Magnetotelluric investigations for imaging electrical structure of Garhwal Himalayan corridor,Uttarakhand, India

Magnetotelluric investigations have been carried out in the Garhwal Himalayan corridor to delineate the electrical structure of the crust along a profile extending from Indo-Gangetic Plain to Higher Himalayan region in Uttarakhand, India. The profile passing through major Himalayan thrusts: Himalayan Frontal Thrust (HFF), Main Boundary Thrust (MBT) and Main Central Thrust (MCT), is nearly perpendicular to the regional geological strike. Data processing and impedance analysis indicate that out of 44 stations MT data recorded, only 27 stations data show in general, the validity of 2D assumption. The average geoelectric strike, N70°W, was estimated for the profile using tensor decomposition. 2D smooth geoelectrical model has been presented, which provides the electrical image of the shallow and deeper crustal structure. The major features of the model are (i) a low resistivity (<50Ωm), shallow feature interpreted as sediments of Siwalik and Indo-Gangetic Plain, (ii) highly resistive (> 1000Ωm) zone below the sediments at a depth of 6 km, interpreted as the top surface of the Indian plate, (iii) a low resistivity (< 10Ωm) below the depth of 6 km near MCT zone coincides with the intense micro-seismic activity in the region. The zone is interpreted as the partial melting or fluid phase at mid crustal depth. Sensitivity test indicates that the major features of the geoelectrical model are relevant and desired by the MT data.     

Proposed seismic hazard maps of Sumatra and Java islands and microzonation study of Jakarta city,Indonesia

This paper presents the development of spectral hazard maps for Sumatra and Java islands, Indonesia and microzonation study for Jakarta city. The purpose of this study is to propose a revision of the seismic hazard map in Indonesian Seismic Code SNI 03-1726-2002. Some improvements in seismic hazard analysis were implemented in the analysis by considering the recent seismic activities around Java and Sumatra. The seismic hazard analysis was carried out using 3-dimension (3-D) seismic source models (fault source model) using the latest research works regarding the tectonic setting of Sumatra and Java. Two hazard levels were analysed for representing 10% and 2% probability of exceedance (PE) in 50 years ground motions for Sumatra and Java. Peak ground acceleration contour maps for those two hazard levels and two additional macrozonation maps for 10% PE in 50 years were produced during this research. These two additional maps represent short period (0.2 s) and long-period (1.0 s) spectra values at the bedrock. Microzonation study is performed in order to obtain ground motion parameters such as acceleration, amplification factor and response spectra at the surface of Jakarta. The analyses were carried out using nonlinear approach. The results were used to develop contour of acceleration at the surface of Jakarta. Finally, the design response spectra for structural design purposes are proposed in this study.     

Asymmetrically zoned reaction rims: assessment of grain boundary diffusivities and growth rates related to natural diffusion-controlled mineral reactions

This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite→garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms ～20‐μm‐wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low‐angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase|garnet|hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe²⁺, Fe³⁺ and Al across the garnet layer; and (ii) micron‐scale compositional variations in the near‐grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast‐path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe²⁺ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast‐path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast‐path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20‐μm‐wide garnet rim is estimated at ～10³–10⁴ years at the inferred reaction conditions of 760 ± 50 °C at 7.6 kbar. Using the same parameterization of the growth law, 100‐μm‐ and 1‐mm‐thick garnet rims would grow within 10⁵–10⁶ and 10⁶–10⁷ years respectively.     

SHRIMP zircon dating and Sm/Nd isotopic investigations of Neoproterozoic granitoids, Eastern Desert, Egypt

There is an increasing evidence for the involvement of pre-Neoproterozoic zircons in the Arabian–Nubian Shield, a Neoproterozoic crustal tract that is generally regarded to be juvenile. The source and significance of these xenocrystic zircons are not clear. In an effort to better understand this problem, older and younger granitoids from the Egyptian basement complex were analyzed for chemical composition, SHRIMP U–Pb zircon ages, and Sm–Nd isotopic compositions. Geochemically, the older granitoids are metaluminous and exhibit characteristics of I-type granites and most likely formed in a convergent margin (arc) tectonic environment. On the other hand, the younger granites are peraluminous and exhibit the characteristics of A-type granites; these are post-collisional granites. The U–Pb SHRIMP dating of zircons revealed the ages of magmatic crystallization as well as the presence of slightly older, presumably inherited zircon grains. The age determined for the older granodiorite is 652.5 ± 2.6 Ma, whereas the younger granitoids are 595–605 Ma. Xenocrystic zircons are found in most of the younger granitoid samples; the xenocrystic grains are all Neoproterozoic, but fall into three age ranges that correspond to the ages of other Eastern Desert igneous rocks, viz. 710–690, 675–650 and 635–610 Ma. The analyzed granitoids have (+3.8 to +6.5) and crystallization ages, which confirm previous indications that the Arabian–Nubian Shield is juvenile Neoproterozoic crust. These results nevertheless indicate that older Neoproterozoic crust contributed to the formation of especially the younger granite magmas.     

Extensional tectonic origin of gneissosity and related structures of the Feiran–Solaf metamorphic belt, Sinai, Egypt

The Feiran–Solaf metamorphic belt consists of low-P high-T amphibolite facies, partly migmatized gneisses, schists, amphibolites and minor calc-silicate rocks of metasedimentary origin. There are also thick concordant synkinematic sheets of diorite, tonalite and granodiorite orthogneiss and foliated granite and pegmatite dykelets. The gneissosity (or schistosity) is referred to as S₁, and is almost everywhere parallel to lithological layering, S₀. This parallelism is not due to transposition. The gneissosity formed during an extensional tectonic event (termed D1), before folding of S₀. S₁ formed by coaxial pure shear flattening strain (Z normal to S₀, i.e. vertical; with X and Y both extensional and lying in S₁). This strain also produced chocolate tablet boudinage of some layers and S₁-concordant sills and veins. S₁ has a strong stretching lineation L₁ with rodding characteristics. Within-plane plastic anisotropy (lower ductility along Y compared to along X) resulted in L₁-parallel extensional ductile shears and melt filled cracks. Continued shortening of these veins, and back-rotation of foliations on the shears produced intrafolial F₁ folds with hinges parallel to the stretching lineation. F₁ fold asymmetry variations do not support previous models involving macroscopic F₁ folds or syn-gneissosity compressional tectonics. The sedimentary protoliths of the Feiran–Solaf gneisses were probably deposited in a pre-800 Ma actively extending intracratonic rift characterizing an early stage of the break-up of Rodinia.     

Paleomagnetism and Detrital Zircon Geochronology of the Upper Vindhyan Sequence, Son Valley and Rajasthan, India: A ca. 1000 Ma Closure age for the Purana Basins?

The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including ⁸⁷Sr/⁸⁶Sr and δ ¹³C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α₉₅ = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at 1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α₉₅ = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the 1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period.     

A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)

New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the ⁴⁰Ar/³⁹Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.     

The role of heterogeneous strain in the development and preservation of a polymetamorphic record in high-P granulites, western Canadian Shield

Mafic rocks in the Chipman domain of the Athabasca granulite terrane, western Canadian Shield, provide the first well‐documented record of two distinct high‐P granulite facies events in the same domain in this region. Textural relations and the results of petrological modelling (NCFMASHT system) of mafic granulites are interpreted in terms of a three‐stage tectonometamorphic history. Stage 1 involved development of the assemblage Grt + Cpx + Qtz ± Pl (M₁) from a primary Opx‐bearing igneous precursor at conditions of 1.3 GPa, 850–900 °C. Field and microstructural observations suggest that M₁ developed synchronously with an early S₁ gneissic fabric. Stage 2 is characterized by heterogeneous deformation (D₂) and synkinematic partial retrogression of the peak assemblage to an amphibole‐bearing assemblage (M₂). Stage 3 involved a third phase of deformation and a return to granulite facies conditions marked by the prograde breakdown of amphibole (Amph₂) to produce matrix garnet (Grt_3a) and the coronitic assemblage Cpx_3b + Opx_3b + Ilm_3b + Pl_3b (M_3b) at 1.0 GPa, 800–900 °C. M₁ and M_3b are correlated with 2.55 and 1.9 Ga metamorphic generations of zircon, respectively, which were dated in a separate study. Heterogeneous strain played a crucial role in both the development and preservation of these rare examples of multiple granulite facies events within single samples. Without this fortuitous set of circumstances, the apparent reaction history could have incorrectly led to an interpretation involving a single‐cycle high‐grade event. The detailed P–T–t–D history constructed for these rocks provides the best evidence to date that much of the east Lake Athabasca region experienced long‐term lower crustal residence from 2.55 to 1.9 Ga, and thus the region represents a rare window into the reactivation and ultimate stabilization processes of cratonic lithosphere.     

Using ALSM to map sinkholes in the urbanized covered karst of Pinellas County,Florida—1, methodological considerations

Comparison of a database of interpreted sinkholes made using airborne laser swath mapping (ALSM) with databases of sinkholes made from interpretation of aerial photographs reveals substantial disagreement. The discrepancies involve the location, number, areas, and circularity of interpreted sinkholes. Methodological factors that contribute to the discrepancies include blockage of ALSM laser returns in thick vegetation, multi-path reflections, and misrepresentation of the true extent of sinkholes. Comparison of two ALSM-derived databases made (1) independently from versus (2) in combination with earlier air photo analysis in undeveloped regions had the following result: one-fourth of the sinkholes interpreted by using the composite method were missed by the independent analysis, and one-third of the sinkholes that were interpreted from the independent analysis were not interpreted as sinkholes using the composite method. Subjective interpretation leads to a high level of uncertainty such that the results of the remote sensing studies are suspect, if not invalid.     

Use of sinkhole and specific capacity distributions to assess vertical gradients in a karst aquifer

The carbonate-rock aquifer in the Great Valley, West Virginia, USA, was evaluated using a database of 687 sinkholes and 350 specific capacity tests to assess structural, lithologic, and topographic influences on the groundwater flow system. The enhanced permeability of the aquifer is characterized in part by the many sinkholes, springs, and solutionally enlarged fractures throughout the valley. Yet, vertical components of subsurface flow in this highly heterogeneous aquifer are currently not well understood. To address this problem, this study examines the apparent relation between geologic features of the aquifer and two spatial indices of enhanced permeability attributed to aquifer karstification: (1) the distribution of sinkholes and (2) the occurrence of wells with relatively high specific capacity. Statistical results indicate that sinkholes (funnel and collapse) occur primarily along cleavage and bedding planes parallel to subparallel to strike where lateral or downward vertical gradients are highest. Conversely, high specific capacity values are common along prominent joints perpendicular or oblique to strike. The similarity of the latter distribution to that of springs suggests these fractures are areas of upward-convergent flow. These differences between sinkhole and high specific capacity distributions suggest vertical flow components are primarily controlled by the orientation of geologic structure and associated subsurface fracturing.