iPlex Lunch spring-2015

Mountains form where the Earth’s plates collide; during this upheaval rocks are deformed by massive forces. The rates and timescales over which these deformational processes occur are determined from tiny accessory minerals that record geological time through radioactive decay. However, there remain major unresolved challenges in linking the dates yielded from these accessory phases to specific deformation events and discerning the effects of deformation on the isotopic and elemental tracers in these phases. The Himalayan orogen represents the ideal natural laboratory to decode the record of the deformational processes encrypted in the rocks. In the eastern region of Sikkim, a unique series of ‘time windows’ are exposed by doming of a major ductile fault, revealing the inner workings of one of the major mountain-building structures that accommodated the India-Asia collision.

Here, we investigate this region, using combined laser ablation (split-stream) U-Pb and REE analysis of deformed monazite along with EBSD imaging and Pressure-Temperature (P-T) phase equilibria modelling to (1) link accessory phase ‘age’ to ‘metamorphic stage’ and (2) to quantify the influence of deformation on monazite (re)crystallisation mechanisms and its subsequent effect on the crystallographic structure, ages and trace-element distribution in individual grains. These data provide links between ages and specific deformation events, thus helping further our understanding of the role of dynamic recrystallisation in producing age variation within and between crystals in a deformed rock.

This study provides new insight into how deformation is accommodated along major thrust faults during mountain building and demonstrates the importance of fully integrating the pressure-temperature-time-deformation history of accessory phases to better interpret the meaningfulness of ages yielded from deformed rocks and thus understand the deformational history of the cores of evolving mountain belts.