Xenoliths of subducted crustal origin hosted by Miocene ultrapotassicigneous rocks in the southern Pamir provide important new informationregarding the geological processes accompanying tectonism duringthe Indo-Eurasian collision. Four types have been studied: sanidineeclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite),felsic granulites (garnet, quartz, sanidine and kyanite), basalticeclogites (omphacite and garnet), and a glimmerite (biotite,clinopyroxene and sanidine). Apatite, rutile and carbonate arethe most abundant minor phases. Hydrous phases (biotite andphengite in felsic granulites and basaltic eclogites, amphibolesin mafic and sanidine eclogites) and plagioclase form minorinclusions in garnet or kyanite. Solid-phase thermobarometryreveals recrystallization at mainly ultrahigh temperatures of10001100°C and near-ultrahigh pressures of 2·52·8GPa. Textures, parageneses and mineral compositions suggestderivation of the xenoliths from subducted basaltic, tonaliticand pelitic crust that experienced high-pressure dehydrationmelting, K-rich metasomatism, and solid-state re-equilibration.The timing of these processes is constrained by zircon agesfrom the xenoliths and 40Ar/39Ar ages of the host volcanic rocksto 5711 Ma. These xenoliths reveal that deeply subductedcrust may undergo extensive dehydration-driven partial melting,density-driven differentiation and disaggregation, and sequestrationwithin the mantle. These processes may also contribute to thealkaline volcanism observed in continent-collision zones. KEY WORDS: xenolith; high-pressure; subduction; Pamir; Tibet相似文献
We have collected 34 hot spring and mineral spring gases and waters in the Chugoku and Kansai districts, Southwestern Japan
and measured the 3He/4He and 4He/20Ne ratios by using a noble gas mass spectrometer. Observed 3He/4He and 4He/20Ne ratios range from 0.054 Ratm to 5.04 Ratm (where Ratm is the atmospheric 3He/4He ratio of 1.39 × 10−6) and from 0.25 to 36.8, respectively. They are well explained by a mixing of three components, mantle-derived, radiogenic,
and atmospheric helium dissolved in water. The 3He/4He ratios corrected for air contamination are low in the frontal arc and high in the volcanic arc regions, which are consistent
with data of subduction zones in the literature. The geographical contrast may provide a constraint on the position of the
volcanic front in the Chugoku district where it was not well defined by previous works. Taking into account the magma aging
effect, we cannot explain the high 3He/4He ratios of the volcanic arc region by the slab melting of the subducting Philippine Sea plate. The other source with pristine
mantle material may be required. More precisely, the highest and average 3He/4He ratios of 5.88 Ratm and 3.8±1.6 Ratm, respectively, in the narrow regions near the volcanic front of the Chugoku district are lower than those in Kyushu and Kinki
Spot in Southwestern Japan, but close to those in NE Japan. This suggests that the magma source of the former may be related
to the subduction of the Pacific plate, in addition to a slight component of melting of the Philippine Sea slab. 相似文献
On April 9, 2001 a Mw 6.7 earthquake occurred offshore of the Chilean coast close to the intersection of the subducting Juan Fernández Ridge (JFR) and the trench near 33°S. The mainshock as well as an unprecedented number of aftershocks were recorded on regional broad-band and short-period seismic networks. We obtained a regional moment tensor solution of the mainshock that indcates a tensional focal mechanism consistent with the Harvard CMT solution. Based on waveform modeling and relocation, the depth of the mainshock was found to be 10–12 km. We relocated 142 aftershocks, which are strongly clustered and restricted to 10–30 km in depth. The seismicity distribution indicates a conjugate normal fault system extending into the lithospheric mantle that correlates with ridge-parallel fractures observed by previous seismic and bathymetric surveys. In conjunction with the historic regional distribution of outer-rise and large interplate seismicity, our results indicate that, with the exception of anomalously large thrust events, preexisting fractures associated with large bathymetric features like ridges have to exist to allow the generation of outer-rise seismicity along the Chilean margin. Hence, flexural bending and time-dependent interplate earthquakes can locally affect the nucleation of outer-rise events. The occurrence of the outer-rise seismicity in the oceanic mantle suggests the existence of lithospheric scale faults which might act as conduits to hydrate the subducting slab.Robert Fromm-Rhim passed away July 31st, 2004. 相似文献
We present a model of the subducting Cocos slab beneath Central Mexico, that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment. Based on the recently developed thermal models for the Central Mexico subduction zone, the thermal stresses due to non-uniform temperature contrast in the subducting slab are calculated using a finite element approach. The slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep. The calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 MPa. On the other hand, the top of the slab experiences high compressive thermal stresses up to 110 MPa, depending on the elastic constants used and location along the flat part of the subducting plate. These compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes, and two different sources of stress are proposed.
The trenchward migration of the Mexican volcanic arc for the last 7 Ma indicates an increase of the slab dip through time. This observation suggests that the gravity torque might exceed the suction torque. Considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0.5 × 1016–1.5 × 1016 N m, the upper half would exhibit tensional stresses of 40–110 MPa that can actually balance the compressive thermally induced stresses.
An alternative stress source might come from the slab pull force caused by the slab positive density anomaly. Based on our density anomaly estimations (75 ± 20 kg/m3), a 350 km slab length, dipping at 20° into the asthenosphere, induces a slab pull force of 1.7 × 1012–4.6 × 1012 N m. This force produces a tensional stress of 41–114 MPa, sufficient to balance the compressive thermal stresses at the top of the flat slab.
The linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60–180 MPa inside the flat slab core. Also, our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40–80 MPa.
This study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath Central Mexico, and also it shows that thermal stresses due to non-uniform reheating of subducting slabs play a considerable role in the total stress field. 相似文献
K–Ar and 40Ar/39Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the 40Ar/39Ar plateau and 36Ar/40Ar versus 39Ar/40Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto-IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by 40Ar/39Ar analysis; (ii) 40Ar/39Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and 40Ar/39Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ~ 15 million years after arc inception concurrently with low-K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto-arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back-arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete. 相似文献
Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of arc volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault-controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features. 相似文献