TECTONIC HISTORY OF THE GREAT BOUNDARY FAULT, RAJASTHAN

Abstract

The Great Boundary Fault is a major tectonic lineament in the northwestern India thatruns close to the contact between the Vindhyan sedimentary rocks (ca. 1400-600 Ma) and the pre-Vindhyan rocks. Despite the fact that the reactivated nature of this fault is indicated in some of the published accounts, the existing opinions on its tectonic evolutionary historyand age are varied andconflicting. This thesis makes an attempt to understand the tectonic history of the Great Boundary Fault by: (i) deciphering the characteristic deformation style of the fault zone and the fault related deformation zone, (ii) analyzing geometry, kinematics and dynamics of the mesoscopic structures developed during its initiation and reactivation in multiple phases, and (iii) analyzing the syntectonic fluid inclusions. The study area consists of three critical sections of the fault around Chittaurgarh, where the Vindhyan sedimentary rocks occurring on the footwall side show well developed fault zones and fault related deformation zone, and the Berach granite (ca. 2500 Ma), occurring on the hangingwall side, does not bear any significant imprints of the fault related deformation. The Great Boundary Fault Zone contains different types of fault rocks, ductile shear zones, three successively developed folds groups (F/ to F3), and multiple sets of fractures and faults. The earliest structures are the mesoscopic scale ductile shear zones containing one or more sets of mylonite foliation. During the course of progressive ductile shearing, the mylonite foliation has been folded and refolded coaxially by the F} group folds. These ductile shear zones, along with the bedding surfaces, have been intensely folded into F2 folds, and the superposition of F2 folds over the Fj folds has resulted into common development of type-2 interference pattern in the Great Boundary Fault Zone. F3 folds are mild and their interference with F2 folds shows development of broad culminations-depressions and coaptation folds. Nature of the fault rocks within the Great Boundary Fault Zone is highly variable. Both, cataclasite and mylonite are commonly developed in the relatively competent lithounit caught up with in the fault zone. Fault breccia is relatively less common. The breccia/cataclasite and the mylonite have developed in alternate cycles, and the brittle deformation has acted as precursor to the ductile deformation, and the Great Boundary Fault Zone has, in general developed in a brittle-ductile transitional zone. It is concluded that the Great Boundary Fault zone has a characteristic deformation style, which is defined bydevelopment of ductile shear zones, F/ folds, type-2 interference patterns, and the cyclic development of cataclasite andmylonite in a brittle-ductile regime. The fault related deformation zone contains two phases of folds, but three groups of brittle faults and fractures that were developed during the three successive phases of reactivation on the Great Boundary Fault. The low angle thrust faults represent the signatures of the first phase of reactivation on the Great Boundary Fault. The second group ofbrittle structures comprise striated fault/fractures, brittle-ductile shear zones and en-echelon veins. This group of structures is inferred to have developed during the second phase of reactivation on the Great Boundary Fault in a major strike-slip regime. The third group of structures include thrust and fault related kink folds that were developed during the third phase of reactivation on the Great Boundary fault in a thrust regime. Paleostress analyses of the striated faults imply that the first andthe third phase of reactivation occurred in thrust regimes in plane deviatoric and axial extensional states of stress, respectively. It is due to a major dextral strike-slip movement during the second phase of reactivation that the N-S trending F2 folds in the fault related deformation zone assume a NNE to NE trend in the Great Boundary Fault Zone. The fluid inclusion analyses on en-echelon veins, occurring within strike-slip shear zone of second phase, reveal that syntectonic fluids were highly saline and dense Na-Ca-Cl brines of intraformational origin. Interpretation of fluid inclusion data together with the estimates of the overburden thickness imply that the strike-slip faulting on Great Boundary Fault occurred at 2 km depth, 53 MPa pressure and 160-202°C temperature. These trapping conditions point to an anomalously high paleogeothermal gradient during the reactivation on the Great Boundary Fault. In summary, amongst the four groups of successively developed mesoscopic structures, the earliest group of structures, i. e., the ductile shear zones and the Fj folds, are related to initiation of the Great Boundary Fault in a possible thrust regime with topto- the-SE sense of movement. The other three groups of structures, comprising successively developed brittle faults, are related to three successive events ofreactivation of the Great Boundary Fault.