首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Thermal history modelling based on zircon‐ and apatite fission track and apatite (U–Th)/He data constrain and refine the near‐surface exhumation of the south‐eastern Tauern Window (Penninic units) and neighbouring Austroalpine basement units in the Eastern Alps. Fast exhumation on both sides of the Penninic/Austroalpine boundary coincides with a period of lateral extrusion and tectonic denudation of the Penninic units in Miocene time (22–12 Ma). The jump to older ages occurs within the Austroalpine unit along the Polinik fault, which therefore defines the boundary between the tectonically denuded units and the hangingwall at that time. According to the different (U–Th)/He ages between the Penninic Hochalm‐ and Sonnblick Domes we demonstrate a differential cooling history of these two domes in the latest Miocene and early Pliocene.  相似文献   

2.
Fission track dating on detrital zircons of Alpine debris in the Swiss molasse basin provides information about the erosion history of the Central Alps and the thermal evolution of source terrains. During Oligocene times, only sedimentary cover nappes, and Austroalpine basement units were eroded. Incision into Austroalpine basement units is indicated by increasing importance of Cretaceous cooling ages in granite pebbles upsection. Erosion of Penninic basement units started between 25 and 20 Ma. Early Oligocene zircon FT ages show that Penninic basement units were exposed at ∼20 Ma. Deeper Penninic units of the Lepontine Dome became exposed first at ∼14 Ma, contemporaneously with the opening of the Tauern window in the Eastern Alps. A middle Miocene cooling rate of 40 °C Myr−1 is deduced for the Lower Penninic units of the Lepontine Dome.  相似文献   

3.
A new set of apatite fission‐track and apatite (U–Th)/He data reveals a hitherto undated late Miocene exhumation pulse in the eastern part of the Eastern Alps. While distinct parts of the study area, including the Seckauer Tauern, have been at near surface conditions (<100 °C) since the Eocene, the neighbouring Niedere Tauern experienced enhanced cooling and exhumation in the middle Miocene and again at the late Miocene/Pliocene boundary. Middle Miocene exhumation is interpreted as a result of tectonic escape and convergence that operated simultaneously during lateral extrusion of the Eastern Alps. As the higher late Miocene/Pliocene exhumation rates are restricted to a single tectonic block, namely the Niedere Tauern, we infer a tectonic trigger that is probably related to a change in the external stress field that affected the Alps during this time.  相似文献   

4.
The Cretaceous-Eocene Xigaze forearc basin is a crucial data archive for understanding the tectonic history of the Asian continental margin prior to and following collision with India during the early Cenozoic Era. This study reports apatite and zircon(U-Th)/He thermochronologic data from fourteen samples from Albian-Ypresian Xigaze forearc strata to determine the degree and timing of heating(burial) and subsequent cooling(exhumation) of two localities along the Yarlung suture zone(YSZ) near the towns of Saga and Lazi. Thirty-seven individual zircon He ages range from 31.5 ± 0.8 Ma to6.06 ± 0.18 Ma,with the majority of grains yielding ages between 30 Ma and 10 Ma. Twenty apatite He ages range from 12.7 ± 0.5 Ma to 3.9 ± 0.3 Ma,with the majority of grains yielding ages between 9 Ma and 4 Ma. These ages suggest that the Xigaze forearc basin was heated to 140-200 ℃ prior to cooling in Oligocene-Miocene time. Thermal modeling supports this interpretation and shows that the samples were buried to maximum temperatures of ～140-200 0 C by 35-21 Ma, immediately followed by the onset of exhumation. The zircon He and apatite He dataset and thermal modeling results indicate rapid exhumation from ～21 Ma to 15 Ma, and at ～4 Ma. The 21-15 Ma thermochronometric signal appears to be regionally extensive, affecting all the lithotectonic units of the YSZ, and coincides with movement along the north-vergent Great Counter Thrust system. Thrusting, coupled with enhanced erosion possibly related to the paleo-Yarlung River, likely drove Early Miocene cooling of the Xigaze forearc basin.In contrast, the younger phase of rapid exhumation at ～4 Ma was likely driven by enhanced rock uplift in the footwall of north-striking rifts that cross-cut the YSZ.  相似文献   

5.
New laser ablation-inductive coupled plasma-mass spectrometry U-Pb analyses on oscillatory-zoned zircon imply Early Miocene crystallization (18.64 ± 0.11 Ma) of the Pohorje pluton at the southeastern margin of the Eastern Alps (northern Slovenia). Inherited zircon cores indicate two crustal sources: a late Variscan magmatic population (~270–290 Ma), and an early Neoproterozoic one (850–900 Ma) with juvenile Hf isotope composition close to that of depleted mantle. Initial εHf of Miocene zircon points to an additional, more juvenile source component of the Miocene magma, which could be either a juvenile Phanerozoic crust or the Miocene mantle. The new U-Pb isotope age of the Pohorje pluton seriously questions its attribution to the Oligocene age ‘Periadriatic’ intrusions. The new data imply a temporal coincidence with 19–15 Ma magmatism in the Pannonian Basin system, more specifically in the Styrian Basin. K-Ar mineral- and whole rock ages from the pluton itself and cogenetic shallow intrusive dacitic rocks (~18–16 Ma), as well as zircon fission track data (17.7–15.6 Ma), gave late Early to early Middle Miocene ages, indicating rapid cooling of the pluton within about 3 Million years. Medium-grade Austroalpine metamorphics north and south of the pluton were reheated and subsequently cooled together. Outcrop- and micro scale structures record deformation of the Pohorje pluton and few related mafic and dacitic dykes under greenschist facies conditions. Part of the solidstate fabrics indicate E–W oriented stretching and vertical thinning, while steeply dipping foliation and NW–SE trending lineation are also present. The E–W oriented lineation is parallel to the direction of subsequent brittle extension, which resulted in normal faulting and tilting of the earlier ductile fabric at around the Early / Middle Miocene boundary; normal faulting was combined with strike-slip faulting. Renewed N–S compression may be related to late Miocene to Quaternary dextral faulting in the area. The documented syn-cooling extensional structures and part of the strike-slip faults can be interpreted as being related to lateral extrusion of the Eastern Alps and/or to back-arc rifting in the Pannonian Basin.  相似文献   

6.
Ar/Ar thermochronology on 24 hornblendes, 3 biotites, 2 muscovites and 2 K-feldspars, collected along a 400 km-long NW-SE geotraverse through the Grenville Province in western Québec, is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the Grenville orogen. In the Grenville Front zone, the c. 1000 Ma time of exhumation previously established from thermobarometric and isotopic studies, is supported by the hornblende age data presented here. From 60 km to 160 km SE of the Front, reworked Archaean migmatites of the parautochthonous Réservoir Dozois terrane (RDT; 1004 Ma-old metamorphic monazites) contain hornblendes with 972– 950 Ma cooling ages. Assuming metamorphic geotherms between 25 and 30 °C km?1, calculated cooling and unroofing rates are about 6 °C Ma?1 and 0.33 km Ma?1 in the P–T range 725 °C–800 MPa and 450 °C–400 MPa. Hornblendes from monocyclic rocks of the Mont-Laurier and Morin terranes (MLT and MT; monazite ages c. 1165 Ma) give ages of about 1040 and 1010 Ma, respectively. Calculation of cooling-unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated Grenville collision which took place approximately between 1060 and 1020 Ma. Cooling ages of c. 900 Ma for muscovite and biotite and 860–810 Ma for K-feldspar, show that cooling rates decreased to around 1.5 °C Ma?1 under retrograde greenschist facies conditions in the MLT. On a time vs. distance diagram, the hornblende data define several distinct age ranges, suggesting that each terrane had a characteristic thermal history. Thus, cooling was diachronous and probably non-homogeneous throughout this segment of the Grenville orogen. The time-lag between the cooling history of the parautochthon (972–950 Ma) and the allochthons (1040–1010 Ma) is compatible with an earlier (pre-1040 Ma) peak of metamorphism in the allochthons. The Réservoir Cabonga allochthon was transported toward the NNW from its probable root zone in the MLT during the 1060–1020 Ma Grenvillian collision as a partially cooled slab. The remobilization of the Archaean parautochthon is attributed to this collision. In the Grenville Front zone, slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid (tectonic?) exhumation shortly after collision. The minor delay (20–30 Ma) in unroofing of the MT compared to the adjacent MLT is most likely related to post-1040 Ma extensional displacement along the Labelle shear zone. In terranes like those described above where metamorphism is diachronous, determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure. However, identification of contrasting cooling histories contributes to unravelling the independent movement of terranes.  相似文献   

7.
Rocks from the Micaschist–Marble Complex of the Wölz Tauern, which are part of the middle Austroalpine unit, contain large (up to 2 cm) garnet crystals that show clear evidence of multistage growth. Isotopic dating indicates that a Variscan (～270 Ma) garnet core was overgrown by new garnet formed during Eo-Alpine metamorphism at Cretaceous times. P-T paths for the Eo-Alpine metamorphism were obtained using the method of pseudosections (Powell and Holland in Metam Geol 16:309–343, 1998) and are consistent with earlier results from independent thermobarometry. Due to the large size of the garnets, growth zoning was preserved during amphibolite facies metamorphism at both Variscan and Alpine times. Full multicomponent diffusion modeling of compositional zoning at the interface of the Alpine and Pre-Alpine garnets in conjunction with the retrieved P-T paths allow average subduction/exhumation as well as heating/cooling rates to be retrieved. The modeling suggests that a minimum subduction/exhumation rate of ～4 cm/a and heating/cooling rates on the order of 100–260°C/Ma for a 60°C subduction angle are required to preserve the observed compositional zoning overall while modifying the zoning at the interface between two garnets to the extent observed. Such rapid rates of burial/exhumation are consistent with the results of direct GPS measurements of convergence rates at several orogenic belts as well as with inferred rates from modeling in the Alps and other areas. In combination, this indicates that such rapid rates are commonplace during metamorphism in collisional orogens and places important constraints on the rheological behavior of crustal blocks in such orogens.  相似文献   

8.
This study uses zircon and apatite fission‐track (FT) analyses to reveal the exhumation history of the granitoid samples collected from the Lesser Hinggan Mountains, northeast China. A southeast to northwest transect across the Lesser Hinggan Mountains yielded zircon FT ages between 89.8 ± 5.7 and 100.4 ± 8.6 Ma, and apatite FT ages between 50.6 ± 13.8 and 74.3 ± 4.5 Ma with mean track lengths between 11.7 ± 2.0 and 12.8 ± 1.7 µm. FT results and modelling identify three stages in sample cooling history spanning the late Mesozoic and Cenozoic eras. Stage one records rapid cooling from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110 °C) during ca. 95 to 65 Ma. Stage two records a period of relative slow cooling (∼110–60 °C) taking place between ca. 65 and 20 Ma, suggesting that the granitoids had been exhumed to the depth of ∼1−2 km. Final stage cooling (60–20 °C) occurred since the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. The maximum exhumation is more than 5 km under a steady‐state geothermal gradient of 35 °C/km. Integrated with the tectonic setting, this exhumation is possibly led by the Pacific Plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

9.
The Cretaceous to Palaeogene Alpine exhumation of previously buried Variscan basements is recorded in the southern portion of the Western Carpathians in the Gemeric and Veporic units. The Meso-Cenozoic collisional processes resulted in basement exhumation of the Tatric Unit from Palaeogene to Neogene times. According to zircon and apatite fission track data, the Gemeric Unit, an uppermost thick-skinned thrust sheet, cooled from depth levels of ∼10 up to 2.5 km (temperature interval of ∼250–60 °C) about 88–64 Ma ago, after the collapse of overlying Meliata-Turňa-Silica Mesozoic accretionary prism. The middle and lower thick-skinned thrust sheets, Veporic and Tatric units, cooled from the depths of ∼10 up to 2.5 km ∼110–40 Ma ago. The process was controlled by unroofing of footwall from beneath the Gemeric Unit. About 50–20 Ma ago, the internal zone of Tatric Unit gradually exhumed to depth of <2 km and some parts of the unit appeared at the surface level. However, the external zone of Tatric Unit was buried beneath the Eocene to Lower Miocene sedimentary successions and exhumed to the subsurface level at ∼21–8 Ma ago, as a result of oblique collision of the Western Carpathians with the European Platform.  相似文献   

10.
(U–Th)/He and fission-track analyses of apatite along deep-seated tunnels crossing high-relief mountain ranges offer the opportunity to investigate climate-tectonic forcing on topographic evolution. In this study, the thermochronologic analysis along the Simplon tunnel (western-central Alps; Italy and Switzerland) constrains in detail the mechanisms controlling the topographic evolution of the Simplon Massif. Cooling rates vary from about 10°C/Ma at about 10 Ma to about 35°C/Ma in the last 3 Ma. Such increase in cooling rates corresponds to the inception of glacial cycles in the northern hemisphere. Age patterns show correlation with faults distribution until 2 Ma, suggesting that tectonics-controlled rocks exhumed up to that age. After 2 Ma thermo-chronometric data show that the Simplon area has experienced primarily erosional exhumation. All age patterns provided are not affected by topographic effects, thus indicating that present-day topography has been carved in the last 2 Ma, most likely controlled by glacial erosion.  相似文献   

11.
The Tauern Window exposes a Paleogene nappe stack consisting of highly metamorphosed oceanic (Alpine Tethys) and continental (distal European margin) thrust sheets. In the eastern part of this window, this nappe stack (Eastern Tauern Subdome, ETD) is bounded by a Neogene system of shear (the Katschberg Shear Zone System, KSZS) that accommodated orogen-parallel stretching, orogen-normal shortening, and exhumation with respect to the structurally overlying Austroalpine units (Adriatic margin). The KSZS comprises a ≤5-km-thick belt of retrograde mylonite, the central segment of which is a southeast-dipping, low-angle extensional shear zone with a brittle overprint (Katschberg Normal Fault, KNF). At the northern and southern ends of this central segment, the KSZS loses its brittle overprint and swings around both corners of the ETD to become subvertical, dextral, and sinistral strike-slip faults. The latter represent stretching faults whose displacements decrease westward to near zero. The kinematic continuity of top-east to top-southeast ductile shearing along the central, low-angle extensional part of the KSZS with strike-slip shearing along its steep ends, combined with maximum tectonic omission of nappes of the ETD in the footwall of the KNF, indicates that north–south shortening, orogen-parallel stretching, and normal faulting were coeval. Stratigraphic and radiometric ages constrain exhumation of the folded nappe complex in the footwall of the KSZS to have begun at 23–21 Ma, leading to rapid cooling between 21 and 16 Ma. This exhumation involved a combination of tectonic unroofing by extensional shearing, upright folding, and erosional denudation. The contribution of tectonic unroofing is greatest along the central segment of the KSZS and decreases westward to the central part of the Tauern Window. The KSZS formed in response to the indentation of wedge-shaped blocks of semi-rigid Austroalpine basement located in front of the South-Alpine indenter that was part of the Adriatic microplate. Northward motion of this indenter along the sinistral Giudicarie Belt offsets the Periadriatic Fault and triggered rapid exhumation of orogenic crust within the entire Tauern Window. Exhumation involved strike-slip and normal faulting that accommodated about 100 km of orogen-parallel extension and was contemporaneous with about 30 km of orogen-perpendicular, north–south shortening of the ETD. Extension of the Pannonian Basin related to roll-back subduction in the Carpathians began at 20 Ma, but did not affect the Eastern Alps before about 17 Ma. The effect of this extension was to reduce the lateral resistance to eastward crustal flow away from the zone of greatest thickening in the Tauern Window area. Therefore, we propose that roll-back subduction temporarily enhanced rather than triggered exhumation and orogen-parallel motion in the Eastern Alps. Lateral extrusion and orogen-parallel extension in the Eastern Alps have continued from 12 to 10 Ma to the present and are driven by northward push of Adria.  相似文献   

12.
Miocene Intrusives and Lower Cretaceous siliciclastic sedimentary rocks from the Basal Complex in western-Fuerteventura were analyzed with low-temperature thermochronometric methods such as fission-track, and (U–Th–Sm)/He dating, in order to reveal the evolution of the island’s exhumation history. The obtained thermochronometric data yields a very slow rate of cooling in the order of 1.5–3°C/Myr from ~50 to 20 Ma for the Early Cretaceous siliciclastic rocks. These sedimentary units have never been heated significantly above 240°C after deposition and still record the submarine onset of the island’s formation in the Eocene. Intrusive bodies associated with the early Miocene magmatic activity of the central volcanic complex of the island show rapid initial cooling rates of 50–70°C/Myr from ~20 to 14 Ma. Contemporaneous with the intrusions the cooling rate of the Cretaceous sedimentary units increased to 25–35°C/Myr and it is inferred that this increase is associated with enhanced uplift and erosion of the Central Volcanic Complex. After ~14 Ma rates slowed down to 3–6°C/Myr. Palaeosols overlying the sedimentary units are themselves covered by Pliocene basalt flows and reveal that the sedimentary rocks reached the surface before ~5 Ma. The thermochronometric data obtained in this study for central Fuerteventura is difficult to reconcile with the cooling history derived from previously obtained fission-track and K–Ar data from the north-western part of the island. This inconsistency is likely to indicate that the exhumation history of Fuerteventura is more complex and regionally subdivided than previously believed.  相似文献   

13.
The combination of magmatic, structural and fission track (FT) data is used to unravel Oligocene/Miocene near-surface tectonics in the internal Western Alps. This includes reburial of parts of the already exhumed Sesia-Lanzo Zone and their subsequent re-exhumation. We define blocks mainly on the base of their Oligocene–Miocene cooling history (FT data) and on published paleomagnetic data. The preservation of a paleosurface allows a detailed reconstruction of the exhumation, burial and re-exhumation of different tectonic blocks. Near-surface, rigid block rotation is responsible for the reburial of the Lower Oligocene paleosurface in part of the Sesia-Lanzo Zone (the Cervo Block) and for the conjugate uplift of deeper portions of the Ivrea-Verbano Zone (the Sessera-Ossola Block). This block rotation around the same horizontal axes produces in the currently exposed portions of the two blocks, quite different temperature/time paths. While the surface of the Cervo Block is buried, the lower part of the Sessera-Ossola Block is uplifted. The rotation is constrained between the age of emplacement of the Biella Volcanic Suite on top of the Sesia-Lanzo Zone (32.5?Ma) and the intrusion of the Valle del Cervo Pluton (30.5?Ma). After this relative fast movements, the concerned blocks remained in (or underneath) the partial annealing zone of zircon until in Aquitanian times they were rapidly uplifted into the partial annealing zone of apatite. The further stage of exhumation out of the partial annealing zone of apatite extends over the entire Miocene. At that time, units of the external Western Alps underwent fast exhumation (external Brian?onnais, Valais). In addition to the well-known post-collisional deformation in the axial- and external Western Alps, the internal units (i.e., the upper plate) hold an apparent stable position in terms of exhumation.  相似文献   

14.
The Beni Bousera peridotite massif and its metamorphic surrounding rocks have been analyzed by the fission track (FT) method. The aim was to determine the cooling and uplift history of these mantle and associated crustal rocks after the last major metamorphic event that dates back to the Lower Miocene–Upper Oligocene time (～22–24 Ma). The zircon FT analyses give an average cooling—i.e., below 320 °C—age of ～19.5 Ma. In addition, the apatite FT data give an average cooling—i.e., below 110 °C—age of ～15.5 Ma. Taking into account the thermal properties of the different thermochronological systems used in this work, we have estimated a rate of cooling close to 50 °C/Ma. This cooling rate constrains a denudation rate of about ～2 mm year?1 from 20 to 15 Ma. These results are similar to those determined in the Ronda peridotite massif of the Betic Cordilleras documenting that some ultrabasic massifs of the internal zones of the two segments of the Gibraltar Arc have a similar evolution. However, Burdigalian sediments occur along the Betic segment (Alozaina area, western Betic segment) unconformably overlying peridotite. At this site, ultramafic rock was exposed to weathering at ages ranging from 20.43 to 15.97 Ma. Since the Beni Bousera peridotite was still at depth until 15.5 Ma, we infer that no simple age projection from massif to massif is possible along the Gibraltar Arc. Moreover, the confined fission track lengths data reveal that a light warming (～100 °C) has reheated the massif during the Late Miocene before the Pliocene–Quaternary tectonic uplift.  相似文献   

15.
Cooling rates based on the retrograde diffusion of Fe2+ and Mg between garnet and biotite inclusions commonly show two contrasting scenarios: a) narrow closure temperature range with apparent absence of retrograde diffusion; or b) high result dispersion due to compositional variations in garnet and biotite. Cooling rates from migmatites, felsic and mafic granulites from Ribeira Fold Belt (SE Brazil) also show these two scenarios. Although the former can be explained by very fast cooling, the latter is often the result of open-system behaviour caused by deformation. Retrogressive cooling during the exhumation of granulite-facies rocks is often processed by thrusting and shearing which may cause plastic deformation, fractures and cracks in the garnet megablasts, allowing chemical diffusion outside the garnet megablast – biotite inclusion system.However, a careful use of garnets and biotites with large Fe/Mg variation and software that reduces result dispersion provides a good correlation between closure temperatures and the size of biotite inclusions which are mostly due to diffusion and compositional readjustment to thermal evolution during retrogression.Results show that felsic and mafic granulites have low cooling rates (1–2 °C/Ma) at higher temperatures and high cooling rates (∼100 °C/Ma) at lower temperatures, suggesting a two-step cooling/exhumation process, whereas migmatites show a small decrease in cooling rates during cooling (from 2.0 to 0.5 °C/Ma). These results agree with previously obtained thermochronological data, which indicates that this method is a valid tool to obtain meaningful petrological cooling rates in complex high-grade orogenic belts, such as the Ribeira Fold Belt.  相似文献   

16.
We present a tectonic map of the Tauern Window and surrounding units (Eastern Alps, Austria), combined with a series of crustal-scale cross-sections parallel and perpendicular to the Alpine orogen. This compilation, largely based on literature data and completed by own investigations, reveals that the present-day structure of the Tauern Window is primarily characterized by a crustal-scale duplex, the Venediger Duplex (Venediger Nappe system), formed during the Oligocene, and overprinted by doming and lateral extrusion during the Miocene. This severe Miocene overprint was most probably triggered by the indentation of the Southalpine Units east of the Giudicarie Belt, initiating at 23–21 Ma and linked to a lithosphere-scale reorganization of the geometry of mantle slabs. A kinematic reconstruction shows that accretion of European lithosphere and oceanic domains to the Adriatic (Austroalpine) upper plate, accompanied by high-pressure overprint of some of the units of the Tauern Window, has a long history, starting in Turonian time (around 90 Ma) and culminating in Lutetian to Bartonian time (45–37 Ma).  相似文献   

17.
New 40Ar/39Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at 42 Ma to 10 kbar and 400 °C at 39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at 35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex.  相似文献   

18.
Abstract 40Ar/39Ar data collected from hornblende, muscovite, biotite and K-feldspar constrain the P-T-t history of the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile. These data show two periods of rapid cooling, the first between c. 500 and c. 325° C at rates ≥25° C Ma-1, and the second between c. 250 and c. 200°C. For high-T cooling, 40Ar/39Ar ages are spatially disparate and depend on metamorphic grade: rocks that record deeper and hotter peak metamorphic conditions have younger 40Ar/39Ar ages. Sillimanite- and kyanite-grade rocks in the south-central part of the complex cooled latest: 40Ar/39Ar Hbl = 73–77 Ma, Ms = 67–70 Ma, Bt = 68 Ma, and oldest Kfs = 65 Ma. Thermobarometry and P-T path studies of these rocks indicate that maximum burial of 26–30 km at 575–625° C may have been followed by as much as 10 km of exhumation with heating of 25–50° C. Staurolite-grade rocks have intermediate 40Ar/39Ar ages: Hbl = 84–86 Ma, Ms = 71 Ma, Bt = 72–75 Ma, and oldest Kfs = 80 Ma. Thermobarometry on these rocks indicates maximum burial of 19–26 km at temperatures of 550–580° C. Garnet-grade rocks have the oldest ages: Ms = 72 Ma and oldest Kfs = 91 Ma; peak P-T conditions were 525–550° C and 5–7 kbar. Regional metamorphic temperatures for greenschist facies rocks south of the Beagle Channel did not exceed c. 300–325° C from 110 Ma to the present, although the rocks are only 2 km from kyanite-bearing rocks to the north. One-dimensional thermal models allow limits to be placed on exhumation rates. Assuming a stable geothermal gradient of 20–25° C km-1, the maximum exhumation rate for the St-grade rocks is c. 2.5 mm yr-1, whereas the minimum exhumation rate for the Ky + Sil-grade rocks is c. 1.0 mm yr-1. Uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades, and so early differential exhumation is inferred to have occurred. Petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c. 20 km of overburden. This behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid- to deep-crustal levels. The P-T-t data from Cordillera Darwin (1) are best explained by a ‘wedge extrusion’model, in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the Magallanes sedimentary basin, (2) suggest that differential exhumation occurred initially, with St-grade rocks exhuming faster than Ky + Sil-grade rocks, and (3) show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions.  相似文献   

19.
A combination of four thermochronometers [zircon fission track (ZFT), zircon (U–Th)/He (ZHe), apatite fission track (AFT) and apatite (U–Th–[Sm])/He (AHe) dating methods] applied to a valley to ridge transect is used to resolve the issues of metamorphic, exhumation and topographic evolution of the Nízke Tatry Mts. in the Western Carpathians. The ZFT ages of 132.1 ± 8.3, 155.1 ± 12.9, 146.8 ± 8.6 and 144.9 ± 11.0 Ma show that Variscan crystalline basement of the Nízke Tatry Mts. was heated to temperatures >210°C during the Mesozoic and experienced a low-grade Alpine metamorphic overprint. ZHe and AFT ages, clustering at ~55–40 and ~45–40 Ma, respectively, revealed a rapid Eocene cooling event, documenting erosional and/or tectonic exhumation related to the collapse of the Carpathian orogenic wedge. This is the first evidence that exhumation of crystalline cores in the Western Carpathians took place in the Eocene and not in the Cretaceous as traditionally believed. Bimodal AFT length distributions, Early Miocene AHe ages and thermal modelling results suggest that the samples were heated to temperatures of ~55–90°C during Oligocene–Miocene times. This thermal event may be related either to the Oligocene/Miocene sedimentary burial, or Miocene magmatic activity and increased heat flow. This finding supports the concept of thermal instability of the Carpathian crystalline bodies during the post-Eocene period.  相似文献   

20.
《Geodinamica Acta》2000,13(5):281-292
The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by 40Ar/39Ar laserprobe mica analyses. Preservation of pre-Alpine (∼ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号