EARLY DUCTILE THRUSTING AND LATE STAGE BRITTLE SHEARING IN THE SOUTH DELHI TERRANE OF ARAVALLI DELHI MOBILE BELT, NW INDIA; IMPLICATION FOR THE EXHUMATION OF GRANULITES

Granulites have been exhumed to the surface by several mechanisms. Ductile thrusts followed by late stage extension through brittle shearing is one of such process that leads to crustal thinning and unroofing of the granulites through erosion. The ductile and brittle shear zones have been studied in the granulites of the South Delhi Terrane in Aravalli-Delhi Mobile Belt, NW India. The Aravalli-Delhi Mobile Belt is a Proterozoic fold belt which extends in NE- SW direction. It has undergone multiple phases of deformation and metamorphism. The South Delhi Terrane occurs at the southern part of the belt. The tectonothermal events of the South Delhi Terrane have been classified as (1) 885.3±6 Ma to 826.6±8 Ma, F₁ folding, granulite metamorphism and emplacement of G₁ granites (2) 793±21 Ma to 780.1±9 Ma, F₂ folding, ductile thrusting, exhumation of granulites and emplacement of G₂ granites (3) 758±45 Ma to 750±21 Ma, F₃ folding, G₃ granite emplacement. Thrusts are oblique to reverse, the obliquity indicates transpression. The Rigid Grain Net analysis of porphyroclast suggests mean kinematic vorticity number (Wm) range from 0.5 to 0.8. This supports the transpressive nature of thrust. Monazite chemical dating has been performed over the ductile thrusts. Biotites and garnets enclose monazite grains. They reflect ages as old as 885 Ma suggesting their growth during peak metamorphism. Other ages are 705.8±12 Ma to 632±33 Ma. The younger ages have been correlated with development of brittle faults. Brittle faults occur in small to large scale in form of strike slip as well as normal slip faults. They strike NNW-SSE, ESE-WNW and NE-SW direction. Strike slip shears have produced ultracataclasites and pseudotachylites. Ultracataclasites are generally foliated and occur at low angle to mylonitic foliation of the dutile thrusts. The paleostress analysis of the brittle shears suggests that a maximum compression/minimum extension (σ₁) in NE-SW direction, intermediate compression (σ₂) vertical/subvertical and minimum compression/maximum extension (σ₃) in NW-SE direction have produced these fractures.

Hence it is suggested that the granulites have been exhumed in a combined processes ductile thrusting at 793 Ma followed by brittle shearing during 632 Ma have brought the granulites to the surface.