Pulses, Sources and Timescales: Tectonic Implications of Melting During Mountain-Belt Formation

Some of the greatest mountain belts on Earth, including the Himalayas, formed as a consequence of the collision between two continents. During such major tectonic upheavals, rocks are buried, deformed, transformed into new rock types or melted, before being transported back to the surface. How long this overall process takes, and whether the affected rocks record evidence of continuous or abrupt episodes of change, is unknown. For instance, processes such as the formation of granitic melts were once considered to be 'instantaneous' on geological timescales, and occurred when fertile source rocks crossed a melting reaction at a specific point in pressure-temperature space. However recent research in the Himalayas has shown that the large granite plutons that pepper the highest structural levels of the orogen may form via a series of pulsed melting events1, and that their source regions may change over time2. As partially molten rocks are much weaker than non-molten rocks, their presence significantly affects how continental-collision mountain belts form.