Balanced Cross‐Sections and Numerical Modeling of the Lithospheric‐Scale Evolution of the Hindu Kush and Pamir

Abstract

AbstractThe role of deformation and removal of continental lithospheric mantle (CLM) during continent‐continent collision, while recognized as fundamental to the evolution of orogenic systems, often remains speculative as direct evidence for these processes is typically absent. Nevertheless, these poorly understood deep lithospheric processes are expected to produce first‐order controls on the style of crustal shortening, magmatism, and metamorphism in large‐scale orogens. The Hindu Kush and Pamir offer an excellent opportunity to constrain these deep processes, as high‐quality seismic images reveal oppositely dipping slabs under the Hindu Kush and Pamir, inferred to be subducting Indian and Asian lithospheres, respectively. Using two‐dimensional mantle‐scale geodynamical models, we show that these along‐strike differences can be explained by the nature of the indenting Indian plate; thinner fringe Indian crust beneath the Hindu Kush, and full‐thickness Indian crust beneath the Pamir. In the Hindu Kush models, steep subduction of India follows early delamination of the Asian lithospheric mantle, whereas in the Pamir models, Asian delamination is followed by flat‐slab underthrusting of Indian CLM and forced subduction of northern Asian terranes. These styles of mantle delamination and subduction have key implications for the ensuing styles of magmatism, metamorphism, and crustal‐scale evolution of these systems. In the Hindu Kush, subduction of fringe India leads to burial of Indian crust under Asia, limiting both Himalayan growth and deformation of the Tajik Basin. In contrast, in the Pamir, forced subduction of the Alai terrane leads to Alai crust underthrusting the Pamir, forming the Pamir orocline.