We documented centimeter to meter scale, isolated ductile shear zones in the Chotanagpur Granite Gneiss Complex, Eastern India. They display distortion patterns of mylonitic foliation, indicating qualitatively little flattening strain in them. Using analog and finite element (FE) model experiments we evaluated the degree of transpression mechanically possible in coherent ductile shear zones within an undeformable host. Our experiments demonstrate that the magnitude of shear zone normal shortening (ϵb) depends mainly on the following geometric factors: (1) the ratio between the length and thickness (Df) of shear zones and (2) the angle between the normal to the shear zone boundaries and the maximum bulk compression (σ1) in direction (α). These geometric parameters show strongly non-linear, inverse relations.We estimate that an appreciable shortening (ϵb˜ 15%) is possible only in shear zones with low Df (˜5), and low α (<20o) values. Shear zones with Df > 10 cannot undergo bulk shortening by more than 5%. We present a detailed quantitative analysis of the ratio of pure and simple shear rates (Sr), and the bulk kinematic vorticity number (Wk) as a function of Df and α. Wk tends towards 1 as Df becomes large (>10) irrespective of α, implying simple shear kinematics in narrow shear zones. Based on FE results, we propose volume reduction as a mechanically effective process to enhance the degree of transpression.
The tectonic history of the Himalaya-Tibet Mountain Range records two important extensional tectonic events: 1) N-S extension in the Himalaya-Tibet transition zone, and 2) E-W extension in the southern and central Tibet, manifested in the form of east-west and north-south striking normal faults, respectively. The N-S extensional event (~22 Ma) started to commence earlier than the E-W extension (~18 Ma) and phased out by ~11 Ma, whereas the E-W extension continued to recent time (~4 Ma), suggesting a temporally overlapping period of ~7 Myr. This article addresses the question- did they originate from the same dynamical process? Using numerical and laboratory experiments, we show that a decreasing India-Asia convergence velocity induced gravitational collapse of the Tibetan plateau is the main driving force for both the extensional tectonic events, but they were controlled by two different mechanisms. Our results show that with a drop in the convergence velocity southern Tibet underwent gravitational collapse due to pressure relaxation in the underlying Himalayan wedge, and the collapse forced the deep crustal materials to extrude up, creating N-S extension (22 – 11 Ma) along the Himalaya-Tibet transition zone. On the other hand, presence of rigid Tarim block in north-western Tibet caused differential topographic uplifts from west to east, resulting in a first-order eastward topographic gradient of 0.1º during the initial fast-stage of India-Asia collision (> 22 Ma). Later on, this topographic slope prompted eastward crustal flows in the course of gravitational collapse, leading to E-W extensional tectonics (~18 - 4 Ma) in Tibet.
Combining field observations with analogue laboratory experiments, this study aims to use surface-roughness characteristics as an indicator of the heterogeneous slip partitioning along shear surfaces. We investigated the roughness of shear surfaces in sheared quartzite of the Singhbhum Shear Zone, eastern India, and identified two distinct kinematic domains: slip zone and stuck zone, marked by strong and weak or no roughness anisotropy, respectively. The experiments, run on brittle-ductile models under a pure shear condition suggest the initial inclination (θ) of shear fractures to the compression direction as a crucial factor in determining their competitive development (measured in terms of their relative area coverage) on the shear surface. Using a laser profilometer we constructed 3D topologies of both field and experimental shear surfaces, which are presented to show their distinctive roughness characteristics. The slip and stuck zones differ from each other in the fractal properties of their surface irregularities. ΔD [difference between across- (D⊥) and along- (D∥) slip direction] is calculated to evaluate the degree of roughness anisotropy. This fractal parameter indicates strong anisotropy in slip (ΔD = 0.0787 – 0.2118) zones, which is virtually absent in stuck zones (ΔD = 0.0024 – 0.0603). We thus propose ΔD as an effective parameter to delineate the slip and stuck zones on a shear surface. Finally, the article presents an in-depth discussion of the geological implications, e.g., earthquake event patterns of this slip-stuck roughness study.
Abstract In peninsular India, the Deccan Traps record massive, continental‐scale volcanism in a sequence of magmatic events that corresponds with the timing of mass extinction at the Cretaceous‐Paleogene boundary. Although the Deccan volcanism is linked with the Réunion hotspot, the origin of its periodic magmatic pulses is still debated. We developed a numerical model replicating the geodynamic scenario of the African superplume underneath a moving Indian plate to explore the mechanism of magmatic pulse generation during the Deccan volcanism. Our model results revealed a connection between the Réunion hotspot and the African large low shear‐wave velocity province (LLSVP), suggesting that the pulses were produced from a thermochemical plume originated in the lower mantle. The ascending plume had stagnation at 660 km due to phase changes in the transition zone, and its head eventually underwent detachment from the tail under the influence of Indian plate movement to produce sequentially four major pulses (periodicity: 5–8 Ma), each giving rise to multiple secondary magmatic pulses at a time interval of ∼0.15–0.4 Ma. This study sheds a new light on the mechanism of periodic hotspot activities from a global perspective.
Ductile yielding of rocks and similar solids localize shear zones, which often show complex internal structures due to the networking of their secondary shear bands. Combining observations from naturally deformed rocks and numerical modelling, this study addresses the following crucial question: What dictates the internal shear bands to network during the evolution of an initially homogeneous ductile shear zone? Natural shear zones, observed in the Chotonagpur Granite Gneiss Complex of the Precambrian craton of Eastern India, show characteristic patterns of their internal shear band structures, classified broadly into three categories: type I (network of antithetic low-angle Riedel (R) and synthetic P-bands), type II (network of shear-parallel C and P/R bands) and type III (distributed shear domains containing isolated undeformed masses). Considering strain-softening rheology, our two-dimensional viscoplastic models reproduce these three types, allowing us to predict the condition of shear band growth with a specific network pattern as a function of the following parameters: normalized shear zone thickness, bulk shear rate and bulk viscosity. This study suggests that complex anastomosing shear-band structures can evolve under simple shear kinematics in the absence of any pure shear component.
<p>Subduction zones witness exhumation of deep crustal rocks metamorphosed under high pressure (HP) and ultra-high pressure (UHP) conditions, following burial to depths of 100 km or more. The exhumation dynamics of HP and UHP rocks still remains a lively issue of research in the Earth science community. We develop a new tectonic model based on the lubrication dynamics to show the exhumation mechanism of such deep crustal rocks in convergent tectonic settings. Our model suggests subducting plate motion produces a dynamic pressure in the subduction wedge, which supports the excess gravitational potential energy of the thicker and relatively denser overriding plate partly lying over the buoyant subduction wedge. A drop in the dynamic pressure causes the overriding plate to undergo gravitational collapse and forces the wedge materials to return to the surface. Using lubrication theory we estimate the magnitude of dynamic pressure (<em>P</em>) in the wedge as a function of subduction velocity (<em>u<sub>s</sub></em>), convergence angle (<em>&#945;</em>) and wedge viscosity (<em>&#181;</em>). We also conduct thermo-mechanical numerical experiments to implement the lubrication model in subduction zones on a real scale. Our analysis suggests that drop in wedge dynamic pressure below a threshold value due to decease in <em>u</em><sub><em>s</em>&#160; </sub>and <em>&#181;</em>, or by other processes, such as slab rollback and trench retreat facilitate exhumation of deep crustal rocks. Finally we discuss their implications for the exhumation of deep crustal rocks in different subduction setups such as the Himalayan continental subduction, the Mediterranean realm (Calabria&#8211;Apennine and Aegean belts) and Western Alps.</p>
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