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1.
New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and  29–26 Ma 40Ar/39Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation.

We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.  相似文献   


2.
The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below  2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature (PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism ( 2.7 Gpa) began at 242.2 ± 0.4 Ma (n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (>  4 Gpa) at 227.3 ± 0.7 Ma (n = 72), before exhumation (<  220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.  相似文献   

3.
The four most recent large mass extinction events in the Phanerozoic – the Cretaceous–Tertiary (KT), the Triassic–Jurassic (TJ), and the Permo-Triassic (PT) and Guadalupian–Tatarian (GT) doublet – are associated with a major flood basalt eruption, with the timing of peak volcanic activity corresponding within measurement uncertainties to the extinction event. Three magnetic superchrons precede the four largest Phanerozoic extinctions. The Cretaceous Long Normal Superchron (duration  35 Myr) precedes the KT and the Permian Kiaman Long Reversed Superchron ( 50 Myr) precedes the PT–GT doublet. In addition, the newly recognized Ordovician Moyero Long Reversed Superchron ( 30 Myr) precedes the end-Ordovician extinction event. There is a 10–20 Myr delay between the end of each superchron and the subsequent mass depletion event, both of which represent distant outliers from their respective populations. We propose that deep mantle plumes link these seemingly unrelated phenomena. Long-term ( 200 Myr) variations in mantle convection possibly associated with the Wilson cycle induce temporal and spatial variations in heat flow at the core–mantle boundary. Polarity reversals are frequent when core heat flow is high and infrequent when it is low. Thermal instabilities in the D”-layer of the mantle increase core heat flow, end the magnetic superchron, and generate deep mantle plumes. The plumes ascend through the mantle on a 20 Myr time scale, producing continental flood basalt (trap) eruptions, rapid climatic change, and massive faunal depletions.  相似文献   

4.
Millimeter-scale amphibole veins in the lower oceanic crust record fracture-controlled fluid flow at high-temperatures but the importance of this fluid flow for the thermal and chemical evolution of the lower oceanic crust is unclear. In the section of lower oceanic crust recovered at Hess Deep from ODP Hole 894G, which formed at the fast-spreading East Pacific Rise, these veins are randomly distributed with an average spacing of  1 m. We unravel the history of fluid flow through one of these veins by combining in situ O-isotope analyses of wall-rock plagioclase with major element analyses, geothermometry and diffusion modeling. Thermometry indicates vein sealing by amphibole at  720 °C over a narrow temperature interval (± 20 °C). In situ O-isotope analyses by ion microprobe, with a precision of < 0.5‰, reveal zoning of O-isotopes in plagioclase adjacent to the vein. The zoning profiles can be reproduced using a diffusion model if the duration of O-isotope exchange was ≤ 100 yr. A similar interval of fluid–rock exchange is suggested by modeling potassium depletion in plagioclase adjacent to the vein. If representative of fracture controlled fluid flow in the lower oceanic crust the limited duration of fluid flow, and its occurrence over a narrow temperature interval, suggest that high-temperature fluid flow in this porosity network does not transport significant heat.  相似文献   

5.
A phase transition in pure CaSiO3 perovskite was investigated at 27 to 72 GPa and 300 to 819 K by in-situ X-ray diffraction experiments in an externally-heated diamond-anvil cell. The results show that CaSiO3 perovskite takes a tetragonal form at 300 K and undergoes phase transition to a cubic structure above 490–580 K in a pressure range studied here. The transition boundary is strongly temperature-dependent with a slightly positive dT / dP slope of 1.1 (± 1.3) K/GPa. It is known that the transition temperature depends on Al2O3 content dissolved in CaSiO3 perovskite [Kurashina et al., Phys. Earth Planet. Inter. 145 (2004) 67–74]. The phase transition in CaSiO3(+ 3 wt.% Al2O3) perovskite therefore could occur in a cold subducted mid-oceanic ridge basalt (MORB) crust at about 1200 K in the upper- to mid-lower mantle. This phase transition is possibly ferroelastic-type and may cause large seismic anomalies in a wide depth range.  相似文献   

6.
We have studied the dependency between incoming plate structure, bending-related faulting, lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic lithosphere is highly hydrated due to the water infiltration through bending-related normal faults outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise processes have yet not affected anisotropy.The first weeks following the megatrust Mw = 8.8 Maule earthquake in 2010 were characterized by a sudden increase of the outer rise seismic activity, located between 34° S and 35°30′ S. We concluded that this phenomenon is a result of an intensification of the water infiltration process in the outer rise, presumably triggered by the main shock, whose epicenter was located some 100 km to the south east of the cluster.  相似文献   

7.
Up to now, tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data. However, large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model.

Cooling of the lithosphere causes a short-wavelength step in the geoid across fracture zones that can also be used to constrain thermal models. We have analyzed geoid data at fracture zones from the SEASAT altimeter measurements in the entire Pacific Ocean and redetermined parameters of the cooling models. We find that the data reveal two distinct regimes of cooling; one for seafloor ages in the range 0–30 Ma, the other beyond 30 Ma; this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied, i.e., the entire south Pacific and northeast Pacific Ocean. These two trends may be interpreted in terms of two different (asymptotic) thermal thicknesses of the plate model. The smaller thermal thickness ( 65 km) found for ages <30 Ma—compared to 90 km in the age range 30–50 Ma—calls for some kind of thermal perturbation in the vicinity of the ridge crest.

From the results obtained in this study, we conclude that the half-space cooling model is unable to explain the data, that beyond 30 Ma, a simple plate model gives a satisfactory fit to the data but in the younger plate portion (ages < 30 Ma) the cooling history of the oceanic lithosphere is much more complex than predicted by the usual cooling models. Furthermore, the depth-age relationship obtained from the geoid-derived thermal parameters departs significantly beyond 30 Ma from the widely used Parsons and Sclater's depth-age curve, predicting a lesser subsidence.  相似文献   


8.
Cratons form the cores of continents and were formed within a narrow window of time (2.5–3.2 Gy ago), the majority having remained stable ever since. Petrologic evidence suggests that the thick mantle roots underlying cratons were built by underthrusting of oceanic and arc lithosphere, but paradoxically this requires that the building blocks of cratons are weak even though cratons must have been strong subsequent to formation. Here, we propose that one form of thickening could be facilitated by thrusting of oceanic lithospheres along weak shear zones, generated in the serpentinized upper part of the oceanic lithosphere (crust + mantle) due to hydrothermal interaction with seawater. Conductive heating of the shear zones eventually causes serpentine breakdown at ~ 600 °C, shutting down the shear zone and culminating in craton formation. However, if shear zones are too thin, serpentine breakdown and healing of the shear zone occurs too soon and underthrusting does not occur. If shear zones are too thick, serpentine breakdown takes too long so healing and lithospheric thickening is not favored. Shear zone thicknesses of ~ 18 km are found to be favorable for craton formation. Because the maximal depth of seawater-induced serpentinization into the lithosphere is limited by the depth of the isotherm for serpentine breakdown, shear zone thicknesses should have increased with time as the Earth's heat flux and depth to the serpentine breakdown isotherm decreased and increased, respectively, with time. We thus suggest that the greater representation of cratons in the late Archean might not necessarily be explained by preferential recycling in the early Archean but may simply reflect preferential craton formation in the late Archean. That is, our model predicts that the early Archean was too hot, the Phanerozoic too cold, and the late Archean just right for making cratons.  相似文献   

9.
Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., (87Sr/86Sr)o = 0.70396–0.70346 and εNd = + 5.5 − + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher εNd (up to + 5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.  相似文献   

10.
智利三联点(CTJ)位于纳兹卡板块、南极洲板块与南美板块的交界处,由南极洲—纳兹卡板块之间的智利洋脊俯冲到智利海沟而形成.巴塔哥尼亚板片窗的发展是智利洋脊长期扩张俯冲的结果之一.随着纳兹卡板块的不断东向俯冲,纳兹卡板块范围逐渐变小,CTJ同时向北移动.本文采用数值模拟方法,建立了关于洋脊海沟碰撞的简单二维模型,来研究智利三联点南部扩张洋脊俯冲区域岩石层的热结构.模拟结果表明,洋脊的位置、板块相对汇聚速度及上覆大陆板块的存在均对俯冲区域海洋板块的温度结构有着很大影响,并且大陆板块下方海洋板块温度变化最大的位置距洋脊的水平距离与洋脊到板片窗范围的水平距离两者之间具有较好的一致性.同时,当存在两两板块间的相对汇聚时,洋脊右侧大陆板块下表面的温度升高,俯冲带内海洋板块温度接近于地幔温度.纳兹卡板块以7.8 cm·a~(-1)的速度急速俯冲于南美板块之下的过程中,同时伴随着智利洋脊的持续扩张俯冲,在智利三联点南部,南美板块之下纳兹卡板块的温度因而可以更快地达到地幔软流层的约1300℃温度,并最终消亡于地幔之中.  相似文献   

11.
Fresh basalt glasses from the North Chile Ridge (NCR) in the southeastern Pacific have Ne isotopic compositions distinctly different from typical mid-ocean ridge basalts (MORB). In a three-isotope plot of 20Ne/22Ne vs. 21Ne/22Ne, the NCR data define a correlation line with a slope smaller than that of the MORB correlation line, i.e. their Ne composition is more nucleogenic than that of MORB. 3He/4He ratios are slightly lower than the MORB average, whereas in a few stepwise heating fractions very high 40Ar/36Ar ratios up to 28,000 are found. One model to explain the data assumes contamination of the NCR mantle source by material from the continental or oceanic crust, but in addition to difficulties with quantitatively reconciling the noble gas patterns with such a model it seems unable to account for some geochemical characteristics of NCR basalts reported earlier [Bach et al., Earth Planet. Sci. Lett. 142 (1996) 223–240], such as depletions in highly incompatible elements and unradiogenic Sr isotope compositions. Therefore we favor the scenario of a mantle source which was depleted and degassed previously, possibly as a residue from mantle melting beneath the southern East Pacific Rise that was transported to the NCR and melted again. The time during which such a depleted reservoir would have to be separated from the MORB mantle is estimated at 10–100 Ma based on U/Th–Ne systematics, in reasonable agreement with the time scale deduced from the formation history of the NCR and the temporal evolution of the southeast Pacific.  相似文献   

12.
The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.  相似文献   

13.
The average secular cooling rate of the Earth can be deduced from compositional variations of mantle melts through time and from rheological conditions at the onset of sub-solidus convection at the end of the initial magma ocean phase. The constraint that this places on the characteristics of mantle convection in the past are investigated using the global heat balance equation and a simple parameterization for the heat loss of the Earth. All heat loss parameterization schemes depend on a closure equation for the maximum age of oceanic plates. We use a scheme that accounts for the present-day distribution of heat flux at Earth's surface and that does not depend on any assumption about the dynamics of convection with rigid plates, which remain poorly understood. We show that heat supply to the base of continents and transient continental thermal regimes cannot be ignored. We find that the maximum sea floor age has not changed by large amounts over the last 3 Ga. Calculations lead to a maximum temperature at an age of about 3 Ga and cannot be extrapolated further back in time. By construction, these calculations are based on the present-day tectonic regime characterized by the subduction of large oceanic plates and hence indicate that this regime did not prevail until an age of about 3 Ga. According to this interpretation, the onset of rapid continental growth occurred when the current plate regime became stable.  相似文献   

14.
The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enéchelon volcanic ridges, located within a rift valley with a regional trend of 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from 32 mm a−1 to 24 mm a−1, as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.  相似文献   

15.
Granoblastic olivine aggregates (GOA) have been discovered in some Type I magnesian chondrules within carbonaceous chondrites by Libourel and Krot [Libourel, G., Krot, A.N., 2007. Evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin. Earth Planet. Sci. Lett. 254, 1–8], who proposed an origin from pre-existing planetesimals. Amoeboid olivine aggregates (AOA), generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites, display similar equilibrium texture, though on a finer scale. For these reasons, we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating. Pressed < 43 µm and < 63 µm San Carlos olivine powder (Fo88.4) was isothermally heated at temperatures ranging from 1350–1550 °C for 1–100 h. The 100 h runs yield olivine aggregates with well-developed granoblastic texture at all temperatures, manifest as a network of randomly-oriented and sutured olivine grains with 120° triple junctions. Individual olivine grains are 4–6 sided and polygonal by 1450 °C and equigranular texture is developed at high temperature (1500–1550 °C). Melting of olivine commences at 1450–1500 °C and aids in ‘ripening’ and suturing (grain coarsening and grain boundary migration). Textural equilibrium is clearly met at 1550 °C. A planetesimal origin cannot be ruled out; however, the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects. GOA may have experienced higher degrees of thermal processing than the finer-grained AOA. If the precursors were the same, grain coarsening would have to be accompanied by modification to bulk and isotopic compositions. However, the precursors could have been olivine condensates formed later than AOA. Annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of GOA prior to their incorporation into chondrules.  相似文献   

16.
We describe the deep structure of the south Colombian–northern Ecuador convergent margin using travel time inversion of wide-angle seismic data recently collected offshore. The margin appears segmented into three contrasting zones. In the North Zone, affected by four great subduction earthquakes during the 20th century, normal oceanic crust subducts beneath the oceanic Cretaceous substratum of the margin underlined by seismic velocities as high as 6.0–6.5 km/s. In the Central Zone the subducting oceanic crust is over-thickened beneath the Carnegie Ridge. A steeper slope and a well-developed, high velocity, Cretaceous oceanic basement characterizes the margin wedge. This area coincides with a gap in significant subduction earthquake activity. In the South Zone, the subducting oceanic crust is normal. The fore-arc is characterized by large sedimentary basins suggesting significant subsidence. Velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing.

Even if the distance between the three profiles exceeds 150 km, the structural segmentation obtained along the Ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation.  相似文献   


17.
We study the foreshocks and aftershocks of the 1 April 2014 Iquique earthquake of Mw 8.1. Most of these events were recorded by a large digital seismic network that included the Northern Chile permanent network and up to 26 temporary broadband digital stations. We relocated and computed moment tensors for 151 events of magnitude Mw?≥?4.5. Most of the foreshocks and aftershocks of the Iquique earthquake are distributed to the southwest of the rupture zone. These events are located in a band of about 50 km from the trench, an area where few earthquakes occur elsewhere in Chile. Another important group of aftershocks is located above the plate interface, similar to those observed during the foreshock sequence. The depths of these events were constrained by regional moment tensor (RMT) solutions obtained using the records of the dense broad band network. The majority of the foreshocks and aftershocks were associated to the interplate contact, with dip and strike angles in good agreement with the characteristics of horst and graben structures (>2000 m offset) typical of the oceanic Nazca Plate at the trench and in the outer rise region. We propose that the spatial distribution of foreshocks and aftershocks, and its seismological characteristics were strongly controlled by the rheological and tectonics conditions of the extreme erosive margin of Northern Chile.  相似文献   

18.
In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.

The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.  相似文献   


19.
Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ18O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ18O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and 87Sr/86Sr and low 143Nd/144Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ18O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.  相似文献   

20.
Synthetic waveform modeling of the anomalous receiver functions for two stations in the eastern San Gabriel Mountains, California, suggests that a flat-topped notch structure exists on the Moho. North of the San Andreas Fault (SAF), the Moho depth is 36–38 km and is 33–35 km south of the San Gabriel Fault (SGF), but in-between under the Mt. Baldy block, it is only  29 km. The inferred notch structure is also supported by the Pms arrival times along the SCSN and the LARSE I profiles. The shallow Moho block correlates well with the surficial exposure of the lower plate Pelona Schist or where it resides at very shallow depth. The large Moho offsets across the two major faults can be either related to differential uplifting reflected by the difference in the denudation rate and the exposure of the lower plate Pelona Schist, or it can be the result of the juxtaposition of the distinctive terranes with initially different Moho depth due to the strike-slip movements along the two major faults.  相似文献   

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