The Balakot Formation foreland basin sediments, located in the Hazara\|Kashmir syntaxis, Pakistan, consist of a >8km thick succession of clastic red beds and calcrete, interpreted as deposited in a tidal environmen...The Balakot Formation foreland basin sediments, located in the Hazara\|Kashmir syntaxis, Pakistan, consist of a >8km thick succession of clastic red beds and calcrete, interpreted as deposited in a tidal environment. Within this sequence are intercalated four distinct grey fossiliferous marl and silt units, 20～60m thick, dated at Late Palaeocene to Mid Eocene (Bossart & Ottiger 1989). Thus the Balakot Formation has been interpreted as the oldest continental foreland basin deposits and has been used to determine the timing of India\|Eurasia collision (Rowley 1996) which has implications for the degree of diachroneity of collision (Burbank et al. 1996; Uddin and Lundberg 1998) and rapidity of metamorphism (Treloar 1997), to interpret the palaeotectonics and palaeogeography of the mountain belt (Critelli and Garzanti 1994; Pivnik and Wells 1996), understand the relationship between mountain belt evolution and exhumation processes (Treloar et al. 1991) and construct models of foreland basin evolution and geometry (DeCelles et al. 1998; Burbank et al 1996).展开更多
This work is based on apatite fission-track analysis of samples(mostly granites)from the basement of the Cretaceous-Tertiary Phosphate and Ganntour Plateaus, exposed in the Jebilet and Rehamna massifs (Western Meseta,...This work is based on apatite fission-track analysis of samples(mostly granites)from the basement of the Cretaceous-Tertiary Phosphate and Ganntour Plateaus, exposed in the Jebilet and Rehamna massifs (Western Meseta,Morocco).This basement展开更多
A key issue in orogenic research today is the recognition and explanation of normal faulting in the heart of collisional mountain belts. The active Himalayan system remains an ideal locality for studying this phenomen...A key issue in orogenic research today is the recognition and explanation of normal faulting in the heart of collisional mountain belts. The active Himalayan system remains an ideal locality for studying this phenomenon, both as E—W synconvergent extension of the Tibetan plateau and normal motion on the South Tibetan Detachment System (STDS). However, these processes are difficult to correlate with the evolution of the northwest Himalaya, particularly the Nanga Parbat syntaxis where a Neogene tectono\|thermal overprint partially obscures the early collisional history. An integrated programme of structural mapping, petrography, thermobarometry and isotopic dating is presented that places important constraints on both the early\| and pre\|Himalayan evolution of the syntaxis. These data include evidence for synconvergent, ductile extension predating syntaxis development, and improved isotopic correlation of the tectonic units with the familiar central Himalayan thrust sheets, building on the work of Whittington et al (1999).Recent studies have focused on the rapid exhumation of the Nanga Parbat\|Haramosh Massif (NPHM) during the last 10Ma, and the related Neogene thermal effects dominating the core of the massif (e.g. Zeitler et al. 1982, 1993). However, the degree of both structural and metamorphic Neogene overprinting varies within the massif, becoming weaker away from the summit region. In addition, the considerable variation in rock\|type outside the gneissic core results in both strain partitioning and various degrees of metamorphic reworking. Thus several workers (e.g. Wheeler et al. 1995) could reconstruct elements of the early and pre\|Himalayan history from field relations and mineral assemblages virtually untouched by Neogene processes. The eastern margin of the massif, in contrast to the active western margin, has remained largely unchanged during the Neogene, except for essentially passive rotation on the limb of the major syntaxial antiform. The original, ductile Main Mantle Thrust (MMT), which emplaced the Ladakh Island Arc (LIA) over the Indian margin in the late Cretaceous, is preserved in a steepened orientation. Dextral shear sense indicators in this steep fabric can be clearly related to southward thrusting on the MMT at peak metamorphic conditions during the early Himalayan stage (600～700°C and 900～1200MPa) once the N—S trending syntaxial antiform is unfolded.展开更多
The well preserved eclogitic rocks of the Tso Morari dome in eastern Ladakh, northwest Himalaya, provide information relevant to the exhumation of high pressure/low temperature rocks, and the early stage of the Himala...The well preserved eclogitic rocks of the Tso Morari dome in eastern Ladakh, northwest Himalaya, provide information relevant to the exhumation of high pressure/low temperature rocks, and the early stage of the Himalayan orogeny. The Tso Morari unit outcrops south of the Indus suture zone (Fig.1). The eclogitic dome is underlined on its eastern part by the Zildat normal fault where serpentinite lenses and partially hydrated peridotites are abundant. The close association of the high pressure rocks and serpentinites suggests a possible role of serpentinites in the exhumation of ultrahigh\|pressure rocks. To evaluate this possibility, geochemical analyses were carried out on the serpentinites closely associated with the Tso Morari eclogites.展开更多
Recently the (U\|Th)/He method on apatite has been applied to studies of cooling and exhumation in mountain belts in order to place constraints on the low temperature cooling history (Wolf et al., 1997; House et al., ...Recently the (U\|Th)/He method on apatite has been applied to studies of cooling and exhumation in mountain belts in order to place constraints on the low temperature cooling history (Wolf et al., 1997; House et al., 1998). Because the closure temperature of the method is 65～75℃, it provides useful information regarding the exhumation of rocks through the upper few kilometers of the earth’s crust. When the (U\|Th)/He method is coupled with methods such as 40 Ar/ 39 Ar with higher closure temperatures (210～500℃), a cooling history can be reconstructed over a wide range of temperatures. In eastern Nepal, in the Dudh Kosi valley a suite of samples from the Main Central Thrust (MCT) to the middle of the Higher Himalayan section was previously dated using 40 Ar/ 39 Ar on K\|feldspar, biotite, muscovite, and hornblende (Hubbard and Harrison,1989). We have applied the (U\|Th)/He method to apatite from these same samples. The structurally highest site, Ngozumba, is in the middle to upper part of the Higher Himalaya in the Gokyo valley. At Ngozumba previous 40 Ar/ 39 Ar ages include: muscovite at (16 6±0 4)Ma (closure temperature (CT)350℃), biotite at (16 8±1 4)Ma (CT 300℃), and K\|feldspar at (15 5±1 8)Ma (CT 250℃).The apatite age for an augen gneiss from this location was (6 3±3 8)Ma suggesting slower cooling for that part of the range during the last 15Ma. Structurally below Ngozumba, in the MCT zone, 40 Ar/ 39 Ar ages include:(20 9±0 2)Ma for hornblende (CT 500℃), (12 0±0 2)Ma for muscovite, and (8 0±0 2)Ma for K\|feldspar (CT 220℃). The apatite age obtained for a biotite schist at this location was (4 6±0 2)Ma and is consistent with relatively constant cooling since 12Ma. In between the MCT and the Ngozumba site samples were collected near the settlement of Ghat that yielded anomalously young 40 Ar/ 39 Ar ages:(7 7±0 4)Ma for muscovite, (7 5±0 6)Ma for biotite, and (3 6±0 2)Ma for K\|feldspar (CT 210℃). The apatite age is consistent with these young ages at (0 8±0 1)Ma. Plausible explanations for the young ages at this site include: post\|MCT age movement on a small\|scale shear zone or late\|stage movement of hydro\|thermal fluids through this localized zone. The MCT zone 40 Ar/ 39 Ar ages were used to constrain a one\|dimensional thermal model aimed at understanding rates of exhumation (Hubbard et al., 1991). The model yielded exhumation rates of 0 7～1 9km/Ma for the MCT zone region. The (U\|Th)/He age for apatite from this region is consistent with the model results. By coupling the (U\|Th)/He ages with the 40 Ar/ 39 Ar ages in this region we can determine that exhumation processes have acted at a relatively constant rate during the last 15～20 million years (with the possible exception of the Ghat location). Major fault activity likely pre\|dates this time period, or has had little affect on the cooling history.展开更多
文摘The Balakot Formation foreland basin sediments, located in the Hazara\|Kashmir syntaxis, Pakistan, consist of a >8km thick succession of clastic red beds and calcrete, interpreted as deposited in a tidal environment. Within this sequence are intercalated four distinct grey fossiliferous marl and silt units, 20～60m thick, dated at Late Palaeocene to Mid Eocene (Bossart & Ottiger 1989). Thus the Balakot Formation has been interpreted as the oldest continental foreland basin deposits and has been used to determine the timing of India\|Eurasia collision (Rowley 1996) which has implications for the degree of diachroneity of collision (Burbank et al. 1996; Uddin and Lundberg 1998) and rapidity of metamorphism (Treloar 1997), to interpret the palaeotectonics and palaeogeography of the mountain belt (Critelli and Garzanti 1994; Pivnik and Wells 1996), understand the relationship between mountain belt evolution and exhumation processes (Treloar et al. 1991) and construct models of foreland basin evolution and geometry (DeCelles et al. 1998; Burbank et al 1996).
文摘This work is based on apatite fission-track analysis of samples(mostly granites)from the basement of the Cretaceous-Tertiary Phosphate and Ganntour Plateaus, exposed in the Jebilet and Rehamna massifs (Western Meseta,Morocco).This basement
文摘A key issue in orogenic research today is the recognition and explanation of normal faulting in the heart of collisional mountain belts. The active Himalayan system remains an ideal locality for studying this phenomenon, both as E—W synconvergent extension of the Tibetan plateau and normal motion on the South Tibetan Detachment System (STDS). However, these processes are difficult to correlate with the evolution of the northwest Himalaya, particularly the Nanga Parbat syntaxis where a Neogene tectono\|thermal overprint partially obscures the early collisional history. An integrated programme of structural mapping, petrography, thermobarometry and isotopic dating is presented that places important constraints on both the early\| and pre\|Himalayan evolution of the syntaxis. These data include evidence for synconvergent, ductile extension predating syntaxis development, and improved isotopic correlation of the tectonic units with the familiar central Himalayan thrust sheets, building on the work of Whittington et al (1999).Recent studies have focused on the rapid exhumation of the Nanga Parbat\|Haramosh Massif (NPHM) during the last 10Ma, and the related Neogene thermal effects dominating the core of the massif (e.g. Zeitler et al. 1982, 1993). However, the degree of both structural and metamorphic Neogene overprinting varies within the massif, becoming weaker away from the summit region. In addition, the considerable variation in rock\|type outside the gneissic core results in both strain partitioning and various degrees of metamorphic reworking. Thus several workers (e.g. Wheeler et al. 1995) could reconstruct elements of the early and pre\|Himalayan history from field relations and mineral assemblages virtually untouched by Neogene processes. The eastern margin of the massif, in contrast to the active western margin, has remained largely unchanged during the Neogene, except for essentially passive rotation on the limb of the major syntaxial antiform. The original, ductile Main Mantle Thrust (MMT), which emplaced the Ladakh Island Arc (LIA) over the Indian margin in the late Cretaceous, is preserved in a steepened orientation. Dextral shear sense indicators in this steep fabric can be clearly related to southward thrusting on the MMT at peak metamorphic conditions during the early Himalayan stage (600～700°C and 900～1200MPa) once the N—S trending syntaxial antiform is unfolded.
文摘The well preserved eclogitic rocks of the Tso Morari dome in eastern Ladakh, northwest Himalaya, provide information relevant to the exhumation of high pressure/low temperature rocks, and the early stage of the Himalayan orogeny. The Tso Morari unit outcrops south of the Indus suture zone (Fig.1). The eclogitic dome is underlined on its eastern part by the Zildat normal fault where serpentinite lenses and partially hydrated peridotites are abundant. The close association of the high pressure rocks and serpentinites suggests a possible role of serpentinites in the exhumation of ultrahigh\|pressure rocks. To evaluate this possibility, geochemical analyses were carried out on the serpentinites closely associated with the Tso Morari eclogites.
文摘Recently the (U\|Th)/He method on apatite has been applied to studies of cooling and exhumation in mountain belts in order to place constraints on the low temperature cooling history (Wolf et al., 1997; House et al., 1998). Because the closure temperature of the method is 65～75℃, it provides useful information regarding the exhumation of rocks through the upper few kilometers of the earth’s crust. When the (U\|Th)/He method is coupled with methods such as 40 Ar/ 39 Ar with higher closure temperatures (210～500℃), a cooling history can be reconstructed over a wide range of temperatures. In eastern Nepal, in the Dudh Kosi valley a suite of samples from the Main Central Thrust (MCT) to the middle of the Higher Himalayan section was previously dated using 40 Ar/ 39 Ar on K\|feldspar, biotite, muscovite, and hornblende (Hubbard and Harrison,1989). We have applied the (U\|Th)/He method to apatite from these same samples. The structurally highest site, Ngozumba, is in the middle to upper part of the Higher Himalaya in the Gokyo valley. At Ngozumba previous 40 Ar/ 39 Ar ages include: muscovite at (16 6±0 4)Ma (closure temperature (CT)350℃), biotite at (16 8±1 4)Ma (CT 300℃), and K\|feldspar at (15 5±1 8)Ma (CT 250℃).The apatite age for an augen gneiss from this location was (6 3±3 8)Ma suggesting slower cooling for that part of the range during the last 15Ma. Structurally below Ngozumba, in the MCT zone, 40 Ar/ 39 Ar ages include:(20 9±0 2)Ma for hornblende (CT 500℃), (12 0±0 2)Ma for muscovite, and (8 0±0 2)Ma for K\|feldspar (CT 220℃). The apatite age obtained for a biotite schist at this location was (4 6±0 2)Ma and is consistent with relatively constant cooling since 12Ma. In between the MCT and the Ngozumba site samples were collected near the settlement of Ghat that yielded anomalously young 40 Ar/ 39 Ar ages:(7 7±0 4)Ma for muscovite, (7 5±0 6)Ma for biotite, and (3 6±0 2)Ma for K\|feldspar (CT 210℃). The apatite age is consistent with these young ages at (0 8±0 1)Ma. Plausible explanations for the young ages at this site include: post\|MCT age movement on a small\|scale shear zone or late\|stage movement of hydro\|thermal fluids through this localized zone. The MCT zone 40 Ar/ 39 Ar ages were used to constrain a one\|dimensional thermal model aimed at understanding rates of exhumation (Hubbard et al., 1991). The model yielded exhumation rates of 0 7～1 9km/Ma for the MCT zone region. The (U\|Th)/He age for apatite from this region is consistent with the model results. By coupling the (U\|Th)/He ages with the 40 Ar/ 39 Ar ages in this region we can determine that exhumation processes have acted at a relatively constant rate during the last 15～20 million years (with the possible exception of the Ghat location). Major fault activity likely pre\|dates this time period, or has had little affect on the cooling history.
