Northwestern Segment Research Articles

Crustal velocity structure beneath Saurashtra, NW India, through waveform modeling: Implications for magmatic underplating

The Saurashtra peninsula in the northwestern segment of the Deccan volcanic province of India, is characterized by several high gravity and magnetic anomalies, which correspond to mafic crustal intrusions. This study attempts to quantify the alterations to the crust caused by the Deccan volcanism, by estimating the crustal Poisson’s ratio, shear wave velocity (Vs) structure and the shear velocity contrast across the Moho, through teleseismic waveform modeling. The P receiver functions (RFs) for six broadband seismic stations were constructed using about 575 high quality (S/N≥2.5) teleseismic waveforms of earthquakes (M≥5.5) recorded during the period 2004–2010. The moveout corrected RF summation stacks were inverted using the Neighborhood algorithm (NA) to estimate the shear velocity structure beneath each station. The crustal thickness is estimated to range from 38km in western Saurashtra to 33km close to the southern extension of the Cambay rift. A low velocity zone possibly corresponding to sub-basaltic sediments is detected beneath all the stations. The average crustal Vs and Poisson’s ratio are estimated to be 3.68km/s and 0.276 respectively. The crustal Poisson’s ratio indicates an intermediate to mafic composition for the crust. Modeling reveals a relatively high velocity lower crust with an average Vs≈3.88±0.1km/s which is consistent with the high Vp ≈ 7.1km/s reported for this region through seismic refraction and wide angle reflection studies, overlying a relatively low velocity (Vs ≈ 4.4±0.1km/s) upper mantle. The shear velocity contrasts across the Moho, derived from the amplitudes of the P-to-s (Pms) conversions from the Moho range from 0.08 to 0.17 which are much smaller than those (>0.20) observed across the Indian shield, implying a gradational Moho due to underplating. This study reveals that the crust beneath Saurashtra is distinctly different from that of the Archean Indian shield largely due to significant alterations by the Deccan volcanism.

Reexamination of the fault slip model of the 1891 M 8.0 Nobi earthquake: The first earthquake detected by a geodetic survey in Japan

The ground deformation and fault slip model for the 1891 M 8.0 Nobi earthquake, central Japan, have been reexamined. The Nobi earthquake appears to have occurred mainly due to the rupture of three faults: Nukumi, Neodani, and Umehara. Since triangulation and leveling had been performed around the Umehara fault, the two geodetic datasets from 1885–1890 and 1894–1908 have been reevaluated. Maximum coseismic horizontal displacements of 1.7 m were detected to the south of the Neodani fault. A fault model of the Nobi earthquake was estimated from the geodetic datasets, taking into account the geometry of the fault planes based on the known surface ruptures. The best fit to the data was obtained from three and four divided fault segments running along the Nukumi, Neodani, and Umehara faults; although, in past studies, the Gifu-Ichinomiya line has been suggested as a buried fault to explain the ground deformation. The detected ground deformation can be well reproduced using a slip model for the Umehara fault, dipping at 61° toward the southwest, with a maximum slip of 3.8 m in the deeper northwestern segment. As this model suitably explains the coseismic deformation, the earthquake source fault does not appear to extend to the Gifu-Ichinomiya line.

Two-stage evolution of mantle peridotites from the Stalemate Fracture Zone, northwestern Pacific

This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65-0.68 and 0.26-0.33, respectively) compared with spinel from the dunites (Mg# = 0.56-0.64 and Cr# = 0.38-0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7-92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10-12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene.

The structure of the lithospheric mantle of the Siberain craton and seismodynamics of deformation waves in the Baikal seismic zone

The structure of the lithospheric mantle of the Siberain craton and seismodynamics of deformation waves in the Baikal seismic zone

Coseismic deformation derived from analyses of C and L band SAR data and fault slip inversion of the Yushu Ms7.1 earthquake, China in 2010

We obtained the coseismic deformation field of the Ms7.1 earthquake in Yushu, Qinghai, China on 14 April 2010 using L band and C band SAR images. The results show that the deformation fields derived from L and C bands separately are consistent in general. They are all characterized by concentric elliptic interference fringes surrounding a NW trending fault with two local intensively deformed regions. Meanwhile they have small differences in area coverage of the deformation field, magnitude of LOS displacements and other details of deformation nearby the fault. The deformation from C band and L band covers 89∗59km square and 77∗43km square, with maximum displacements of about 45cm and 65cm in the radar line of sight, respectively. Using these two kinds of InSAR measurements separately and jointly, we constructed a three-segment fault model and inverted the coseismic fault slip of the Yushu event. The inversion results show that the slip distributions constrained by the two kinds of inSAR data are in agreement on the whole. They all indicate two concentration regions of slip distribution, one is located near the Jiegu town on the southeastern segment of the Yushu fault with a large area and the maximum slip of 2.4m, and the other is at the epicenter on the northwestern segment of the fault with a relatively small area. There are also discrepancies between the two kinds of slip distribution models. In general, the scale, magnitude, and depth of slip distribution constrained by C band ASAR data are larger than that by L band PALSAR data. The slip distribution from a joint inversion of the two kinds of SAR data is considered to be the most robust model compared to the results of that constrained by L band SAR data or C band SAR data alone.

Role of the asthenosphere in transfer and deformation of the lithosphere: The Ethiopian-Afar superplume and the Alpine-Himalayan Belt

Seismic tomographic data showing the mantle structure of the Ethiopian-Afar superplume and various segments of the Alpine-Himalayan Orogenic Belt and their relationships with the adjacent mega� structures of the Earth are presented. These data and their correlation with the geological evidence lead to the conclusion that lateral flows of mantle material are crucial for the evolution of the Tethys and its closure in the Cenozoic with transformation into an orogenic belt. The lateral flow of hot upper mantle asthenospheric matter spreading from the stationary superplume extending in the meridional direction (in presentday coor� dinates) was responsible for the accretion of the fragments torn away from Gondwana to Eurasia and for the development of subduction at the northeastern flank of the Tethys. The characteristic upper mantle structure of cold slabs passing into nearly horizontal lenses with elevated seismic wave velocity in the lowermost upper mantle is currently retained in the Indonesian segment of the orogenic belt. In the northwestern segments of this belt, a hot asthenospheric flow reached its northern margin after closure of the Tethys and onset of colli� sion, having reworked the former structure of the upper mantle and enriched it in aqueous fluids. The effect of this active asthenosphere on the lithosphere gave rise to intense Late Cenozoic deformation, magmatism, and eventually resulted in mountain building.

Inter-seismic deformation field of the Ganzi-Yushu fault before the 2010 Mw 6.9 Yushu earthquake

The 14 April 2010 Mw 6.9 Yushu earthquake ruptured the northwestern segment of the Ganzi-Yushu fault in Qinghai Province, China. Using GPS data obtained from 1999 to 2007 in the vicinity of the Ganzi-Yushu fault, we estimate the slip rates of the Fenghuoshan and Ganzi-Yushu faults, and the northwestern segment of the Xianshuihe fault as 6.1±1.9, 6.6±1.5, and 9.7±0.7mm/a for left lateral components, and 2.8±1.9, 1.7±1.6, and −2.0±0.9mm/a for shortening components, respectively. The Maduo-Gande fault slips left laterally at a rate of about 1–2mm/a, and ~3mm/a sinistral shear motion is left unexplained, possibly caused by deformation across one or more unknown faults in the region. These results agree with geological estimates of the fault slip rates, and show a progressive increase of shear motion from northwest to southeast across segments of the Xianshuihe–Ganzi-Yushu fault zone, implying variation in transferring and absorbing deformation in different regions in and around the Tibetan plateau.

Numerical simulation on the influences of Wenchuan earthquake on the surrounding faults

On 12 May 2008, the devastating Wenchuan earthquake struck the Longmenshan fault zone, which comprised the eastern margin of the Tibetan Plateau, and this fault zone was predominantly a convergent boundary with a right-lateral strike-slip component. After such a large-magnitude earthquake, it was crucial to analyze the influences of the earthquake on the surrounding faults and the potential seismic activity. In this paper, a complex viscoelastic model of western Sichuan and eastern Tibet regions was constructed including the topography. Based on the findings of co-seismic static slip distribution, we calculated the stress change caused by the Wenchuan earthquake with the post-seismic relaxation into consideration. Our preliminary results indicated that: (1) The tectonic stressing rate was relatively high in Kunlun mountain pass-Jiangcuo, Ganzi-Yushu, Xianshuihe and Zemuhe faults; while in the east Kunlun and Longriba was medium; also the value was less in the Minjiang, Longmenshan, Anninghe and Huya faults. As to the Longmenshan fault, the value was 0.28×10−3 MPa/a to 0.35×10−3 MPa/a, which is coincident with the previous long recurrence interval of Wenchuan earthquake; (2) The Wenchuan earthquake not only caused the Coulomb stress decrease in the source region, but also the stress increase in the two terminals, especially the northeastern segment, which is comparatively consistent with the aftershock distribution. Meanwhile, the high concentration areas of the static slip distribution were corresponding to the Coulomb stress reductions; (3) The Coulomb stress change caused by Wenchuan earthquake showed significant increase on five major faults, which were northwestern segment of Xianshuihe fault, eastern Kunlun fault, Longriba fault, Minjiang fault and Huya fault respectively; also the Coulomb stress on the fault plane of the Yushu earthquake was faintly increased; (4) We defined the recurrence interval as the time needed to accumulate the magnitude of the stress drop, and the recurrence interval of Wenchuan earthquake was estimated about 1 714 a to 2 143 a correspondingly.

Tectonostratigraphic overview of the Zagros Suture Zone, Kurdistan Region, Northeast Iraq

ABSTRACTThe northwestern segment of the Zagros Orogenic Belt of the Kurdistan Region of Iraq includes the Zagros Suture Zone which is consisting of allochthonous Tethyan Triassic–Eocene thrust sheets. The zone is bounded by the Zagros Main Reverse Fault in the northeast, and the Zagros Thrust Front in the southwest. Parts of this zone’s rocks are represented by stacks of thrust megasheets obducted over the autochthonous Cretaceous–Cenozoic sequence of the Arabian Plate margin. Each sheet represents a specific Tethyan tectonostratigraphic facies, and includes (from older to younger): isolated Triassic carbonate platforms (Avroman Limestone), Jurassic carbonate imbricates (Chia Gara, Sargelu and other formations), radiolarites (Qulqula Group), sedimentary mélange (sedimentary-volcanic units of the Qulqula Group), ophiolites (Mawat and Penjwin ultramafics complexes), and Cenozoic fore-arc volcano-sedimentary sequences (Walash Group). Petrography, facies interpretation and lithostratigraphic correlation of these allochthons along four traverses across the Zagros Suture Zone of the examined area indicate that they evolved during the closure of the Neo-Tethys Ocean. Their stacking pattern and tectonic association resulted from two important events: the Late Cretaceous obduction processes, and the Late Miocene–Pliocene collision, uplift, folding and suturing between the Arabian Plate and the Sanandaj-Sirjan Block of Iran. Based on these field observations and by using the model of the Iranian Zagros evolution, a tectonic scenario is proposed to explain the history and evolution of the Zagros Suture Zone in this area.

Silicification of peridotites at the stalemate fracture zone (Northwestern Pacific): Reconstruction of the conditions of low-temperature weathering and tectonic interpretation

During cruise SO201-1b of the joint Russian–German expedition on the R/V Sonne in 2009, mantle peridotites affected by varying secondary alteration were dredged on the eastern slope of the north- western segment of the Stalemate transverse ridge adjacent to the eponymous fracture zone. The collection discussed in this paper included four samples of silicified serpentinites after dunites and 11 lherzolite samples serpentinized to a varying degree. The abundance of amorphous silica and quartz, very high SiO2 content (up to 88.7 wt %), and unusually low MgO (up to 1.4 wt %) in the serpentinized dunites strongly distinguish these rocks from the known products of hydrothermal alteration and low-temperature (seafloor) weathering of peridotites in the oceanic crust. In order to determine the conditions and processes resulting in the silicifica- tion of peridotites at the Stalemate Fracture Zone, thermodynamic modeling accounting for the kinetics of mineral dissolution implemented in the GEOCHEQ program package was used in this study. The results of modeling allowed us to suppose that the geochemical and mineralogical effects observed in the silicified ser- pentinized dunites of the Stalemate Fracture Zone are consequences of low-temperature deserpentinization of oceanic materials under subaerial conditions.

The 2010MW6.8 Yushu (Qinghai, China) earthquake: Constraints provided by InSAR and body wave seismology

[1] By combining observations from satellite radar, body wave seismology and optical imagery, we have determined the fault segmentation and sequence of ruptures for the 2010 Mw 6.8 Yushu (China) earthquake. We have mapped the fault trace using displacements from SAR image matching, interferometric phase and coherence, and 2.5 m SPOT-5 satellite images. Modeling the event as an elastic dislocation with three segments fitted to the fault trace suggests that the southeast and northwest segments are near vertical, with the central segment dipping 70° to the southwest; slip occurs mainly in the upper 10 km, with a maximum slip of 1.5 m at a depth of 4 km on the southeastern segment. The maximum slip in the top 1 km (i.e., near surface) is up to 1.2 m, and inferred locations of significant surface rupture are consistent with displacements from SAR image matching and field observations. The radar interferograms show rupture over a distance of almost 80 km, much larger than initial seismological and field estimates of the length of the fault. Part of this difference can be attributed to slip on the northwestern segment of the fault being due to an Mw 6.1 aftershock two hours after the main event. The remaining difference can be explained by a non-uniform slip distribution with much of the moment release occurring at depths of less than 10 km. The rupture on the central and southeastern segments of the fault in the main shock propagated at a speed of 2.5 km/s southeastward toward the town of Yushu located at the end of this segment, accounting for the considerable building damage. Strain accumulation since the last earthquake on the fault segment beyond Yushu is equivalent to an Mw 6.5 earthquake.

Significance of angular unconformities between Cretaceous and Tertiary strata in the northwestern segment of the Zagros fold–thrust belt, Kurdistan Region, NE Iraq

AbstractIn this study, two angular unconformities are found and analysed for the first time in the Mesozoic–Cenozoic succession in the northwestern segment of the Zagros fold–thrust belt (ZFTB) in the Kurdistan Region. The first unconformity exists between Lower Cretaceous and Paleocene–Eocene rocks and the second between the Campanian Shiranish Formation and the Maastrichtian Tanjero Formation. Each of these unconformities is found in two different localities in the Zagros Imbricate Zone (i.e. the highly deformed zone immediately SW of the Zagros Suture) of the ZFTB of the Kurdistan Region near the border with Iran. The study uses recent geological mapping, structural and stratigraphic analyses in addition to using previous biozonation of the stratigraphic units that bound the two unconformities. The first unconformity was initiated with obduction of the ophiolite and Lower Cretaceous radiolarite onto the passive margin of the Arabian plate. This unconformity formed during an early phase of the Zagros orogeny, which is associated with the developing of a foreland basin, and resulted in the folding of the radiolarites and their uplift to form high-relief land. The erosion of this high-relief land resulted in the formation of the Paleocene–Eocene Red Bed Series and their deposition on the folded radiolarite. The timing of the deformation that caused this unconformity is hard to determine; however, its stratigraphic position may suggest that it possibly is related to post-Cenomanian movements. The second unconformity is between the tilted Campanian Shiranish Formation (hemipelagite) and Tanjero Formation (500 m of conglomerate in the more proximal area). These unconformities indicate that deformation and uplift of the sedimentary units was variable during ophiolite obduction in this part of the ZFTB. We argue that deformation, ophiolite obduction and collision are likely to have varied in space and time along thec. 2000 km long ZFTB.

Displaced South Alpine and Dinaridic elements in the mid-Hungarian zone

The Mid-Hungarian Zone is a WSW-ENE trending composite structural unit in the basement of the Pannonian Basin that is made up of displaced crustal fragments (terranes) of South Alpine and Dinaridic origin. In the early stage of the Alpine evolution these fragments were located in various sectors of the NW Neotethys region, representing different paleogeographic settings from passive margin through continental slope to oceanic basement. Middle to Late Jurassic closure of the Neotethys led to the development of a suture zone made up of subduction-related complexes that can be followed all along the strike of the Dinarides. During the Cretaceous compressional stages, nappe stacks were formed from the accretionary complex and the fragments of the previously disrupted passive margin. Eastward extrusion (escape) of the ALCAPA Mega-unit during the Oligocene to Early Miocene led to large-scale displacement of fragments of this nappe stack, transporting them to their present-day position, and resulted in dispersal of the northwestern segment of the suture zone. The paper summarizes the basic characteristics of the dislocated blocks, evaluates their relationships and determines their original setting.

Fault tectonics, neotectonic stresses, and hidden uranium mineralization in the area adjacent to the Strel’tsovka Caldera

The scheme of recent fault tectonics and neotectonic stresses of the area adjacent to the Strel’tsovka Caldera in the southeastern Transbaikal region has been compiled for the first time on the basis of structural and geomorphic study. The faults were ranked by criteria of slip direction stability along separate segments and their expression in topography. Neotectonic stresses of corresponding ranks were ascertained as well. The heterogeneity of neotectonic stress field is related to the mosaic development of compression, extension, and the three-axial stress state. Comparison of morphogenetic features of recent and older faults shows the different character of the deformation mechanism and orientation of tectonic displacements. It has been established that the Strel’tsovka Caldera and its northwestern segment, in particular, developed as an echeloned system of pull-apart grabens, but the caldera itself is situated in a recent rise, whereas the northwestern segment is located in a neotectonic depression corresponding to the Dry Urulyungui Basin filled with volcanic and sedimentary rocks. Such a structure markedly expands the outlook for discovery of hidden uranium mineralization in the studied area. The elaborated scheme of neotectonic faults and stresses reflects the postore geodynamic setting and completes the reconstruction of geodynamic conditions pertaining to the periods of preore preparation and ore-forming tectonomagmatic reactivation.

Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results

The North Tabriz Fault is a major seismogenic fault in NW Iran. The last damaging earthquakes on this fault occurred in 1721, rupturing the southeastern fault segment, and in 1780, rupturing the northwestern one. The understanding of the seismic behavior of this fault is critical for assessing the hazard in Tabriz, one of the major cities of Iran; the city suffered major damage in both the 1721 and 1780 events. Our study area is located on the northwestern fault segment, west of the city of Tabriz. We performed geomorphic and trenching investigations, which allowed us to recognize evidence for repeated faulting events since the Late Pleistocene. From the trenches, we found evidence for at least four events during the past 3.6 ka, the most recent one being the 1780 earthquake. On the basis of different approaches, horizontal slip per event and slip rates are found in the ranges of 4 ± 0.5 m and 3.1-6.4 mm/yr, respectively. We also attempted an estimate of the average recurrence intervals which appears to be in the range 350-1430 years, with a mean recurrence interval of 821 ± 176 years. On the basis of these results, the northwestern segment of the North Tabriz Fault does not appear to present a major seismic potential for the near future, however, not enough is known about the southeastern segment of the fault to make a comparable conclusion.

Chuchkhur xenoliths and tectonic position of Middle Paleozoic volcano-sedimentary sequences in the Peredovoi Range, northern Caucasus

The Peredovi (Frontal) Range zone is character� ized by the most complete geological record as com� pared with other preAlpine structures of the Greater Caucasus. Its section in the wide northwestern seg� ment (Laba River basin) includes the following struc� tural units (from the base upward): Blyb metamorphic complex and its analogues; Devonian-Lower Car� boniferous to, locally, Silurian (Urup-Tokhan Com� plex) greenstone volcanosedimentary sequences; Atsgara nappe of metamorphic rocks and granitoids; and ophiolites of the Marukh nappe (figure). The neoautochthonous cover of the zone is composed of upper Visean-Permian molasses. The existence of the

Stress state of the Earth’s crust of the Kuril Islands and Kamchatka before the Simushir earthquake

The field of modern tectonic stresses was reconstructed for the Earth’s crust of the northwestern segment of the Pacific subduction zones. For this purpose, we used the method of cataclastic analysis and data on the magnitude of the stresses released at the source of the Simushir earthquake of 2006, which allowed us to determine both the orientation of the principal stress axes and the magnitude of the stresses and to estimate the effective strength of rock masses. The effective cohesion was estimated for this region of the Earth’s crust as 12 bar, and the maximum shear stresses are no higher than 300 bar. The analysis of the reconstructed stress field in the zone of the preparation of the Simushir earthquake showed that this region was almost free of domains with high stresses where brittle failure requires considerable energy inputs. The medium level of effective pressure indicates that this region is most favorable for the development of a large-scale brittle failure.

Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults’ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.

Record of 1.82 Ga Andean-type continental arc magmatism in NE Rajasthan, India: Insights from zircon and Sm–Nd ages, combined with Nd–Sr isotope geochemistry

We document a Palaeoproterozoic Andean-type continental arc granitoid emplacement event in the northwestern segment of the Aravalli mountain range, NW India. Our study is focused on the Jasrapur pluton which is dominated by monzogranites with a subordinate amount of granodiorite and quartz monzodiorite. These rocks carry typical continental arc geochemical signatures, such as high-K calc-alkaline, mildly peraluminous, I-type and magnesian features, as well as high Rb and low Nb, Y and Rb/Zr ratios, decoupling between LILE and HFSE, and negative Nb–Ta and Ti anomalies. Conventional geothermobarometric estimates suggest high temperature (~ 800 °C) and pressure (≥ 600 MPa) conditions for the crystallisation of the Jasrapur granitoids. Whole-rock Sm–Nd isotope dating yielded an isochron age of 1800 ± 59 Ma, in good agreement with a zircon Pb–Pb evaporation age of 1821.7 ± 0.4 Ma, which we interpret to reflect the age of emplacement. A Ba-enrichment trend (400 to 1600 ppm), together with mantle-like initial Sr ratios (Sr i = 0.7060 ± 0.0007), suggest that the primary Jasrapur magma was generated from partial melting of a depleted mantle source enriched by slab-induced LILE fluids. Nevertheless, negative values of initial Nd (ε Nd(t) = − 5.01 ± 0.13) and a 2.6 Ga mean crustal residence age suggest the involvement of an Archaean component in the petrogenesis of Jasrapur magma. The eventual Jasrapur magma was generated from a zone of melting, assimilation, storage, and homogenisation, i.e. a MASHed zone at the base of an Archaean continental crust. Sr–Nd isotope mixing modelling, assessment of major element data based on various melts produced by experimental dehydration melting as well as comparison of the trace element data with those from the Central and Southern volcanic zones (CVZ and SVZ) of the Andes suggest that the Jasrapur magma underwent crustal contamination while traversing through a relatively thick continental crust. Appraisal of the REE patterns of the granitoids and tectonically similar and contiguous amphibolites signify that the granitoid magma was produced from a garnet-free but plagioclase-rich MASHed zone formed at a relatively shallow depth. Isotopic comparisons with other correlated events documented farther south in the central Rajasthan indicate a 1850–1822 Ma event of continental arc magmatism in the northwestern peninsular Indian shield which also corresponds in age to the amalgamation of the Palaeoproterozoic supercontinent Columbia.

Geometria e evolução do feixe de zonas de cisalhamento Manhuaçu - Santa Margarida, Orógeno Araçuaí, MG

Feixe de Zonas de Cisalhamento Manhuaçu-Santa Margarida de orientação geral NS estende-se por, aproximadamente, 300 km na região leste de Minas Gerais. Como uma das principais estruturas do núcleo cristalino do Orógeno Araçuaí, é constituído por um conjunto de zonas de cisalhamento dúcteis, que, em mapa, mostram traços sigmoidais. O seu segmento norte é composto por zonas de cisalhamento reversas que promovem o transporte de material em direção a oeste e, subordinadamente, por zonas transcorrentes dextrais. Os segmentos central e sul são dominados por zonas transcorrentes dextrais. O desenvolvimento do feixe deu-se em quatro fases deformacionais neoproterozóicas. A primeira fase promoveu a nucleação do conjunto de zonas reversas, que ficaram parcialmente preservadas no segmento norte do feixe. Durante a segunda fase, as zonas preexistentes experimentaram, de norte para sul, uma rotação progressiva para a vertical e para a direção NE-SW, além de intensa reativação transcorrente dextral. A terceira fase gerou zonas de cisalhamento normais de ocorrência restrita ao segmento norte. Na quarta fase, formaram-se falhas transversais e juntas. Em sua terminação sul, o feixe, dominado por zonas transcorrentes dextrais, rotaciona-se até confundir-se com as estruturas da porção NW da Faixa Ribeira. Tal fato implica que as estruturas dessa porção do feixe são de mesma idade, ou algo mais velhas que as transcorrências dextrais, que marcam o quadro tectônico do segmento NW da Faixa Ribeira.