Process Of Orogeny Research Articles

A uniform basement: Implications for the tectonics of Beishan Orogenic Belt in the southern Central Asian Orogenic Belt

The orogen is a significant window for studying the history and architecture of Earth; its architecture involves many significant issues. Currently, research on the architecture of orogenic belts mainly focuses on the process of orogeny, and few studies focus on the tectonics before the orogeny, which somewhat constrains orogen evolution. The nature of the basement(s) of microcontinents in the Beishan Orogenic Belt (BOB) in the middle Central Asian Orogenic Belt (CAOB) has been debated for a long time, which causes confusion about the tectonics of the BOB. This study conducted field surveys, geochronology, geochemistry, and Hf isotope analysis on the major microcontinents of BOB. The age information of ca. 1.4 Ga and the Paleoproterozoic-Archean with small scale were found in the northern BOB. The northern BOB (NBOB) and southern BOB (SBOB) (separated by the Hongliuhe-Niujuanzi-Xichangjing ophiolitic mélange belt) have experienced similar magmatic thermal events at ca. 1.4 Ga and ca. 0.9 Ga, which are revealed by the analogical tectonic setting, formation ages, and Hf isotopic features. The Precambrian basements of the NBOB and SBOB are analogous in terms of rock assemblages, detrital zircon ages, magmatic rock ages, εHf(t) of Precambrian rocks, and Hf crustal model ages TDMC of Paleozoic igneous rocks, which imply that they shared similar history and provenance and might originate from a uniform basement. The basement has no close tectonic affinity to the adjacent cratons or blocks, except the Baltica Craton and Laurentia Craton. And it might have been involved in the amalgamation of the Columbia Supercontinent and be situated on the periphery of the Rodinia Supercontinent, and then fragmented when the Rodinia Supercontinent rifted apart.

Geochronology and Geological Implications of Paleoproterozoic Post-Collisional Monzogranitic Dykes in the Ne Jiao-Liao-Ji Belt, North China Craton

Hardly any previous studies have focused on the granitic dykes which intrude into the Paleoproterozoic Liaohe Group in the Liaodong Peninsula, northeast of the North China Craton. In situ zircon U-Pb dating, Lu-Hf isotopic and geochemical analyses on three representative monzogranite dykes were taken in this study. These dykes have relatively high content of SiO2 (72.20%–74.78%) and K2O (2.83%–6.37%), and have characteristics of high-K calc-alkaline to shoshonite series. Two dyke samples have I-type granite features and have high Sr/Y ratios and positive Eu anomalies, showing an adakitic feature. Another dyke has a high ratio of Ga/Al, but has a low Zr saturation temperature, which differs from the typical A-type granite. Zircon grains from these three dykes have typical magmatic zoning in CL images and yield consistent U-Pb ages of ~1859–1852 Ma, which are interpreted as the crystallization ages of these dykes. Hf isotopic analyses yield mainly negative εHf(t) values and TDM2 ages of 2782–2430 Ma, similar to those of the 2.2–2.1 Ga granitoids and meta-sedimentary rocks (the Liaohe Group), indicating these monzogranitic dykes may have been sourced from melting of Paleoproterozoic granitoids and meta-sedimentary rocks. The monzogranitic dykes were generated under a post-collisional geological setting after the Jiao-Liao-Ji orogeny process.

Tectonic evolution of Southeast Asia documented by two periods of high-grade metamorphism in the Kon Tum Massif, Central Vietnam

The Kon Tum Massif in the eastern Indochina Block (ICB) contains extensive Ordo-Silurian and Permo-Triassic metamorphic rocks. These two episodes of metamorphism not only affected the Kon Tum Massif, but were also widely developed in the whole of Southeast and East Asia, including the South China Block (SCB). Comparisons of the metamorphic characteristics among these blocks can reveal the crustal evolution and their affinity in supercontinents. Two gneiss samples in the eastern part of the Kan Nack Complex of the Kon Tum Massif have similar peak metamorphic ages of 488–482 Ma and mineral assemblages of garnet + plagioclase + K-feldspar + biotite + sillimanite + ilmenite + quartz. P-T calculations and thermodynamic modelling constrain a pressure peak (M1) at 770–820 °C and ∼1.1–1.2 GPa, and a temperature peak (M2) at 790–850 °C and 0.85–0.98 GPa, suggesting a near-isothermal decompression. The post-peak M3 assemblage developed at 650–760 °C and 0.48–0.74 GPa. The clockwise P-T-t path with medium and variable geothermal gradients probably reflects an orogeny process from compression to extension, which was triggered by the closure of the Tam Ky-Phuoc Son Ocean in the northern flank of the Kon Tum Massif. The P-T-t path and tectonic setting of this period of metamorphism in the Kon Tum Massif is similar to that in the SCB, but occurred a little earlier than that in the SCB, suggesting that the ICB and the SCB were adjacent on the northern margin of East Gondwana and together experienced a diachronous early Paleozoic orogeny.A granulite sample from the western Kan Nack Complex contains garnet + orthopyroxene + plagioclase + quartz + ilmenite ± K-feldspar. Decomposition of garnet to plagioclase and orthopyroxene suggests a decompression process. Coupled with features of a gneiss sampled at the same spot, P-T calculations and thermodynamic modelling indicate a pressure peak of 885–960 °C and 0.83–0.95 GPa and a temperature peak at 890–970 °C and 0.61–0.73 GPa, corresponding to ultrahigh-temperature (UHT) metamorphism. LA-ICP-MS monazite U-Th-Pb dating constrains a peak metamorphic age of 254 Ma, and Sm-Nd mineral isochrons date the retrograde stage at 243–236 Ma. This UHT metamorphism with a clockwise P-T-t path probably resulted from the excess heat supplied by asthenospheric upwelling following the break-off of the subducting slab of the Paleo-Tethyan Ocean. The Permo-Triassic metamorphism and other geological records in the ICB are significantly different from those in the SCB, implying that the geological processes in the two blocks were controlled by different geodynamic regimes during Late Permian to Early Triassic.

Multi-Level Detachment Deformation of the West Segment of the South Dabashan Fold-and-Thrust Belt, South China: Insights From Seismic-Reflection Profiling

The South Dabashan arcuate tectonic belt located at the northern margin of the Yangtze Block in South China, which primarily comprises a series of northwestern (NW)-trending foreland fold-and-thrust belts (FTBs), is useful for determining the intracontinental orogeny processes of the Yangtze Block. In this study, we integrated the latest pre-stack depth migration of three- and two-dimensional seismic profiles, drill hole, and outcrop data to explore the structural geometric and kinematic features of the west segment of the South Dabashan FTB. This belt is characterized by multi-level detachment structures due to the presence of three predominant sets of weak layers: the Lower Triassic Jialingjiang Formation gypsum interval, Silurian mudstone beds, and Cambrian shale beds. The belt is accordingly subdivided vertically into three structural deformation systems. The upper system appears above the Jialingjiang Formation gypsum layer and exhibits Jura-type folds, which were formed by alternating anticlines and synclines that are parallel to each other. The middle system comprises Silurian shale as the base and Jialingjiang Formation gypsum interval as the passive roof and exhibits NW-striking imbricate thrusts. The lower system is bounded by Cambrian and Silurian detachment layers, forming a duplex structure. The Sinian and Proterozoic basements below the Cambrian were not involved in deformation. The west segment of the South Dabashan FTB underwent four periods of tectonic evolution: Late Jurassic to Early Cretaceous, Late Cretaceous, Paleogene, and Neogene to Quaternary. The deformation was propagated southward in imbricate style, resulting in the passive uplifting of the overlying strata. Based on the magnetotelluric and deep seismic profile, the tectonic processes of the west segment of the South Dabashan FTB are inferred to be primarily controlled by the Yangtze Block northward subduction under the Qinling Orogenic Belt and the pro-wedge multi-level thrusting during the Late Jurassic to Cretaceous.

Geochemistry of two types of Palaeoproterozoic granites, and zircon U–Pb dating, and Lu–Hf isotopic characteristics in the Kuandian area within the Jiao‐Liao‐Ji Belt: Implications for regional tectonic setting

Palaeoproterozoic granites accompanied with granulite‐facies gneisses can be recognized in the Kuandian area, northern Jiao‐Liao‐Ji Belt (JLJB). This study highlights two episodes of Palaeoproterozoic magmatism, identified by granites and biotite monzogranitic gneisses herein. Zircon grains from biotite monzogranitic gneiss with homogeneous oscillatory zoning yielded a U–Pb age of 2,153 ± 16 Ma, in accordance with the age of zircon cores from the monzogranitic gneiss. Zircon grains from the two granites yielded upper intercept ages of 1,892 ± 38 Ma and 1,859 ± 36 Ma respectively. Monzogranitic gneiss samples are enriched in Th, Rb, Pb, U, and REE, but depleted in Sr and P. They have high 1,000 × Ga/Al ratios versus Zr, Ce, Nb, Y contents, which are usually interpreted as the features of classical A‐type granite that originated in a lithospheric extension setting. The ~1.89–1.86 Ga granite samples have relatively high Al, Rb, Th, U and low Sr, Zr, and Hf contents. These features, combined with the observation of minor apatite grains under the microscope are similar with the I‐type granites. In situ zircon Lu–Hf isotopic analyses revealed that ~1.89–1.86 Ga granite had a similar range and TDM2 ages with the monzogranitic gneiss, but with much lower εHf(t) values. The monzogranitic gneiss may have originated from an Archean crust, while the later granite may be derived from melting of the partial melting of the monzogranitic gneiss or the Archean crust that generated the monzogranitic gneiss. The ~1.89–1.86 Ga granites, together with the coeval S‐type granites in the JLJB, represent an orogeny process that may involve a Palaeoproterozoic subduction.

A geophysical perspective on the lithosphere–asthenosphere system from Periadriatic to the Himalayan areas: the contribution of gravimetry

The structure of the lithosphere–asthenosphere system as imaged from the complementary inversion of seismological and gravimetric data can provide significant clues to understand the processes of orogeny and cratonization and to better formulate the theory of plate tectonics within a sound geodynamic context. In this paper, we review some studies sharing a common synergic methodology that uses the geometry of cellular shear-wave velocity models of the crust and uppermost mantle to constrain a 3D density model obtained by means of linear inversion of gravimetric data. These studies, which were applied to selected areas along an ideal stream running from the Periadriatic region to the Himalayas, despite having different resolution, show some basic common features in the density distribution of the upper mantle whose unveiling may be precluded to gravity models achieved by assuming velocity–density relationships. In fact, one of the main conclusions of the discussed studies is that lithospheric slabs under the investigated orogenic belts, as identified by seismic wave velocity, are, as a rule, not marked by high density. This fact casts serious doubts about the effectiveness of slab-pull and confirms one of the key issues of polarized plate tectonics, mostly fuelled by tidal drag.

Age, sediment source and tectonic setting of the ore-hosting Jinwozi Formation at the Jinwozi gold deposit in Beishan Orogen, NW China: Evidence from detrital zircon U–Pb ages and Lu–Hf isotopes

The Jinwozi deposit is the most economically important gold (Au) deposit in the Beishan Orogen (Eastern Xinjiang, NW China). The Au orebodies occur as quartz veins or altered lodes in the Jinwozi Formation clastic rocks and the Jinwozi granite. The age, detrital source and tectonic setting of the Jinwozi Formation, however, have not been well understood. In this paper, we reported new LA–ICP MS detrital zircon U–Pb ages and Lu–Hf isotopes of ore-hosted clastic rock of the Jinwozi Formation. Detrital zircons from three samples yield 334 U–Pb ages varied from 330 to 2598 Ma, showing complex detrital sources. The youngest detrital zircon population suggests that the Jinwozi Formation deposited no earlier than 330 Ma in the Late Carboniferous or Permian. The majority of the detrital zircons (291 out of 334) have ages between 330 and 533 Ma, suggesting a dominant Paleozoic detrital source. Much of these age data (282 out of 334) range from 470 to 370 Ma, broadly coeval with the zircon U–Pb ages of the Jinwozi granite. Zircon Hf isotope data also implicate their similar origin to the Jinwozi granite. The source rocks, as well as the Jinwozi granite, may have formed in the Early Paleozoic Caledonian Orogeny, during which the supercontinents of Gondwana and Laurasia assembled. This orogeny process may have provided plenty of detritus for the clastic rocks of the Jinwozi Formation. Precambrian detrital zircon ages from the Jinwozi Formation cluster around 2400–2600 Ma, 1800–2100 Ma, 800–1300 Ma, which corresponds to the supercontinent assembly events of Kenor, Columbia and Rodinia, respectively. The age populations of 2200–2300 Ma and 600–660 Ma are possibly related to the breakup of Kenor and Rodinia supercontinents. This understangding is supported by zircon Hf isotope data.

New insight into crustal and lithospheric variability beneath the central Tien Shan (NW China) revealed by P-wave receiver functions

The Tien Shan has experienced intensive crustal shortening and lithospheric deformation, making it an ideal site to study the orogeny processes. We applied P-wave receiver functions to get new insight into crustal and lithospheric variability beneath the central Shan region. Clear P-to-S converted phases are observed at many recording stations exhibiting double positive converted phases at 5–9 s. The corresponding crustal thickness varies from 35 km to 70 km, which appears to be relatively thinner beneath the Kazakh Shield and Tarim Basin and thicker beneath the southern and northern flanks of the central Tien Shan. The negative converted phases are observed at depths of 30–45 km beneath the central Tien Shan, corresponding to the extremely low seismic velocities in the middle to lower crust revealed by seismic tomography. The receiver function results, coupled with a regional shear-wave velocity model, suggest that the lithosphere of the central Tien Shan has been significantly modified. The thickening and thinning of lithosphere may be related to up-warped and down-warped of mantle and could be associated with the mantle upwelling triggered by bilateral underthrusting of the Tarim Basin and the Kazakh Shield, respectively, beneath the central Tien Shan.

Constraints of eclogites from the Marun-Keu metamorphic complex on the tectonic history of the Polar Urals (Russia)

Knowing the protoliths of eclogites is important to understanding the nature and process of orogeny. It has been debated whether the eclogites of the Marun-Keu complex, Russian Polar Urals represent a fragment of metamorphosed oceanic crust or a fragment of metamorphosed continental crust. Here, we focus on eclogites from the garnet- peridotite-eclogite unit in the complex at the Mica Hill. The eclogites have high MgO contents (mostly greater than 8 wt%) and Mg# (Mg/[Mg + Fe2+] = 0.7–0.8), their Al2O3 contents vary from 16 wt% to 26 wt%, and CaO contents vary from 10 wt% to 15 wt%, respectively. TiO2 contents less than 0.5 wt% and a large variation of chromium concentrations (282–1745 ppm). The eclogites are enriched in the light rare earths (LREE), have positive Eu anomalies (δEu = 1.0–2.3), and varying εNd (t) from −3 to +2. Such geochemical features are evidence that the protoliths of these eclogites are troctolite and gabbro, which may have formed in a rift environment of a continental margin. Using the SHRIMP method to date zircons two groups of ages were obtained: 500–490 Ma and 370–360 Ma, respectively. The older population is interpreted as protolith formation age while the later population is interpreted as the metamorphic age of the eclogite facies.So the eclogites are likely the result of high pressure metamorphism due to subduction of the newly formed Eastern European continental margin under the Syum-Keu intra-oceanic arc, and not metamorphosed ophiolite (Paleo-Urals oceanic crust).

Alb-Cenomanian granitoid magmatism and copper ore genesis of the Sikhote-Alin

Late Albian-early Cenomanin epoch of Au-Cu porphyry mineralization has been distinguished within the Sikhote-Alin. It is associated with the Alb-Cenomanian granitic rocks which emplacement coincided with the processes of orogeny and neoformation of continental lithosphere caused by compressive stress in the setting of transform continental margins of that time. The intrusion of the granitic magma into the crust of Jurassic accretionary wedge terranes and Early Cretaceous terrane of epicontinental turbidite basin provoked development of Au-Mo-Cu and Cu-Au-W ore genesis, respectively. U-Pb dating of zircons from host granites of the Malmuzh Au-Cu deposit yielded Alb-Cenomania age of 100-95 Ma. This age harmonizes with the age data reported by other researchers on the granitic rocks of East and Southeast Asia which are productive for hydrothermal mineral deposits of copper, gold, tin and other metals. Such age consistency suggests that there is Albian-Cenomanian metallogenic megabelt extending throughout the entire East Asian continental margin.

A Preliminary Review of Metallogenic Regularity of Tin Deposits in China

China is rich in tin resources, and contains many types of tin deposits. Among the tin deposit types, the cassiterite‐sulfide type, skarn type and quartz vein type occupy a large proportion of tin resources and reserves. From the aspect of exploitation and utilization, the most important types are cassiterite‐sulfide type and quartz vein type. The cassiterite‐sulfide type tin deposits are mainly located in Northern Guangxi and Eastern Yunnan, skarn type deposits are mainly distributed in the ore‐concentration areas of South Hunan in Middle Nanling, and the quartz vein type tin deposits are mainly distributed in South China, such as Western Fujian, Middle Jiangxi, Northern Guangzhou and Southern Hunan. The most important metallogenic epoch for tin deposits is the Mesozoic era. The metallogenic geotectonic background is mainly continental environments after orogeny process, with strong tectonic changes, interlaced deep fracture and frequent magmatism. And the most distinctive feature is the well developed Mesozoic granites, which have a close relationship with tin mineralization. Based on the detailed study of the data from 873 tin deposits in China, this paper summarized the metallogenic regularity of tin deposits, classified 20 important metallogenic series of tin or tin‐associated deposits, and inferred that the cassiterite‐sulfide type, skarn type, quartz vein type and greisen type are the main prediction types of tin resources. Forty‐four tin‐mineralization belts were divided, among which, 19 belts are the most important. In addition, a series of maps about tin metallogenic belts and tin metallogenic regularity were compiled, aiming to provide theoretical basis for potential estimation and prediction of tin mineral resources.

Tethyan Composite Orogeny Process and its Constraints on Metallogenesis in Sanjiang Region, SW China

Tethyan Composite Orogeny Process and its Constraints on Metallogenesis in Sanjiang Region, SW China

Investigation of the Various Spectral Decomposition Methods to Detect and Explore Hidden Complex Reef Reservoir Structures and Their Hydrocarbon Potentials in Northwestern Part of the Persian Gulf

The Persian Gulf and its coastal areas are recognized as one of the biggest hydrocarbon basins in the world as world's largest petroleum reservoirs are located through its offshore and onshore oil and gas fields. Coastline countries of the Persian Gulf produce about 25% of the world's oil yearly and they have nearly two-thirds and one-third of the world's crude oil and natural gas reserves, respectively. In the Persian Gulf and south of Iran, Zagros orogeny process has a most important structural role as it causes many petroleum reservoirs into folds with northwest-southeast trends that are parallel with Zagros orogeny direction. However, there is good potential to explore new petroleum reservoirs in the Persian Gulf yet. One of these potential oilfields located northwestern of the Persian Gulf includes high petroleum potential, but with structural complexity and this investigation is focused to explore reservoir in its other horizons, specially Sarvak formation as the famous reservoir in the area. To achieve essential information from reservoir and locating it, we applied two useful seismic methods. Authors considered different advantages of the fast Fourier transform (FFT) and the continuous wavelet transform (CWT) as the most commonly applicable spectral analysis of seismic methods in an experimental investigation besides geological new findings. We found that the FFT can provide a good image from reservoir stratigraphic characteristics; it has specifically proven efficient even in carbonate reservoirs allowing individualization of geological bodies not easily defined in amplitude maps. Besides that, the CWT can provide an accurate and clear image from reservoir structural characteristics and low frequency shadows under petroleum reservoirs. We will prove our claims by using theoretical and experimental evidences. Objectives of this paper, beside geological evidences, are to show abilities of these two methods and to describes how, (1) the FFT method shows the distribution of the hidden reef boundaries and (2) the CWT shows the probable presence of hydrocarbons in purpose structure much better than the original seismic data and others methods.

Seismic anisotropy and implications for mantle deformation beneath the NE margin of the Tibet plateau and Ordos plateau

The northeastern Tibet and the Ordos plateau connects east China and west China, which are dominated by horizontal extension and compression, respectively. Knowledge of seismic anisotropy beneath this transitional region can provide important constraints on deformation pattern of the crust and lithosphere mantle during an orogeny process. We measured SKS wave splitting parameters from the new installed regional networks and obtained a detailed anisotropy map for 119 broadband stations. Beneath the NE margin of Tibet, the observed WNW fast polarization direction is parallel to the surface geological features. The coherence between the observed geodetic motion of the crust and fast axis direction suggests that the vertical coherent deformation of the lithosphere is the dominate source for the observed seismic anisotropy. Further east to the Qinling orogenic belt, the fast axis direction tracks change in strike of the faults, with the fast direction aligning almost EW. We suggest that both the lithosphere mantle and east extrusion of mantle material contribute to the observed anisotropy. Small value of delay time is observed interior the thick-rooted Ordos plateau, suggesting a rigid and stable lithosphere with little deformation. The deviation of fast polarization direction beneath the adjacent rifting regions might be caused by the edge flow induced by sharp changes in the thickness of lithosphere. Our results propose that the upper crust and mantle lithosphere beneath the northeastern Tibet are at least partly coupled, or subjected to the same boundary conditions, which is inconsistent with the channelized lower crustal flow model.

Re–Os and U–Pb Geochronology of the Erlihe Pb–Zn Deposit, Qinling Orogenic Belt, Central China, and Constraints on Its Deposit Genesis

Abstract:The Erlihe Pb–Zn deposit is an important mine of the Pb–Zn metallogenic zone in the South Qinling Orogen. It has been considered a sedimentary exhalative deposit in previous investigations because the ore body occurs concordantly at the transitional location of an upright fold. Re and Os isotopic analyses for paragenetic pyrites with sphalerite and galena from the ore body have been used to determine the timing of mineralization and to trace the source of metallogenic materials. The Re–Os isotopic data of four pyrite samples construct an isochron, yielding a weighted average age of 226±17 Ma (mean square weighted deviation=1.7), which is considered the main mineralization age. A dioritic porphyrite vein sample, showing weaker mineralization, was also dated using the SHRIMP zircon U–Pb isotopic method to constrain the youngest metallogenic age of the ore deposit, because it distributes along a group of tensional joints cutting not only the upright fold in the deposit field, but also the main ore bodies. The dioritic porphyrite sample yields a weighted mean 206Pb/238U age of 221±3 Ma, which is slightly younger than the Re–Os isotopic isochron age of the pyrites, considered as the upper age limit of the mineralization, namely the ending age of the mineralization. The Os isotopic compositions of sulfide minerals distribute within a range between Os isotopic compositions of the crust and the mantle, indicating that the ore deposit can be derived from magma‐related fluid, and the metallogenic materials are most likely derived from the mixing source of the crust and the mantle. The Erlihe Pb–Zn deposit and associated dioritic porphyrite vein, important records of Qinling tectonic–magmatism–mineralization activities, were formed during the Triassic collisional orogeny processes.

Early Miocene continental subduction and rapid exhumation in the western Mediterranean

New Lu-Hf garnet growth ages from high-pressure rocks from the structurally lowest terrane (Nevado-Filabride Complex) in the Betic Cordillera of southern Spain indicate early Miocene subduction of continental crust, requiring significant revision of the timing and processes of orogeny in the western part of the Alpine orogenic system. Mafic eclogite and pelitic schist give Lu-Hf ages in the range 18–14 Ma, and fission-track data show that the terrane was exhumed by 12–8 Ma. The overlying terrane (Alpujarride Complex) shows early Tertiary high pressure-temperature metamorphism; this was followed by a thermal event that accompanied rapid crustal extension at 22–18 Ma, leading to the formation of the Alboran Sea backarc basin. The evidence therefore suggests two phases of continental subduction separated by a phase of large-scale intraorogenic extension. The two terranes are now separated by a mid-late Miocene low-angle extensional fault. The timing and paleogeographic setting suggest that the Nevado-Filabride terrane originally was close to the rifted South Iberian margin, and was subducted during westward motion of the extending upper terrane between the converging African and Iberian plates.

The role of deep basement during continent-continent collision: a review

Abstract Structural, geophysical and metamorphic studies show that collisional orogeny thickens the crust by a factor of two or more. A large volume of continental material at the base of the orogen is, therefore, subject to eclogite facies conditions. Phase equilibration results in a loss of buoyancy and thermodynamic heating of this crustal root. This dense crustal material may be partially subducted, as in the Alps or the Himalayas, and lost to the system. Alternatively, it may rest isostatically below the Moho until it is partially exhumed during orogenic collapse, as in the Scandinavian Caledonides or the Tonbai-Dabie Mountains. Remnant orogenic roots may exist as seismically reflective mantle and provide a locus for subsequent Wilson Cycle rifting. The rate at which these phase transformations take place may have a profound buffering effect on the amount and duration of orogenic contraction. Isostatically compensated transient 2-dimensional finite element thermal models are presented, which seek to place some limits on these processes. It is interesting to speculate whether more is learnt about the process of orogeny from a single exhumed eclogitic boudin or from mapping nappe complexes.

Modeling mountain building and the seismic cycle in the Himalaya of Nepal

A host of information is now available regarding the geological and thermal structure as well as deformation rate across the Himalaya of central Nepal. These data are reconciled in a two‐dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes. Over geological timescale (5 Ma) the ∼20 mm/yr estimated shortening rate across the range is accommodated by localized thrust faulting along the Main Himalayan Thrust fault (MHT). The MHT reaches the surface along the foothills, where it is called the Main Frontal Thrust fault (MFT). The MHT flattens beneath the Lesser Himalaya and forms a midcrustal ramp at the front of the Higher Himalaya, consistent with the river incision and the anticlinal structure of the Lesser Himalaya. Farther northward the MHT roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector. Aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust, increasing the Coulomb stress beneath the front of the Higher Himalaya, where most of the microseismic activity clusters. Negligible deformation of the hanging wall requires a low apparent friction coefficient (μ) less than ∼0.3 on the flat portion of the MHT. On the ramp, μ might be as high as 0.6. Sensitivity tests show that a rather compliant, quartz‐rich rheology and a high radioactive heat production in the upper crust of ∼2.5 μW/m3 is required. Erosion affects the thermal structure and interplays with crustal deformation. A dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure, topography, and deformation field. Using a linear diffusion model of erosion, we constrain the value of the mass diffusivity coefficient to 0.5–1.6×l04 m2/yr. This study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period.

Magnetic fabrics of dike swarms from SE Bahia State, Brazil: their significance and implications for Mesoproterozoic basic magmatism in the São Francisco Craton

Magnetic fabrics were determined by applying two techniques: anisotropy of low-field magnetic susceptibility (AMS); and anisotropy of anhysteretic remanence (AAR). AMS was performed in 81 dikes from Ilhéus, Olivença and Itaju do Colônia swarms located in SE Bahia. The dikes intrude high-grade metamorphic terrains as old as 3.2 Ga. Dikes are vertical or subvertical in dip and trend mainly E–W. They are tholeiitic in composition and are classified into two groups, characterized by high TiO 2 (>2%) and incompatible elements contents, and low TiO 2 and incompatible elements contents, respectively. Thicknesses vary from a few centimeters to ca. 20 m (average of 3 m). 40Ar/ 39Ar data indicate ages of 1.012 (Ilhéus dikes) and 1.078 Ga (Olivença dikes). Rock magnetism shows that Ti-poor titanomagnetites up to pure magnetites pseudo-single-domain/multidomain grain sizes carry the magnetic fabrics. Three types of AMS fabric are recognized: (i) ‘Normal’ fabric (64 dikes) defined by the clustering of K 1– K 2 axes on the dike plane whereas K 3 axes are nearly perpendicular to it. This fabric is interpreted as due to magma flow. (ii) ‘Intermediate’ fabric (seven dikes) characterized by K 1– K 3 axes clustering close to dike plane and K 2 axes perpendicular to the dike wall. It was interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike direction; and (iii) ‘Inverse’ fabric (nine dikes) defined by K 2– K 3 plane parallel to the dike plane and K 1 perpendicular to dike wall. AAR measurements were performed in three dikes; two of them with ‘inverse’ and one with ‘normal’ AMS fabrics. AAR was coaxial for ‘normal’ AMS fabric and it resulted in ‘intermediate’ and ‘normal’ for ‘inverse’ AMS fabrics. A combination of AMS and AAR fabrics suggests that the magmatic fabric for these dikes that should be flow, was overprinted by some local event, probably related to Brasiliano Orogeny processes, after dike emplacement. ‘Inverse’ AMS fabric would record this event.‘Normal’ fabric is the most important one found for the studied swarms. The analysis of K 1 inclination permitted one to infer that the dikes from Ilhéus were fed dominantly by horizontal fluxes ( K 1<30°) whereas those from Olivença and Itaju do Colônia were mainly fed by inclined up to vertical fluxes ( K 1>60°). Dikes from Itaju do Colônia region have the steepest K 1 suggesting that this region could be closer to a magma source. On the other hand, a magma source near to Olivença area could be also inferred since the majority of the dikes were fed by inclined flow. The existence of more than one source is supported by geochemical, geochronological and paleomagnetic data which have shown that contemporaneous dikes were fed by compositionally distinct magmas. The AMS data also suggest that the majority of dikes were fed from upward flow coming from west to east and only a few of them were fed from upward flow from east to west.