The Yakataga fold-thrust belt has been treated as a dominantly 2D system, yet our work indicates that 3D processes are prominent. In the Duktoth transect, we interpret a group of northeast-trending thrusts as younger, out-of-sequence structures formed in response to the rapid destruction of the orogenic wedge by glacial erosion and deposition immediately offshore. We infer that these northeast-trending thrusts transfer slip downdip into a duplex system that forms the antiformal stack modeled in cross-section restorations, and we infer that these structures represent thrusting stepping back from the active thrust front attempting to rebuild an orogenic wedge that is being destroyed as rapidly as, or more rapidly than, it is being rebuilt. In the Icy Bay transect, we use the relative chronology provided by an angular unconformity beneath the syntectonic Yakataga Formation to infer that early, northwest-trending fold-thrust systems were formed along the Fairweather transform as transpressional structures. Continued strike slip carried these structures into the tectonic corner between the Fairweather and Yakataga segments of the orogen, producing a counterclockwise rotation of the shortening axis until the rocks reached their present position.

LA-ICPMS U–Pb zircon age constraints on the provenance of Cretaceous sediments in the Yichang area of the Jianghan Basin, central China

Detrital zircon U–Pb ages have been shown to be a powerful tool for provenance analysis and determining the exhumation of sediment source areas. This paper presents the results of detrital zircon LA-ICPMS U–Pb ages for Cretaceous sediments from the Yichang area of the Jianghan Basin, central China. The results provide new information on the provenance of these sediments and the detailed exhumation process of the Huangling Dome. Zircons with different age populations have been derived from the strata of the Huangling Dome. The Liantuo, Gucheng and Nantuo formations and the Kongling complex were exhumed, leading to deposition of the early Cretaceous Wulong Formation, which provides the sources of zircons with age peaks at 3.1–3.0, 2.5 and 1.8 Ga. Exhumation of the Huangling granitoid and contemporary volcanics provided the source of the late Cretaceous Luojingtan Formation, which contains zircons with age peaks at 1.1–0.95 and 0.83–0.74 Ga. The Qinling-Dabie orogeny supplied zircons with an age cluster of 0.27 to 0.18 Ga. These results indicate the timing of initial exhumation for the Huangling granite. They also show how overlying strata was first uplifted and eroded, followed by exposure of underlying strata at the surface during continued exhumation.

Phengite geochronology of crystalline schists in the Sakuma–Tenryu district, central Japan

AbstractThe Sakuma–Tenryu district consists mainly of pelitic and basic schists. Its metamorphic sequence has been divided into two units, the Shirakura and the Sejiri units. We carried out K–Ar analyses of phengite separates and X‐ray diffraction analyses of carbonaceous materials from the pelitic schists of both units. The age–d002 relationships show that the ages become older (66–73 Ma) in the Shirakura unit and younger (57–48 Ma) in the latter with increasing metamorphic temperature. The former has a positive relationship observed in the Sanbagawa meta‐Accretionary Complex (meta‐AC) (Sanbagawa metamorphic belt sensu stricto) in central Shikoku and the latter, a negative one in the Shimanto meta‐AC (a subunit of traditional Sanbagawa belt) of the Kanto Mountains. These contrasting age–temperature relationships are due to different tectonic styles relating to the exhumation of the metamorphic sequences. The duration from the peak metamorphism to the closure of the phengite K–Ar system was significantly different between the two metamorphic sequences: longer than 31 my in the Sanbagawa meta‐AC and shorter than 13 my in the Shimanto meta‐AC. The different natures of subducted plate boundaries may cause the different exhumation processes of metamorphic belts.

Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Syn-orogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell'Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27 Ma obtained at the Monti dell'Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell'Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13–10 Ma rocks were already at shallow crustal levels (< 9 km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at ~ 6–4 Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6–4 Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional “out-of-sequence” or “re-activation” event occurring in the internal sector of Tuscany already affected by a regional extensional regime.

Deciphering tectonic‐ and erosion‐driven exhumation of the Nevado–Filábride Complex (Betic Cordillera, Southern Spain) by low temperature thermochronology

Terra Nova, 23, 257–263, 2011AbstractNew apatite (U‐Th)/He and fission‐track data from the Nevado–Filábride complex in the Sierra Nevada of southern Spain are used to constrain the Neogene exhumation history. Apatite (U‐Th)/He ages are close to fission‐track ages in western Sierra Nevada indicating that rapid cooling occurred at 8–6 Ma, consistent with exhumation due to extension along the Mecina Fault. The western Sierra de los Filabres cooled rapidly at c. 12 Ma, while the central Sierra de los Filabres experienced less rapid cooling at 8 Ma. The age distribution in the Sierra de los Filabres can be explained by exhumation due to flexural uplift to the south‐west of the footwall of the Mecina Fault. The lower cooling rates in the central Sierra de los Filabres suggest that folding and erosion were the main exhumation processes during the Late Tortonian.Terra Nova, 00, 1–7, 2011

The uplifting and denudation of main Huangshan Mountains, Anhui Province, China

The Huangshan Mountain Geopark is an important geological landmark and tourist attraction. In this paper, we apply fission track dating to examine the uplift and exhumation processes that created the Huangshan Mountains and provide a timeline for their development. In addition to being an important scientific contribution, this information can be used on guided tours and to promote tourism. The results of eight apatite fission-track analyses suggest three age groups: 56, 45–30, and 15 Ma. These age groups are related to the uplifting-erosion processes involved in forming ancient multilevel denudation planes at 85–50, 45–30, and 24–5 Ma, respectively. The scenic area has experienced three stages of thermal evolution. The first stage occurred before 80 Ma and the third stage after 13 Ma. The three stages had varying cooling rates: 2.69, 0.62, and 4.23°C/Ma, respectively. Uplifting rates also varied: 0.08, 0.02, and 0.12 mm/a. The uplifting ranges were also variable: 1.14, 1.57, and 4.00 km, respectively. Significant uplifting differences between diverse areas are obvious and there were four magnitudes of cumulative uplifting range: 4060–3950, 3760–3490, 3190–3070, and 2650 m. The surface uplifting range varied from 450 to 2230 m. Denudation and uplifting amount averaged 2340 and 3400 m, respectively. The 1060 m difference between them is the average elevation at present.

Testing modes of exhumation in collisional orogens: Synconvergent channel flow in the southeastern Canadian Cordillera

This study investigates exhumation processes in collisional orogens. A critical test between three modes of exhumation is presented based on a review of quantitative numerical and analogue modeling studies. The test is applied to the large tract of migmatites surrounding the Monashee Complex in the southeastern Canadian Cordillera. It reveals that the extensive and multidisciplinary database of this region is entirely compatible with the synconvergent channel-flow mode but not with critical wedge and gravitational collapse modes. We propose that a partially molten channel decoupled from its lid and base and started to flow in the middle crust toward the foreland at 100–90 Ma. A steady-state channel-flow system was established for the following ∼30 m.y., during which rocks at the front of the channel were exhumed to upper-crustal levels as they flowed above an underthrusting basement ramp. Flow was accommodated by oppositely verging shear zones bounding the channel, by internal ductile deformation, and by shortening in the foreland belt. The locus of flow migrated downward to exhume rocks of the Monashee Complex between 60 and 50 Ma by a similar process. The southeastern Canadian Cordillera thus constitutes an excellent natural analogue for the channel-flow model. In contrast to the commonly held view of large-magnitude extension and core complex formation, the role of extension was limited to the final ∼10–15 km of exhumation after 50 Ma.

Granite intrusion in a metamorphic core complex: The example of the Mykonos laccolith (Cyclades, Greece)

The Aegean domain is a well-suited place to study the formation of metamorphic core complex (MCC) and to investigate the role of syn-tectonic granites on their development. In the northern Cyclades, the Mykonos–Delos–Rhenia MCC is characterized by the intrusion of a kilometer-scale Late Miocene pluton of I-type granitoids within a migmatitic gneiss dome. New combined AMS (anisotropy of magnetic susceptibility) and microstructural studies on the Mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers. The Mykonos granitoids form a laccolith-like intrusion with a N70°E long axis. The laccolith is strongly asymmetric with an outlying root zone to the SW and a major body mainly developed to the NE. The laccolith construction is due to successive pulses of more or less differentiated magma that intruded the Cycladic Blueschist Unit. The attitude of stretching markers suggests an important (about 60°) vertical-axis local rotation phenomenon in the cycladic upper crust during the exhumation of the Mykonos MCC. Structural data suggest a four-stage evolution of the Mykonos MCC: (i) a first stage characterized by flat shearing toward the N–NE and by the formation of a domal structure in migmatitic paragneisses with multi-scale generation of folds with axes either perpendicular or parallel to the regional stretching, as a result of the interplay between regional N20°E-directed extension and EW shortening; (ii) a second stage marked by the emplacement of the Mykonos laccolith at 13.5 ± 0.3 Ma at the top of the migmatitic paragneisses; (iii) the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions; and (iv) the late stage marked by an acceleration of the exhumation of the Mykonos MCC. This exhumation was accommodated by important rotations of upper crustal blocks. During the end of the exhumation processes, around 10 Ma, deformation localized at the top of the laccolith in semi-ductile conditions and then in brittle conditions in the major detachment plane. Our study shows that the Cycladic plutonism event had no role on the initiation of the MCC. However, the geometry of the Mykonos intrusion supports that the magmas are “sucked” into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the MCC development. This acceleration was marked by a very fast exhumation of the laccolith after its emplacement.

Petrology and geochemistry of chromian spinel-bearing serpentinites in the Hida Marginal Belt (Ise area, Japan): characteristics of their protoliths

The Ise area is located in the western part of the Hida Marginal Belt (central Japan), which includes several sporadic exposures of ultramafic rocks, sometimes forming a serpentinite melange of the Paleozoic age. Ultramafic rocks in the Ise area, enveloped by Paleozoic-Mesozoic sedimentary rocks, are completely serpentinized; however, the abundance of bastite after orthopyroxene suggests harzburgite protoliths. The bastite- and mesh-textured serpentines are distinguished from each other in Al2O3, Cr2O3 and NiO contents. The bastite-textured serpentine is high in Al2O3 (up to 4.0 wt%) and Cr2O3 (up to 1.2 wt%), but low in NiO (<0.3 wt%) relative to the mesh-textured one. Relic chromian spinel, vermicular in shape, shows an inter-grain chemical homogeneity and is sometimes altered to ferritchromite at the margin. It has a narrow range of Cr# [= Cr/(Cr + Al) atomic ratio] from 0.38 to 0.51 and low YFe {=[Fe3+/(Cr + Al + Fe3+) atomic ratio, <0.03]}, similar to chromian spinel in Kotaki and Oeyama ultramafic masses. It is also similar in chemistry to spinels in forearc and abyssal peridotites, suggesting two possibilities for the derivation of the Ise serpentinite's protoliths. The degree of melting using Cr# versus TiO2 of chromian spinel is ∼ 20-25%, which is in accordance with the harzburgite protoliths obtained by whole-rock chemistry models. We found that Cr, Al, Ni and Ca, preserved in bastite and mesh-textured serpentine, are conservative during serpentinization, confirmed from the similarity in the whole-rock A1 and Ca of the Ise serpentinites to Horoman harzburgites. The occurrence of dolomite can stabilize the Ca and limited its mobility to escape outside the serpentinites. The harzburgite protoliths were possibly serpentinized by slab-derived fluids at a shallow depth relative to the Happo-O'ne serpentinites during the exhumation process.