Paleomagnetic constraint on the Age of the Shyok Suture Zone

The India-Eurasia collision is a key case study for understanding the influence of plate tectonic processes on Earth’s crust, atmosphere, hydrosphere and biosphere. However, the timing of the final India-Eurasia continental collision is debated due to significant uncertainty in the age of the collision between the Kohistan-Ladakh arc and Eurasia along the Shyok suture zone. Here we present paleomagnetic results that constrain the Karakoram terrane in northwest India to a paleolatitude of 19.9 ± 8.9 °N between 93 – 75 million years ago (Ma). Our results show that the Karakoram terrane was situated on the southern margin of Eurasia in the Late Cretaceous. Our results indicate that the Kohistan-Ladakh arc and Eurasian continent had not converged until < 61.6 Ma, placing a Paleocene older limit on the age of final closure of the Shyok suture zone. This suggests that the India-Eurasia collision in northwestern India likely occurred after the closure of the oceanic basin between the Kohistan-Ladakh arc and Eurasia. The Paleocene collision event affecting India that has been widely interpreted to represent the final India-Eurasia collision instead records the arc-continent collision between the Kohistan-Ladakh arc and the northern edge of India prior to the final India-Eurasia collision. The final India-Eurasia collision in northwest India most likely occurred after the closure of the oceanic basin between the Kohistan-Ladakh arc and Eurasia.

Methods
We collected 7–10 oriented core samples from 34 horizons distributed throughout the Saltoro Formation stratigraphy near Charasa for paleomagnetic analysis. Bedding orientations at each site were measured repeatedly over a 20–40 m along-strike, and the bedding orientation used for structural corrections is the mean of 5–15 individual measurements. Andesite bedding horizons were identified using chilled upper surfaces of beds. Core samples were collected in the field using a water-cooled electric hand drill and oriented using an ASC Industries Pomeroy orienting fixture. Nonmagnetic brass tools were used to extract cores from the outcrop to prevent damaging the remanent magnetization of samples during sample collection. Optical microscopy was used to identify magnetic carriers and textural relationships within the samples. One specimen was cut from each core sample using an ASC Scientific dual-blade rock saw. The natural remanent magnetization was measured on a 2G Enterprises Superconducting Rock Magnetometer equipped with an automated sample handler (Kirschvink et al., 2008) inside a mu-metal magnetically shielded room in the MIT Paleomagnetism Laboratory applying a <200-nT DC field. Specimens were subjected to stepwise alternating field (AF) and thermal demagnetization. AF steps were applied in increments of 2 mT up to 12 mT followed by thermal steps starting at 100 °C and increasing up to 600 °C or 680 °C in variable interval sizes from a maximum of 100 °C and minimum of 2 °C close to the Curie temperatures of the suspected principal magnetic carriers (magnetite: 580 °C, and hematite: 680 °C). Stable components of magnetization were isolated using principal component analysis (PCA)(Kirschvink, 1980) (see supplementary Table S2). Origin trending components were anchored to the origin and components were only fitted where the maximum angular deviation (MAD) was <15°.