Western Shear Zone Research Articles

Structural Analysis of Kirthar Fold Belt, Lower Indus Basin, Balochistan, Pakistan; Implications from Compression and Inversion Tectonics

The current research work mainly focuses on tectonic evolution of the Central Kirthar Fold Belt. It also deals with the structural impact of the Late Cretaceous/Paleocene, Indian-Eurasian collision on the Central Kirthar Fold Belt which is situated in the western shear zone of Indian Plate. The Kirthar Fold Belt is different from the rest of western shear zone because it is mainly deformed by compressional tectonics which has created wide anticlines, narrow synclines and overturned folds as shown by geological mapping. It has been observed that thin-skinned deformation has played a major role in structural disharmony which is mainly seen in the Early Eocene Ghazij and Oligocene Nari formations. Seismic data shows thick-skinned deformation because the faults seem to penetrate deeper stratigraphic levels. Involvement of the basal decollement and the penetration of the interpreted faults to the level of basement rocks is also evident from the exposure of Jurassic/Cretaceous rocks at surface on the western part of the mapped area. Cross sections data shows that Kirthar Thrust Fault has a structural throw around 3-4 km in the Kirthar Folded Zone featuring deformational front while the seismic data and well data shows that this intensity decreases in the Kirthar Foredeep Zone towards east where the structural throw is about 2-3 km. Restoration of cross sections show an overall 17.3% shortening which shows that the area is under influence of compressional stresses.

Exhumation of the Ronda Peridotite During Hyper‐Extension: New Structural and Thermal Constraints From the Nieves Unit (Western Betic Cordillera, Spain)

AbstractThe Ronda peridotite (Betic Cordillera, Southern Spain) is the largest alpine‐type peridotite massif worldwide. Yet, the emplacement mechanism of these mantle rocks is still a highly debated topic. In this study, we aim at better constraining their context of exhumation by focusing on the Nieves Unit, which is mostly composed of Mesozoic metasediments displaying HT metamorphism and deformation along the contact with peridotite. The topmost parts of the peridotites and the bottom parts of the Nieves Unit share the same planar/linear fabric parallel to the contact accompanied by component of sinistral kinematic indicators. Raman Spectroscopy on Carbonaceous Material geothermometry then shows a contraction of peak‐temperature conditions within metasediments of the Nieves Unit, as well as a north‐eastward decrease along strike, highlighting a strong thinning of the unit and a significant obliquity of the isograds to the contact with the peridotites. Local magnetite ores and ophicalcite bodies are finally observed nearby the contact, suggesting a hyper‐extension context for the mantle exhumation. Indeed, the Nieves Unit, the Ronda peridotite and the Jubrique Unit, where a strong thinning has also been documented, are all in contact at a triple junction between the western contact of the peridotite and the Nieves/Jubrique contact. Altogether, these features support the idea that the western shear zone limiting the peridotites from the Nieves and Jubrique units is a tilted large‐scale extensional detachment that has exhumed mantle rocks from beneath the continental basement (Jubrique Unit) and its Mesozoic cover (Nieves Unit) during an episode of hyper‐extension.

Three-dimensional seismic tomographic imaging beneath the Sea of Marmara: evidence for locked and creeping sections of the Main Marmara Fault

SUMMARYWe present 3-D Vp and Vp/Vs tomographic images for depths down to 22 km along with precise earthquake locations beneath the Sea of Marmara using the seismological data sets collected during the Ocean Bottom Seismometer (OBS) experiments conducted in 2001 and 2006. The unique data set collected by 52 OBSs and augmented by 14 land stations include 3852 P and 2643 S arrival times from 434 earthquakes and 4744 P arrival times from 557 air-gun shot records. From the tomographic depth sections and relocated seismicity, we identify two shear zones which are marked by narrow-width (∼10 km), low Vp (4.3–5.3 km s–1) and high Vp/Vs ratio (∼2.0) anomalies coinciding with the surface trace of the Main Marmara Fault (MMF) and extending near vertically from 8 km depth below the basins into the deeper part of the crust. We consider these shear zones to be the strike-slip deformation signature of the MMF at depth. The western shear zone imaged lies between the Western High and the eastern end of the Central Basin including the 60-km-long segment of the MMF and contains higher seismicity and is visible in the 8–22 km depth range. The eastern shear zone, coinciding with the NW–SE trending surface trace of the MMF, is imaged beneath the Çınarcık Basin with lower seismicity and tomographic resolution. Furthermore, these shear zones may contain fractures filled with deep crustal fluids allowing partial aseismic creep. Within these shear zones, we also identify small-scale, high Vp (6 km s–1) anomalies with low seismicity indicative of strong patches of the fault at depth that can be considered as asperities. Between these shear zones, our tomographic images clearly show a central zone that has homogenously high Vp (∼6.0–6.5 km s–1) and low Vp/Vs ratio (∼1.7) over a large area including the 50-km-long central segment of the MMF lying between the eastern end of the Central Basin and western end of the Çınarcık Basin. Thus, we infer that the central MMF is in a locked state based on its high shear strength, low seismicity and a lack of detectable geodetic slip. Furthermore, a large earthquake might not nucleate on the central MMF since it may be strongly locked at the current level of stress loading, and thus it may behave as a seismic barrier or anti-asperity. On the other hand, the central MMF is a good candidate for a supershear rupture because it has nearly uniform elastic properties and low background seismicity indicating the homogeneity of friction and pre-stress across the fault, which are presented as the main characteristics of supershear fault ruptures. From the seismic hazard perspective, we speculate that a large earthquake may be expected to nucleate within either of the imaged shear zones with the possibility of the central MMF acting as a seismic barrier to stop or to join the rupture in a supershear mode. The imaged features have important implications for various aspects of seismic hazard and crustal dynamics in the Sea of Marmara region.

MINERALOGY, CHEMISTRY AND RADIOACTIVITY OF THE ANOMALOUS QUARTZ VEIN ACCOMPANYING THE WESTERN SHEAR ZONE OF RAS ABDA GRANODIORITE, NORTH EASTERN DESERT, EGYPT

The quartz vein cutting the granodiorite of Ras Abda, along the western shear zone, exhibits highradioactive potentiality (up to 3000 ppm for Th and 1600 ppm for U. The microscopic investigation ofthe quartz vein revealed that it is composed mainly of quartz and iron oxides enclosing squadrons ofthe accessory minerals. Granitic fragments are corroded and digested from the wall-rock affecting thechemical composition of the studied rock. Chemically, it is characterized by low alumina and mediumpotassium contents with peralkaline affinity. It is also characterized by high concentrations of the traceelements (Zr >10000, Nb 3481, Y 8621, U 903 and Th 2340 ppm) and the total rare earth elements (up to24246 ppm) specially the HREEs with very low degree of fractionation in the melt (0.014) and in turn,high degree of fractionation in the accessory minerals.The mineralogical investigation using ESEM and XRD techniques revealed the minerals areresponsible for the radioactivity in the anomalous rock such as zircon, thorite, uranothorite and Nb-Taminerals (columbite, euxenite and uranopolycrase).Experimental work in this study agreed with the previous experiments and concluded that metamictizationis attributed to the heat of self-annealing that responsible for transformation of U-euxenite to metamictizedeuxenite and transformation of Ti-U-euxenite to metamictized uranopolycrase.The present study concluded that the studied quartz vein originated from silicic magma rich in the traceand rare earth elements; hence it is considered as good hostile for the radioelements and possesses highradioactive potentiality.

Interference fold patterns in regional unidirectional stress fields: A result of local kinematic interactions

Folds are commonly found in association with ductile shear zones. However, superimposed folding contemporaneous with shearing is rarely reported. Here, we describe a macroscopic fold interference pattern with geometry intermediate between types 1 and 2 related to development of adjacent transcurrent shear zones. The interference fold pattern is located in a compartment bounded, on the eastern side, by an ENE-trending, dextral shear zone that connects with a NE-trending, sinistral shear zone, and, on the western side, by a NE-trending sinistral shear zone. A first folding episode produced NW-trending, SW-verging inclined folds and a later one NE-trending, upright folds. Folds of the first event are attributed to a local contractional strain field induced by the growth of the shear zones with opposed kinematics whereas the later folding episode and nucleation of the western shear zone reflects regional NW-SE contraction. Folding of a macro-scale NW-trending fold by NE-trending folds produced the fold interference structure. In contrast with most cases of types 1 and 2 fold interference patterns, in the present case no change on the orientation of regional stress axes is required.

Systematic mineralogical diversity in A-type granitic intrusions: Control of magmatic source and geological processes

The origin and causes of mineralogical diversity of A-type granites are debated. The series of A-type granite plutons, with distinct mineralogical differences, emplaced along an Upper Paleozoic crustal-scale shear zone in the Cobequid Highlands, Nova Scotia, provide an opportunity to examine the origin of different A-type plutons in a similar tectonic setting. Based on the ferromagnesian minerals present, the plutons are classified into sodic granites with sodic amphibole, calcic granites with calcic amphibole, and biotite granites. Sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone, whereas plutons in the western shear zone, with lesser gabbro, are solely biotite granites. Trace elements and radiogenic isotopes show that the three granite types have different sources. Intensive parameters including temperature, pressure, and water-in-melt contents were estimated from mineralogical and geochemical data. Modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20%–40% partial melting of intracrustal feldspathic rocks, whereas the sodic granites are extreme fractionates (90%) of coeval mafic magma. We propose that supply of Upper Paleozoic mafic magma, probably related to regional extension and decompression melting beneath the Magdalen Basin, created a deep crustal hot zone in the eastern Cobequid Highlands, and extreme fractionation of underplated mafic sills produced the sodic granites. Heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust, creating batches of biotite and calcic granitic melts in different depths. Fractionated and crustally derived melts segregated along crustal-scale faults, constructing the complex plutons in the east. Melting of the crust was further facilitated by the release of water from the crustal rocks upon heating. In the eastern shear zone, water was released predominantly by magmatic rocks and in lesser amounts compared to the west, where Neoproterozoic sedimentary and volcaniclastic rocks are more abundant. The volatile-rich granitic melts in the western part of the shear zone were crystallized rapidly, stabilizing only biotite. This study demonstrates that the mineralogical variations in A-type granites arise from rather similar magma compositions, but they are important petrogenetic indicators of varying sources, specific magmatic processes, and emplacement conditions.