Constraints on the collision and the pre-collision tectonic configuration between India and Asia from detrital geochronology, thermochronology, and geochemistry studies in the lower Indus basin, Pakistan

Highlights

• The paper focuses on the collision and pre-collision tectonic configuration.

• We studied late Cretaceous–Pleistocene sediments on the lower Indus Basin.

• Detrital zircon U–Pb & FT analyses captured the change in post-Paleocene sediments.

• Nd–Sr isotopes reveal clear input of Asian detritus to the Indian plate by 50 Ma.

• The terminal India–Asia collision was accomplished by 50 Ma.

Abstract

Knowledge of the timing of India–Asia collision is a fundamental prerequisite for understanding the evolution of the Himalayan–Tibetan orogen and its role in global climate, oceanic chemistry, and ecological evolution. Despite much active research, the basic pre-collision tectonic configuration and the timing of terminal India–Asia suturing remain debated. For example, debates regarding when and how the intervening Kohistan–Ladakh arc was sutured with India and Asia still remain elusive; some models propose the arc collided with Asia at about 100 Ma, with India–Asia collision at ca. 55 Ma, whilst a newer model proposed the arc's collision with India at 50 Ma and subsequently with Asia at 40 Ma. Another example is the recent proposition that an oceanic Greater India Basin separated the Tethyan Himalaya microcontinent from the remaining Indian plate until $20\text{–}25\text{Ma}$ with the consumption of this oceanic basin marking the final collision at this time. These controversies relate to whether the commonly documented 50 Ma contact represents the terminal India–Asia suturing or the amalgamation between various arcs or microcontinents with India or Asia. Here we present an integrated provenance study of geochronology, thermochronology, and geochemistry on the late Cretaceous–Pleistocene sediments from the lower Indus basin on the Indian plate. The detrital zircon U–Pb and fission track data show a reversal in sediment source from a pure Indian signature to increasing inputs from the suture zone and the Asian plate between the middle Paleocene and early Oligocene. The Nd and Sr isotopes narrow down this change to 50 Ma by revealing input of Asian detritus and the establishment of a Nd & Sr isotopic pattern similar to the present-day Indus Fan by 50 Ma, with no significant variations up section, contrary to what might be expected if later major collisions had occurred. Our isotopic data indicate that Greater India was occupied by a fluvial-deltaic system, analogous to the present-day Indus and named as the Paleo-Indus, which has been transporting Asian detritus southward across the suture zone and Kohistan–Ladakh arc since 50 Ma, suggesting no other ocean basins intervened between India and Asia after this time in this region. Our data require that in the west the India–Asia collision were accomplished by ∼50 Ma.

Introduction

The timing of India–Asia collision is an important factor in determining the size of Greater India and the post-collision crustal shortening budget. It is therefore essential information to our understanding of the growth history of the Himalayas and Tibetan Plateau (e.g. Guillot et al., 2003; Van Hinsbergen et al., 2011, 2012) and thus crucial to our understanding of the associated changes in paleogeography, paleoclimatology, and ocean geochemistry (Ruddiman and Kutzbach, 1991). The past three decades have witnessed great advances in understanding the India–Asia collision (Fig. 1) (see a review in Wang et al., 2014), for example from sedimentary records (e.g. Beck et al., 1995, Cai et al., 2011; DeCelles et al., 2004, DeCelles et al., 2014; Garzanti et al., 1987, Hu et al., 2012, Najman et al., 2010, Orme et al., 2014, Roddaz et al., 2011, Rowley, 1996, Wang et al., 2011, Zhu et al., 2005), paleomagnetism studies (e.g. Dupont-Nivet et al., 2010, Huang et al., 2015, Klootwijk et al., 1992, Lippert et al., 2014, Meng et al., 2012, Patzelt et al., 1996, Yi et al., 2011), and ultra-high pressure metamorphism studies (e.g. Donaldson et al., 2013, Leech et al., 2005, O'brien et al., 2001). Most studies agree that a collision occurred at ca. $55\pm 10\text{Ma}$, but recently there has been debate as to whether this collision, traditionally taken to be the timing of India–Asia collision (Garzanti et al., 1987, Rowley, 1996), instead may represent the time of collision between India or Asia and various intraoceanic island arcs or microcontinents, with India–Asia collision occurring later (Bouilhol et al., 2013, S.D. Khan et al., 2009, van Hinsbergen et al., 2012). This is especially true in the western Himalayan region where the Indian and Asian plates are separated by the intervening intraoceanic Kohistan–Ladakh arc (Fig. 1B, Bouilhol et al., 2013, S.D. Khan et al., 2009). Based on hafnium isotopes and detrital zircon U–Pb ages, Bouilhol et al. (2013) proposed that a 50 Ma collisional event represents collision between the intraoceanic Kohistan–Ladakh arc and the Indian plate, rather than between Indian and Asian plates, with a subsequent collision between the assembled Indian plate & Kohistan–Ladakh arc and the Asian plate at ca. 40 Ma. In addition, the Greater India Basin model (van Hinsbergen et al., 2012) considers the collision at ca. 50 Ma to be the accretion of the Tethyan Himalayan microcontinent with the Asian plate, with the final collision with contiguous Indian continental crust not until ca. 25–20 Ma, after the consumption of an intervening oceanic Greater India Basin (van Hinsbergen et al., 2012). In the western part of the orogen, the debates as to the pre-collision tectonic configuration and thus when final continent-continent collision occurred are related to the uncertainties regarding the amalgamation history of Kohistan–Ladakh arc and the proposed existence of the Greater India Basin.

The existence of the Greater India Basin was challenged by DeCelles et al. (2014) who argued, in the Central Himalayan region, that Asian-derived material was deposited in the Tethyan Himalaya no later than 58.5 Ma and in the Himalayan foreland basin prior to 45 Ma, indicating that any intervening ocean must have closed by this time. Recognizing the potential pivotal role of the proposed Greater India Basin and Kohistan–Ladakh arc in understanding the India–Asia collision in the west, we conducted an integrated detrital provenance study in Sulaiman–Kirthar Province in the lower Indus basin (Fig. 1, Fig. 2) which lies on the western margin of the main body of Indian continental crust. The study includes analysis on fluvial-deltaic samples of Maastrichtian to Pleistocene age (Fig. 2) using detrital zircon U–Pb geochronology, double dated with fission track analysis where zircon crystals were of sufficient size, and Nd and Sr isotopes (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8). We compare our data with up-stream terrestrial (Roddaz et al., 2011) and down-stream marine archives (Clift and Blusztajn, 2005) (Fig. 2, Fig. 7) and determine the timing of the first arrival of Asian detritus onto the Indian plate. In doing so, we 1) place an upper limit on the time by which oceanic crust was eliminated between India and Asia and terminal collision had occurred in the west, and 2) constrain the pre-collisional tectonic configuration concerning both the accretion history of the Kohistan–Ladakh arc and the existence of the putative Greater India Basin.