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1.
The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 to 8.2 ± 1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4 ± 0.4 to 9.2 ± 0.3 Ma and 10.7 ± 1.0 to 8.9 ± 0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1 ± 1.4 Ma to 11.1 ± 1.0 Ma and 12.7 ± 2.8 Ma to 10.5 ± 2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3 ± 0.4 Ma and 9.8 ± 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of ∼ 6–8 km Myr− 1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at ∼ 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of ∼ 50 km between ∼ 16 and 8 Ma.  相似文献   

2.
This study reconstructs the late stages in the exhumation history of a nappe derived from the Meliatic accretionary wedge in the Western Carpathians by means of zircon and apatite (U-Th)/He dating. The Meliatic accretionary wedge formed due to the closure of the Neotethyan Triassic–Jurassic Meliata–Hallstatt Ocean in the Late Jurassic. The studied fragments of the blueschist-bearing Meliatic Bôrka Nappe were metamorphosed at low-temperature and high- to medium-pressure conditions at ca. 160–150 Ma and included into the accretionary wedge. The time of the accretionary wedge formation constrains the beginning of the Bôrka Nappe northward thrusting over the Gemeric Unit of the evolving Central Western Carpathians (CWC) orogenic wedge. The zircon (U-Th)/He data on four samples recorded three evolutionary stages: (i) cooling through the ∼180 °C isotherm at 130–120 Ma related to starting collapse of the accretionary wedge, following exhumation of the high-pressure slices in the Meliatic accretionary wedge; (ii) postponed exhumation and cooling of some fragments through the ∼180 °C isotherm from 115 to 95 Ma due to ongoing collapse of this wedge; and (iii) cooling from 80 to 65 Ma, postdating the thrusting (∼100–80 Ma) of the Bôrka Nappe slices during the Late Cretaceous compression related to formation of the CWC orogenic wedge. The third stage already documents cooling of the Meliatic Bôrka Nappe slices in the CWC orogenic wedge. The apatite (U-Th)/He data may indicate cooling of a Bôrka Nappe slice to near-surface temperatures at ∼65 Ma. The younger AHe age clusters indicate that at least one, or possibly two, reheating events could have occurred in the longer interval from ∼40 to ∼10 Ma during the Oligocene–Miocene. These were related to sedimentary burial and/or the magmatism as documented in other parts of the CWC.  相似文献   

3.
This paper reports ten new surface heat-flow density (qs) values for central and southern Israel (central Sinai Microplate), whose crystalline crust and lithosphere formed as part of the Neoproterozoic Arabian-Nubian Shield. Heat flow was calculated in Mesozoic sediments using the classical approach of heat-flow determination by implementing in the analysis high-precision continuous temperature logs obtained in air- and/or water-filled boreholes. Thermal conductivity (TC) measured for a large suite of rock samples of lithotypes making up the sequence was assigned to temperature gradients in intervals for which the lithology was known. The heat-flow values obtained for different depth intervals in a borehole as well as the average values for the individual borehole locations cover a narrow range, attesting heat-conduction conditions. A steady-state thermal model along an E–W crustal cross section through the area shows that the observed systematic spatial distribution of the qs values, which range between 50 and 62 mW m−2, can primarily be explained by variations in the thickness of the upper crust and in the ratio between sedimentary and crystalline rocks therein. Given the time lapse of thermal heat transfer through the lithosphere, the qs data monitor the crustal thermal conditions prior to rift- and plume-related lithospheric thermal perturbations that have started in the larger area ca. 30 Ma ago. Observed and modeled qs display the best fit for a pre-Oligocene lithosphere–asthenosphere boundary (LAB) at ∼150 km, which would be at the upper end of LAB depths determined from stable areas of the Arabian Shield (150–120 km) not affected by the young, deep-seated thermal processes that have caused a further uprise of the LAB. Our data imply or predict that the surface heat flow of the Sinai Microplate generally tends to increase along N–S and W–E traverses, from ∼45–50 mW m−2 to ∼55–60 mW m−2. Surface heat flows on the order of 55–60 mW m−2 may be common in the northern Arabian Shield, where it exhibits typical lithosphere structure and composition and is unaffected by young heating processes, compared to values of ≤45 mW m−2 recently determined in the southern Arabian Plate for the Arabian Platform.  相似文献   

4.
In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.  相似文献   

5.
The Alisitos arc is an approximately 300 × 30 km oceanic arc terrane that lies in the western wall of the Peninsular Ranges batholith south of the modern Agua Blanca fault zone in Baja California. We have completed detailed mapping and dating of a 50 × 30 km segment of this terrane in the El Rosario to Mission San Fernando areas, as well as reconnaissance mapping and dating in the next 50 × 30 km segment to the north, in the San Quintin area. We recognize two evolutionary phases in this part of the arc terrane: (I) extensional oceanic arc, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions at the onset of arc rifting, and (II) rifted oceanic arc, characterized by mafic effusive and hydroclastic rocks and abundant dike swarms. Two types of units are widespread enough to permit tentative stratigraphic correlation across much of this 100-km-long segment of the arc: a welded dacite ignimbrite (tuff of Aguajito), and a deepwater debris-avalanche deposit. New U–Pb zircon data from the volcanic and plutonic rocks of both phases indicate that the entire 4000-m-thick section accumulated in about 1.5 MY, at 111–110 MY. Southwestern North American sources for two zircon grains with Proterozoic 206Pb / 207Pb ages support the interpretation that the oceanic arc fringed North America rather than representing an exotic terrane.The excellent preservation and exposure of the Alistos arc terrane makes it ideal for three-dimensional study of the structural, stratigraphic and intrusive history of an oceanic arc terrane. The segment mapped and dated in detail has a central major subaerial edifice, flanked by a down-faulted deepwater marine basin to the north, and a volcano-bounded shallow-water marine basin to the south. The rugged down-faulted flank of the edifice produced mass wasting, plumbed large-volume eruptions to the surface, and caused pyroclastic flows to disintegrate into turbulent suspensions that mixed completely with water. In contrast, gentler slopes on the opposite flank allowed pyroclastic flows to enter the sea with integrity, and supported extensive buildups of bioherms. Caldera collapse on the major subaerial edifice ponded the tuff of Aguajito to a thickness of at least 3 km. The outflow ignimbrite forms a marker in nonmarine to shallow marine sections, and in deepwater sections it occurs as blocks up to 150 m long in a debris-avalanche deposit. These welded ignimbrite blocks were deposited hot enough to deform plastically and form peperite with the debris-avalanche matrix. The debris avalanche was likely triggered by injection of feeder dikes along the basin-bounding fault zone during the caldera-forming eruption.Intra-arc extension controlled very high subsidence rates, followed shortly thereafter by accretion through back-arc basin closure by 105 Ma. Accretion of the oceanic arc may have been accomplished by detachment of the upper crust along a still hot, thick middle crustal tonalitic layer, during subduction of mafic–ultramafic substrate.  相似文献   

6.
One of the main morphological changes along the Southern Central Andes occurs from 36° to 39°S. The northern portion is characterized by prominent basement structures and a thick-skinned orogenic front with relief of over 2000 m with a deep level of exhumation where more than 4 km of section has been eroded. Contrastingly, the southern part is formed by mildly inverted basement structures restricted mainly to the hinterland zone, which reaches only 1500–1700 m relief. We quantify the variable contributions of two main contractional stages through the construction of three regionally balanced sections across the Andes, constrained by field and geophysical data. Extensional re-activation described for this segment in late Oligocene-early Miocene and Pliocene to Quaternary times, after the two main contractional episodes, suggests only 3 km of stretching that represents 30–10% of the original longitude. We, therefore, conclude that while initial Late Cretaceous to Eocene compression was similar along strike (∼10–7 km), it is the contrasting degrees of Neogene shortening (∼16–6 km) that have played the largest role in the along strike differences in structure and morphology along this portion of the southern Andes. Variable Neogene arc expansion could be responsible for the contrasting contractional deformation: In the north, late Miocene arc-related rocks cover most of the retroarc zone (>200 km with respect to the late Miocene arc front in the south), presumably driven by a shallow subduction episode in the area, whereas to the south they remain restricted to the continental drainage divide. Other factors involving architecture of previous rift structures, are proposed as additional mechanisms that accommodated variable shortening magnitudes through inversion.  相似文献   

7.
Two late Mesozoic granitoids in the Luanchuan area of the East Qinling orogen are considered; ore-bearing rocks are granite porphyries and granodiorite, with K2O > Na2O, appearing in the form of stocks. The Laojunshan rocks contains dominantly monzonitic granite, with K2O ≈ Na2O, in the form of a batholith. Both the ore-bearing rocks and the Laojunshan rocks are highly siliceous and shoshonitic, high-K calc-alkaline, similar to some I-type granites. Light rare earth elements (LREEs) are enriched in both rock suites, although the Luanchuan ore-bearing granitoids have higher concentrations, with (La/Yb)N ratios twice that of the barren Laojunshan granite suite. Ore-bearing rocks have, therefore, undergone greater fractionation of heavy rare earth elements. All Laojunshan rocks have negative Eu anomalies, indicating plagioclase fractionation. δEu values are different in both rock suites, the values in the ore-bearing granites, ranging from 0.52 to 1.04, which are much higher than that of Laojunshan batholith, ranging from 0.4 to 0.65. (La/Sm)N values of ore-bearing granites are 5.32–8.28, while that of Laojunshan batholith are 3.75–5.77, confirming the observation that the ore-bearing granites have undergone a higher degree of strong differentiation than that of Lanjunshan batholith.Major and trace element data, and REE data, combined with isotope data from previous work and the close relationships between the tectonic settings of the barren and ore-bearing rocks indicate that both groups of rocks were derived from the lower crust. At ∼157 Ma, with the tectonic regime in transition from a syn-collisional to a post-collisional setting, highly fractionated granites ascended from their storage area via faults; at ∼145 Ma, ore-bearing plutons, which are triggered by slab melts, formed at the junctions of fault planes trending WNW-ESE and NE-SW. At ∼115 Ma, the tectonic regime changed from compression to extension; in this environment, the barren Laojunshan batholith was emplaced, representing the end of the collisional event.  相似文献   

8.
Large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events. Less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera-forming events produce dominantly effusive eruptions of small-volume rhyolite domes and flows. The Bearhead Rhyolite and associated Peralta Tuff Member in the Jemez volcanic field, New Mexico, represent small-volume eruptions from a large silicic magma system in which no caldera-forming event occurred, and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long-term evolution of continental silicic magma systems.40Ar/39Ar dating reveals that most units mapped as Bearhead Rhyolite and Peralta Tuff (the Main Group) were erupted during an ∼540 ka interval between 7.06 and 6.52 Ma. These rocks define a chemically coherent group of high-silica rhyolites that can be related by simple fractional crystallization models. Preceding the Main Group, minor amounts of unrelated trachydacite and low silica rhyolite were erupted at ∼11–9 and ∼8 Ma, respectively, whereas subsequent to the Main Group minor amounts of unrelated rhyolites were erupted at ∼6.1 and ∼1.5 Ma.The chemical coherency, apparent fractional crystallization-derived geochemical trends, large areal distribution of rhyolite domes (∼200 km2), and presence of a major hydrothermal system support the hypothesis that Main Group magmas were derived from a single, large, shallow magma chamber. The ∼540 ka eruptive interval demands input of heat into the system by replenishment with silicic melts, or basaltic underplating to maintain the Bearhead Rhyolite magma chamber.Although the volatile content of Main Group magmas was within the range of rhyolites from major caldera-forming eruptions such as the Bandelier and Bishop Tuffs, eruptions were smaller volume and dominantly effusive. Bearhead Rhyolite domes occur at the intersection of faults, and are cut by faults, suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera-forming eruption.  相似文献   

9.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

10.
Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Pálmason, G., Björnsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Rückens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rücken, Island, Norwegischer Kontinentalrand. Abschlusskolloquium, Hamburg zur Meteor-Expedition, vol. 45. DFG, Bonn, pp. 53–61], and here we report on the data and results of interpretation. Clear refracted phases to 90 km distance permit crustal and uppermost mantle structure to be modelled by ray tracing. The apparent P-wave velocities are around 4.5, 6–6.5, 7–7.6 and 8.2–8.7 km/s, but no wide-angle reflections have been clearly seen. Accompanying sparker reflection data reveal thin sediment ponds in the axial zone and up to 400 m thick sediments at 10 Ma crustal age. Ray tracing reveals the following model below the sediments: (1) a distinct, 1–2 km thick upper crust (layer 2A) with Vp increasing with age (to 10 Ma) from <3.4 to 4.9 km/s and with a vertical gradient of 0.1–0.2 km/s/km, (2) a lower crust or layer 3 beginning at depths of 2 (axis) to 4 km (10 Ma age) below sea level with 6.1–6.8 km/s and similar vertical gradients as above, (3) the lower crust bottoms at 5.2–9.5 km depth below sea level (0–10 Ma) with a marked discontinuity, underneath which (4) Vp rises from about 7.5–7.8 km/s (0–10 Ma) with a positive vertical gradient of, again, 0.1–0.2 km/s/km such that 8 km/s would be reached at 12 km and deeper near the axis. Our preferred interpretation is that the mantle begins at the distinct discontinuity (“Moho”), but a deeper “Moho” of Vp ≈ 8 km/s cannot be excluded. From Iceland southward to 60°N several experiments show a decrease of crustal thickness from 14 to 8 km. Velocity trends with age across the ridge reflect cooling and filling of cracks, and thickness trends probably suggest volcanic productivity variations as previously suggested.Gravity inversion concentrates on a profile across the ridge with the above seismic a priori information; with 0.2–0.5 km depth uncertainty it leads to a good fit (±2.5 mGal where seismic data exist). Best fitting densities are (in kg/m3) for sediments, 2180; upper crust, 2450–2570; lower crust, 2850–2940; mantle lithosphere, 3215–3240 with a deficit for an asthenospheric wedge of no more than −100 kg/m3. The morphological ridges and troughs superimposed on the SE ridge flank are partly correlated, partly anti-correlated with the Bouguer anomaly and suggest that variable crustal density variations accompany the morphology variations.  相似文献   

11.
The mid-Carboniferous Pelhřimov core complex, Bohemian Massif, is a crustal-scale elongated granite–migmatite dome interpreted to have formed by gravity-driven diapiric upwelling of the metapelitic middle crust. The vertical diapiric flow is evidenced by outward-dipping foliation and lineation patterns, deformation coeval with the widespread presence of melt, rapid exhumation of the dome center from depths corresponding to pressure of about 0.6 GPa to shallow levels (pressure less than 0.2 GPa) within 2 M.y., and kinematic indicators of downward return flow of the mantling rocks. As compared to common diapirs, however, the Pelhřimov complex exhibits a more complicated inferred strain pattern with two perpendicular, irregularly alternating directions of horizontal extension in what is interpreted as the diapir head. Comparison of structural data from migmatites with anisotropy of magnetic susceptibility (AMS) data in granites also reveals that only final increments of strain are recorded in the granites. The map dimensions and gravity image of the complex suggest that the diapiric upwelling affected a large portion of the orogen's interior between two microplates brought together during continental collision. The northwesterly microplate (the upper-crustal Teplá–Barrandian unit) collapsed vertically as an ‘elevator’ at around 346–337 Ma whereas the easterly microplate (Brunia) was underthrust beneath the Moldanubian rocks during ∼346–330 Ma (the indentor). It is suggested that these microplates then acted as cold and rigid margins localizing mid-crustal diapirism and associated voluminous S-type granite plutons inbetween, parallel to the edge of the Brunia indentor.We conclude that bringing together soft metapelitic middle crust with two rigid lithospheric blocks during collision resulted in significant lateral temperature and strength variations across the orogen's interior. A general conclusion from these inferences is that granite–migmatite domes delineating margins of collided microplates may form as crustal-scale structures accommodating late-orogenic isostatic reequilibration.  相似文献   

12.
Long Valley Caldera is an active volcanic region in east central California. Surface deformation on the resurgent dome within the caldera was an order of magnitude higher for the five-month period September 1997 through January 1998 compared to the previous three-year average. However, the location of the immediate (shallow) source of deformation remained essentially constant, 5–7 km beneath the dome, near the top of a region of probable magma accumulation defined by seismic data. Similarly, although the rate of seismic moment release increased dramatically, earthquake locations remained similar to earlier periods. The rate of deformation increased exponentially between April–May 1997 and late November 1997 with a time constant of ∼55–65 days, after which it decreased exponentially with about the same time constant. We develop a model consistent with these observations and also consistent with independent constraints on sub-surface rheology from thermal, geochemical and laboratory data. Deformation at sites on the resurgent dome most sensitive to the shallow deformation source are well fit by a model with a single pressure source at 6 km depth which experienced a pressure pulse that began in late 1996, peaked in November 1997, close to the time of major seismic moment release, and essentially ended in mid-1999. The pressure source in our model is surrounded by a 1 km thick “shell” of Maxwell viscoelastic material (shell viscosity 1016 Pa s) within an elastic half space, and has peak values that are much lower than corresponding purely elastic half space models. The shell viscosity is characteristic of a weak, deformable solid, e.g. quartz-bearing country rock surrounding the magma chamber at temperatures in the range 500–600°C, i.e. above the brittle–ductile transition, and/or largely crystallized rhyolite near its solidus temperature of ∼670°C, material that probably exists near the top of the zoned magma chamber at Long Valley.  相似文献   

13.
All-sky camera (ASC), Global Positioning System (GPS), and ionosonde measurements were used to investigate the upper atmospheric variations at mid-latitude during the strong geomagnetic storm on October 29–31, 2003. An arc-shaped 630.0 nm emission was observed in the northern sky on all-sky images taken at Mt. Bohyun (36.2°N, 128.9°E, GMLAT=29°N) in Korea during 17:48–8:58 UT in the main phase of the geomagnetic storm on October 29. The NmF2 and hmF2 from the ionosonde show strong disturbances at that time. The vertical profiles of electron densities, calculated by the ionospheric tomographic method using ground-based GPS slant total electron contents measurements, show the largest value at ∼440 km height at 18:30 UT on October 29 when the enhancements of OI 630.0 nm emission were observed. The arc-shaped red emission observed during the main phase of the magnetic storm is likely a low-latitude red aurora due to its short duration of ∼1 h. The result implies that the plasmapause was at L=1.4–1.6 during the geomagnetic storm. The fact that the arc did not follow a constant L-value appears to suggest that neutral precipitation and a traveling ionospheric disturbance could also be the cause of the arc.  相似文献   

14.
《Geofísica Internacional》2014,53(4):435-456
The Rancho Nuevo semi-circular structure is a geomorphological structure defined by drainage patterns in coastal Sonora, about 160 km NW of Hermosillo. The structure is about 15 by 30 km, and it is cored by felsic to intermediate plutons (granodiorite, monzogranite, quartz-porphyry) covered by Miocene volcanic rocks. This work is focused on the deformation of the intrusives which cover most of study area.The plutons are a co-magmatic suite dated between 71 ± 1.1 and 67.9 ± 1.0 Ma (U-Pb zircon, LA-ICPMS). The most voluminous unit is a granodiorite characterized by conspicuous sigmoidal fractures at the scale of high resolution satellite images, along which rhyolite dikes were emplaced about 13.2 Ma. Magnetic fabric (AMS) and paleomagnetic data were collected from 27 sites in the granodiorite. Magnetic fabrics are weak but well developed, and are characterized by steep foliation planes with strikes that follow the sigmoidal fracture pattern and suggest NE-SW to NW-SE flattening after emplacement. The characteristic magnetization is of dual polarity, but it is dominantly reverse consistent with emplacement during chron C31r. The prevalent magnetization is southwest and moderately steep negative (ten sites), a discordant direction rotated clockwise about 41 ° ± 11 with respect to the expected Late Cretaceous reference direction, also indicating gentle southward tilt. There is, however, paleomagnetic evidence suggesting that the structure did not rotate as a rigid body, but it deformed internally instead. These data are interpreted to indicate that the Rancho Nuevo semicircular structure is a large-scale, dextral, brittle-ductile shear indicator. The age of the dikes and the fact that they are covered discordantly by rocks assigned to the tuff of San Felipe indicate that northwest, srike-slip, motion of Baja California peninsula (and thus the Pacific plate relative to North America) was accommodated by faults in coastal Sonora about 13 Ma ago.  相似文献   

15.
There are two main ocean-ridge discontinuities in Iceland: the Tjörnes Fracture Zone (TFZ) and the South Iceland Seismic Zone (SISZ). The TFZ is a 120-km-long and as much as 70-km-wide WNW-trending zone of high seismicity. It has three main seismic lineaments: the Husavik-Flatey Fault (HFF), the Dalvik lineament, and the Grimsey lineament. The HFF, a dextral strike-slip fault and active as a transform fault for about 9 Ma, has a cumulative transform-parallel displacement of some 60 km. Offshore, the HFF is marked by a transform (fracture-zone) valley, 5–10 km wide and 3–4 km deep. Onshore the Flateyjarskagi Peninsula the HFF is marked by a 3–5-km-wide zone of intense crustal deformation with numerous strike-slip and normal faults, transform-parallel dykes, dense sets of mineral veins, and subzones of completely crushed rocks, that is, fault cores. Where the HFF comes on land on Tjörnes there is a similar, but much thinner, zone of crushed rocks. The seismic lineaments are located a few tens of kilometres south (Dalvik) and north (Grimsey) of, and run subparallel with, the HFF. Both lineaments are composed of sets of NNW-trending sinistral faults arranged en echelon.The SISZ is a 70-km-long and 10–20-km wide zone of almost continuous seismicity located between the overlapping West and East Volcanic Zones. It produces the largest earthquakes in Iceland, some of which exceed M7, during which the N–S width of the zone may be as great as 50–60 km. The SISZ is partly covered with Holocene lava flows where the seismogenic faults occur as dextral NNE-trending and sinistral ENE-trending conjugate arrays with push-ups between their nearby ends. The same fault-segment trends occur in the Pleistocene pile north of the Holocene lava flows.The HFF is neither perpendicular to the nearby ridge segments nor parallel with the spreading vector. As a consequence, the North Volcanic Zone has propagated to the north and the Kolbeinsey Ridge to the south during the past 1 Ma, resulting in the development of the Grimsey and Dalvik lineaments. Similarly, the tip of the East Volcanic Zone has been propagating rapidly to the southwest during the past 3 Ma. The tip has been at its present location for no more than several hundred thousand years, thus making the SISZ less stable than the HFF. If the propagation of the tip of the East Volcanic Zone continues, it will eventually reach the Reykjanes Ridge, whereby either the West or the East Volcanic Zone becomes extinct. Then the SISZ dies out as a major seismic zone.  相似文献   

16.
《Continental Shelf Research》2006,26(17-18):2241-2259
The Amazon River spawns a vast mobile mudbelt extending ∼1600 km from the equator to the Orinoco delta. Deposits along the Amazon–Guianas coastline are characterized by some of the highest Corg remineralization rates reported for estuarine, deltaic, or shelf deposits, however, paradoxically, except where stabilized by mangroves or intertidal algal mats, they are usually suboxic and nonsulfidic. A combination of tides, wind-driven waves, and coastal currents forms massive fluid muds and mobile surface sediment layers ∼0.5–2 m thick which are dynamically refluxed and frequently reoxidized. Overall, the seabed functions as a periodically mixed batch reactor, efficiently remineralizing organic matter in a gigantic sedimentary incinerator of global importance. Amazon River material entering the head of this dynamic dispersal system carries an initial terrestrial sedimentary Corg loading of ∼ 0.7 mg C m−2 particle surface area. Total Corg loading is lowered to ∼ 0.2 mg C m−2 in the proximal delta topset, ∼60–70% of which remains of terrestrial origin. Loading decreases further to 0.12–0.14 mg C m−2 (∼60% terrestrial) in mudbanks ∼600 km downdrift along French Guiana, values comparable to those found in the oligotrophic deepsea. DOC/ΣCO2 ratios in pore waters of French Guiana mudbanks indicate that >90% of metabolized organic substrates are completely oxidized. Within the Amazon delta topset at the head of the dispersal system, both terrestrial and marine organic matter contribute substantially to early diagenetic remineralization, although reactive marine substrate dominates (∼60–70%). The conditional rate constant for terrestrial Corg in the delta topset is ∼0.2 a−1. As sedimentary Corg is depleted during transit, marine sources become virtually the exclusive substrate for remineralization except very near the mangrove shoreline. The δ13C and Δ14C values of pore water ΣCO2 in mudbanks demonstrate that the primary source of remineralized organic matter within ∼1 km of shore is a small quantity of bomb signature marine plankton (+80‰). Thus, fresh marine organic material is constantly entrained into mobile deposits and increasingly drives early diagenetic reactions along the transit path. Relatively refractory terrestrial Corg is lost more slowly but steadily during sedimentary refluxing and suboxic diagenesis. Amazon Fan deposits formed during low sea level stand largely bypassed this suboxic sedimentary incinerator and stored material with up to ∼3X the modern high stand inner shelf Corg load (Keil et al., 1997b. Proceedings of the Ocean Drilling Program, Scientific Results. Vol. 155. pp. 531–537). Sedimentary dynamics, including frequency and magnitude of remobilization, and the nature of dispersal systems are clearly key controls on diagenetic processes, biogeochemical cycling, and global C storage along the continental margins.  相似文献   

17.
Garnet geochronology was used to provide the first direct measurement of the timing of eclogitization in the central Himalaya. Lu–Hf dates from garnet separates in one relict eclogite from the Arun River Valley in eastern Nepal indicate an age of 20.7 ± 0.4 Ma, significantly younger than ultra-high pressure eclogites from the western Himalaya, reflecting either different origins or substantial time lags in tectonics along strike. Four proximal garnet amphibolites from structurally lower horizons are 14–15 Ma, similar to post-eclogitization ages published for rocks along strike in southern Tibet. P–T calculations indicate three metamorphic episodes for the eclogite: i) eclogite-facies metamorphism at ～ 670 °C and ≥ 15 kbar at 23–16 Ma; ii) a peak-T granulite event at ～ 780 °C and 12 kbar; and iii) late-stage amphibolite-facies metamorphism at ～ 675 °C and 6 kbar at ～ 14 Ma. The garnet amphibolites were metamorphosed at ～ 660 °C. Three models are considered to explain the observed P–T–t evolution. The first assumes that the Main Himalayan Thrust (basal thrust of the Himalayan thrust system) cuts deeper at Arun than elsewhere. While conceptually the simplest, this model has difficulty explaining both the granulite-facies overprint and the pulse of exhumation between 25 and 14 Ma. A second model assumes that (aborted) subduction, slab breakoff, and ascent of India's leading edge occurred diachronously: ～ 50 Ma in the western Himalaya, ～ 25 Ma in the central Himalaya of Nepal, and presumably later in the eastern Himalaya. This model explains the P–T–t path, particularly heating during initial exhumation, but implies significant along-strike diachroneity, which is generally lacking in other features of the Himalaya. A third model assumes repeated loss of mantle lithosphere, first by slab breakoff at ～ 50 Ma, and again by delamination at ～ 25 Ma; this model explains the P–T–t path, but requires geographically restricted tectonic behavior at Arun. The P–T–t history of the Arun eclogites may imply a change in the physical state of the Himalayan metamorphic wedge at 16–25 Ma, ultimately giving rise to the Main Central Thrust by 15–16 Ma.  相似文献   

18.
The southernmost segment of the Andes of southern Patagonia and Tierra del Fuego forms a ～ 700 km long orogenic re-entrant with an interlimb angle of ～ 90° known as Patagonian orocline. No reliable paleomagnetic evidence has been gathered so far to assess whether this great orogenic bend is a primary arc formed over an articulated paleomargin, or is due to bending of a previously less curved (or rectilinear) chain. Here we report on an extensive paleomagnetic and anisotropy of magnetic susceptibility (AMS) study carried out on 22 sites (298 oriented cores), predominantly sampled in Eocene marine clays from the external Magallanes belt of Tierra del Fuego. Five sites (out of six giving reliable paleomagnetic results) containing magnetite and subordinate iron sulphides yield a positive fold test at the 99% significance level, and document no significant rotation since ～ 50 Ma. Thus, the Patagonian orocline is either a primary bend, or an orocline formed after Cretaceous–earliest Tertiary rotations. Our data imply that the opening of the Drake Passage between South America and Antarctica (probably causing the onset of Antarctica glaciation and global climate cooling), was definitely not related to the formation of the Patagonian orocline, but was likely the sole consequence of the 32 ± 2 Ma Scotia plate spreading. Well-defined magnetic lineations gathered at 18 sites from the Magallanes belt are sub-parallel to (mostly E–W) local fold axes, while they trend randomly at two sites from the Magallanes foreland. Our and previous AMS data consistently show that the Fuegian Andes were characterized by a N–S compression and northward displacing fold–thrust sheets during Eocene–early Miocene times (50–20 Ma), an unexpected kinematics considering coeval South America–Antarctica relative motion. Both paleomagnetic and AMS data suggest no significant influence from the E–W left-lateral Magallanes–Fagnano strike–slip fault system (MFFS), running a few kilometres south of our sampling sites. We thus speculate that strike–slip fault offset in the Fuegian Andes may range in the lower bound values (～ 20 km) among those proposed so far. In any case our data exclude any influence of strike–slip tectonics on the genesis of the great orogenic bend called Patagonian orocline.  相似文献   

19.
The inner Northern Apennines (western Tuscany and Tyrrhenian basin) is characterized by a relatively thin continental crust (∼20–25 km), high heat flow (>100 mW m−2), and the presence of relevant tectonic elision of stratigraphic sequences, a setting known as Serie Ridotta. These features are normally ascribed to an extensional deformation that affected the back-arc area above the subducting Adria plate since the Early-Middle Miocene (∼16 Ma). However, various geophysical studies image the continental crust to be currently affected by W-dipping thrust faults (and associated basement uplifts) that have not been obliterated by this claimed long-lasting extensional process. These observations raise the question whether the thrusts are older or younger than the continental extension. To address this question we have reprocessed and interpreted the deep seismic reflection profile CROP03/c that crosses the onshore hinterland sector, and investigated the structural setting of some of the Late Miocene-Pliocene hinterland basins (Cinigiano-Baccinello, Siena-Radicofani, Tafone, Albegna and Radicondoli basins) that are situated at the front or in-between the basement uplifts. The analysis of field structures and commercial seismic profiles has allowed the recognition that both substratum and basins’ infill have been intensely shortened. These findings and the architecture of the basins suggest that the latter developed under a contractional regime, which would have started around 8.5 Ma with the onset of the continental sedimentation. This compressive stress state followed an earlier phase of continental extension that presumably started at ∼16 Ma (with the blocking of the Corsica-Sardinia rotation), and thinned both the continental crust and sedimentary cover producing most of the Serie Ridotta. The main phases of basin shortening are bracketed between 7.5 and 3.5 Ma, and thus overlap with the increase in the exhumation rate of the metamorphic cores at ∼6–4 Ma determined through thermochronological data. We therefore propose a correlation between the basin deformation and the activity of the nearby basement thrusts, which would have thus shortened a previously thinned continental crust. This chronology of deformation may suggest a geodynamic model in which the back-arc and hinterland sector of the Northern Apennines was recompressed during Late Miocene-Early Pliocene times. This evolution may be explained through different speculative scenarios involving a blockage of the subduction process, which may vary between end members of complete slab detachment and stalled subduction.  相似文献   

20.
Located on the margin of the west Alboran basin, the Gibraltar Arc (Betic-Rif mountain belt) displays post-Pliocene vertical movements evidenced by uplifted marine sedimentary basins and marine terraces. Quantification of vertical movements is an important clue to understand the origin of present-day relief generation in the Betic-Rif mountain chain together with the causes of the Messinian Salinity Crisis. In this paper, we present the results of a pluridisciplinary study combining an analysis of low temperature thermochronology and Pliocene basins evolution to constrain the exhumation history and surface uplift of internals units of the Rif belt (Northern Morocco). The mean (U-Th)/He apatite ages obtained from 11 samples are comprised between 14.1 and 17.8 Ma and display a wide dispersion, which could be explained by a great variability of apatite chemistries in the analyzed samples. No correlations between altitude and age have been found along altitudinal profile suggesting a rapid exhumation during this period. Thermal modeling using our (U-Th)/He apatite ages and geochronological data previously obtained in the same area (40Ar/39Ar and K/Ar data on biotite, zircon and apatite fission track) allow us to propose a cooling history. The rocks suffered a rapid cooling at 60–100 °C/Ma between 22.5 and 19 Ma, then cooled to temperatures around 40 °C between 19 and 18 Ma. They were re-heated at around 110 °C between 18 and 15 Ma then rapidly cooled and exhumed to reach the surface temperature at around 13 Ma. The re-heating could be related to a renewal in thrusting and burying of the inner zones. Between 15 and 13 Ma the cooling resumed at a rate of 50 °C/Ma indicating an exhumation rate of 0.8 mm/y considering an average 40 °C/km geothermal gradient. This exhumation may be linked to the extension in the Alboran Sea. Otherwise biostratigraphic and sedimentological analysis of Pliocene basins of the internal Rif provided informations on the more recent events and vertical movements. Pliocene deposits of the Rifian coast represent the passive infilling of palaeo-rias between 5.33 and 3.8 Ma. The whole coastal area was uplifted at slow average rates (0.01–0.03 mm/y) in relation with a northeastward tilting of 0.2–0.3° since the Lower-Pliocene. A late Pliocene to present extensional tectonics associated to uplift has been identified all along the coastal ranges of the Internal Zone of the Rif chain. This extension was coeval with the major late Pliocene to Pleistocene extensional episode of the Alboran Sea and appears to be still active nowadays. No significant late Messinian uplift was evidenced, thus calling into question the geodynamic models relating the closure of the marine gateways and the MSC to slab roll back.  相似文献   

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