Evolution and characterization of fracture patterns: Insights from multi-scale analysis of the Buxa dolomite in the Siang Valley, Arunachal Lesser Himalayan fold-thrust belt

摘要

Fractures at different scales are an integral component of shallow crustal deformation associated with the progressive evolution of a fold-thrust belt (FTB). We characterize the evolution of fractures from the Buxa dolomite of the Main Boundary thrust (MBT) sheet in the frontal segment of the Arunachal Lesser Himalayan FTB. Regionally, the MBT sheet forms a fault-bend antiform. Based on unfolding of strata, early formed low- and moderate-angle fractures are interpreted to be pre folding layer-parallel shortening structures, while the dominant late-stage high-angle fractures are inferred to have formed synchronous to folding, tracking the progressive deformation of the orogenic wedge. The high-angle fracture set is the dominant one at both outcrop and microscopic scales, forming similar to 44% and similar to 45% of the total number of fractures, respectively. Microfracture analysis indicates that similar to 41% of the high-angle fractures, similar to 22% of the moderate-angle fractures and similar to 10% of the low-angle fractures form veins, respectively. Cumulative plots of fracture spacing values collected from outcrops and under the microscope reveal that this parameter is best described by power-law distributions that indicate scale-independence of fracture spacing. However, the coefficients of variation (Cv) of spacing of these high-angle fractures reflect scale-dependent clustering.