Why are geologists poking around the challenging environs of southern Tibet trying to figure out the age of a sandstone formation there?
This seemingly niche pursuit, reported in a recent paper, tells a wider story of geological processes at work just before the rise of the Himalayan range. For those who want to know more about geological analysis, this study provides a good example of how geologists combine the results of field mapping, detailed rock descriptions and sophisticated laboratory methods to arrive at a richer understanding of earth processes.
Sedimentary rocks ranging in age from the Cambrian to the Eocene (540 million to 33 million years ago) form majestic mountain ranges spanning the northern borders of Pakistan, India and Nepal, and across into southern Tibet. These rocks were laid down in the ancient Tethys Ocean. The southern parts of these ranges are composed of sediments deposited in relatively shallower water, while the northern ranges contain sediments deposited in the abyssal ocean depths.
Geologists recognise a more fundamental distinction between them. The southern ranges represent sediments deposited on the Indian continental shelf made up of lighter continental crust. Sediments of the northern ranges were deposited on denser oceanic crust.
Granitic crust, which is richer in silica, aluminium, sodium and potassium, is lighter and forms thicker, elevated continents. Basaltic crust, which is richer in iron and magnesium and poorer in silica, is thinner and denser, and forms the oceanic basins. These two types of crust, together with the uppermost part of the mantle, form a rigid layer known as the lithosphere. It is about 100 km thick and is broken up into blocks, more famously called tectonic plates. Rigid plates ride on a more ductile part of the mantle called the asthenosphere.
In the Cretaceous Period, the Indian plate, which had been moving northwards since the breakup of Gondwanaland, was approaching the Asian continent. The southern edge of the Asian continent was lighter continental crust, whereas the leading part of the Indian plate was denser oceanic crust. As a result, in the zone where the the two plates converged, the denser Indian plate slid below the Asian plate, forming a subduction zone.
As the Indian lithosphere sunk deeper into the mantle, it heated up and released water trapped in sediments and hydrated oceanic crust. This water penetrated the overlying Asian plate, lowering the melting point of its rocks and triggering magma generation. This buoyant magma rose through the Asian continental crust. Some of it reached the surface, resulting in extensive volcanism. The rest solidified in the subsurface, forming giant bodies of granite known as batholiths.
Such terrains have the grandiose name of magmatic arcs. The town of Leh and the surrounding settlements in the Indian region of Ladakh are situated partly on a magmatic arc.
Two sedimentary basins since developed south of this arc. Immediately adjacent to the magmatic arc was the forearc basin. A deeper depression, known as the trench, formed further away on the Indian oceanic lithosphere, at the junction where the Indian plate had slid under the Asian plate. Both were receiving sediments derived from the erosion of the Asian continent.
During subduction, slices of the Indian plate were scraped off and thrust to the surface. Such fault-bounded piles of sediment and oceanic crust are called accretionary wedges, and they, along with a chain of oceanic volcanoes that formed to the west in the region between Ladakh and Kohistan, would have been the first island ranges formed in the Tethys Ocean between India and Asia. This process continues even today towards the south, in the Bay of Bengal. The Andaman and Nicobar islands are accretionary wedges formed by the subduction of the oceanic part of the Indian plate below the oceanic part of the South East Asian plate.
In the Tethyan domain in the north, subduction continued throughout the late Cretaceous (100-66 million years ago). Eventually, in the early Palaeocene Epoch (60 million years ago), the Indian oceanic lithosphere disappeared entirely. The Indian continental crust collided with the Asian continental crust, forming two distinct terrains.
The zone of tectonic contact between the two is known as the Indus-Tsangpo suture zone; to the east, in the Brahmaputra catchment, it is called the Yarlung-Zangbo suture zone. This suture consists of ophiolites, which are fault-bounded sheets of ocean crust and the uppermost mantle, along with mangled and crushed trench and forearc sediments. South of this suture zone are most continuous successions of sediments deformed into the northern Tethyan Himalayan terrain.
The schematic cross section below shows the subduction zone complex that existed during the Late Cretaceous between the Indian and Asian continents.
A recent study by Hanpu Fu and colleagues, published in the July 2018 issue of the journal Science China Earth Sciences, examines sediments of the Jiachala Formation exposed just south of the suture zone in the northern Tethyan ranges in detail. Their aim was to resolve the sediments’ age and the environment in which they were deposited. With this information, the scientists hoped to understand the sequence of tectonic events that led to their uplift during the earliest stages of Himalaya mountain building.
The Jiachala Formation consists of thin sandstone beds inter-fingering and alternating with mud-rich sediments. They contain flute casts, which are scours or grooves dug into soft sediment by fast-moving currents and later filled by sand. These sedimentological characters and the position of the Jiachala Formation to the south of the forearc basin sediments shows that they represent a deep sea fan, a cone-shaped sedimentary body deposited by currents laden with sand and mud mixtures. This fan formed in the deep trench, where the Indian lithosphere slid under Asia.
In the schematic above, the Jiachala Formation is marked (a), while (b) and (c) are similar sandstones deposited in the trench further to the west.
The mineral content of the sandstone reveals its source. The sandstone contains quartz, feldspar and volcanic and metamorphic rock fragments. This indicates that the sediments were derived from the erosion of granites and volcanic rocks of the magmatic arc to the north, as well as metamorphic rocks of the south Tibet continental crust, collectively called the Lhasa Terrain.
But how old is the Jiachala Formation? Earlier work had reported the presence of dinoflagellate cysts and pollen grains from the Palaeocene-Eocene age (60-55 million years ago). On the basis of these fossils, the Jiachala Formation had been interpreted to represent deposits formed after the continental crusts of India and Asia had collided, timed by independent analysis to have been around 58-60 million years ago.
However, in the current study, the scientists did not observe any of the previously reported fossils. A review of earlier reports showed that the published photographs of the fossils could not be used to confirm a Palaeocene-Eocene age for the sandstone. Instead, a method called detrital zircon geochronology was used to establish the timing of deposition. Zircons (i.e. zirconium silicate) crystallise out of magma. As they grow, they incorporate radiogenic uranium-238 atoms, which decay to lead-206. This radioactive clock, locked up in zircon crystals, is used to estimate the age of zircon as well as that of the igneous rocks that host them.
Zircons from the Jiachala Formation sandstones were sampled and analysed. The youngest crystals were found to be about 84 million years old. Magmatism had persisted in the Lhasa Terrain until as late as 45 million years ago. If the Jiachala Formation had been younger, as the fossils data suggested, it should have contained zircons derived from the erosion of igneous rocks formed during later times, say at 70 million years, 65 million years, 58 million years, and so on. Their absence strongly suggests that the Jiachala Formation is not much younger than 84 million years. Indeed, other sandstone formations deposited in this subduction zone complex contain zircons with ages as young as 59 million years.
Another type of geochemical analysis was carried out to zero in on the provenance of this deposit. Along with uranium-238, growing zircon crystals also incorporate lutetium-176, which decays to hafnium-176. The ratio of hafnium-176 to hafnium-177 (which also occurs in the zircon) is used to distinguish between magmas that originated via different processes and from chemically distinct sources. The specific composition of hafnium isotopes in the Jiachala Formation’s zircons indicated origins in the magmatic arc and the central parts of the Lhasa Terrain.
Such accurate age estimate and provenance fingerprinting gives us a broader understanding of how the subduction zone between India and Asia evolved. During the late Cretaceous Period, when the Jiachala Formation was being deposited, the forearc basin and the trench system were receiving sediments only from the Asian continent. The Jiachala Formation specifically contains sediments sourced from the magmatic arc and the central Lhasa Terrain. But other sandstone bodies in the suture zone point to different sources within the Lhasa Terrain – suggesting that along the length of the arc and subduction zone, different sediment routing systems were in operation.
At this time, the Indian continental crust was too far to the south for it to be a source for these northern depocenters. However, by 65 million years ago or so, the Indian oceanic lithosphere had nearly been consumed by subduction and the Indian continental crust had neared the trench. Soon after, India and Asia collided. Sandstones from the Indian continent deposited on the northern edge of the Indian continental crust were moved by faults and slammed against sandstones sourced from the Asian terrains. These dismembered bodies of rocks containing sandstone blocks from different sources are known as mélange.
Subsequent uplift caused the Tethys Ocean to drain away. In the Early Eocene (55-50 million years ago), deformed sediments of the long disappeared subduction zone and succeeding terrestrial basins were incorporated into the growing ranges of the Yarlung Zangbo Suture Zone and the northern Tethyan Himalaya.
Suture zones are an important archive of the sedimentation, deformation and uplift history of convergent plate margins. Crucial to unraveling this geological history is establishing an accurate geochronology of the rock sequence. This study resolves an old problem about the age of a Himalayan sedimentary deposit by using detrital zircon. The more reliable age estimate, along with supplementary information about the environment of deposition, enabled scientists to reconstruct the sequence of sedimentation and tectonic events that affected the subduction zone that existed below the Tethys Ocean in the Late Cretaceous. They were able to throw light on the palaeo-geography and sediment distribution processes taking place across the zone of plate convergence before India collided with Asia.
As a geologist, I have always taken delight in studies that take the minutiae and draw inferences about processes in operation on a much larger scale. My own work in carbonate sedimentology relied in large part on measuring isotopes of oxygen in calcite and then interpreting the history of sea level change and ancient groundwater aquifers. In this study too, Hanpu Fu and colleagues look at the small to understand the large. A story of subducting tectonic plates stored in natural radioactive clocks beating inside zircon crystals is something to wonder at.