Geology & Geophysics Editors' Vox

Competing Models of Mountain Formation Reconciled

The author of a prize-winning paper published in Geochemistry, Geophysics, Geosystems describes new insights into crustal mechanics and the formation of the Himalaya.


Andy Parsons was recently awarded the 2017 Ramsay Medal by the Geological Society of London for his research article published in Geochemistry, Geophysics, Geosystems [Parsons et al., 2016]. The Ramsay Medal is an annual international award for the best publication in the field of tectonics and structural geology during the last year from a postgraduate or recent postgraduate. His study tested datasets against different models of mountain formation to explain the evolution of the Himalaya, resolving some long-standing arguments between advocates of competing models. Andy answers some questions about research in this field.

How did the Himalayan mountain chain form?

50 million years ago, tectonic plate motion caused India to collide with Asia. This movement has continued ever since, resulting in formation of the world’s largest mountain range; the Himalaya. Understanding the geological processes responsible for the formation of mountain belts is challenging and, for many years, researchers have been debating two seemingly incompatible ideas for the formation of the Himalaya: channel flow and duplex formation.

The channel flow model proposes that the Himalaya formed by southwards horizontal flow of a weak mid-crustal layer. Flow was driven by the weight of the overlying thickened crust, similar to the squeezing of a tube of toothpaste. On the other hand, duplex formation involves vertical stacking of multiple layers of the crust in a similar fashion to a wedge of snow piling upward as it is pushed by a plow. This model requires strong and rigid crust with deformation occurring along planes of weakness that allow slices of the crust to slide over and on top of each other.

The mechanical differences between these models led many researchers to believe that they were mutually exclusive, and arguments for and against both models have been equally strong. More recent research in the Himalaya is beginning to show that these models are not mutually exclusive but rather operate at different times and positions during formation of the mountain belt. Key to this reconciliation is the recognition that the mechanical properties of the crust can vary in both space and time during the development of large, long-lived mountain belts.

What particular aspects of mountain formation are the focus of your research?

During continental collision, crustal thickening and erosion leads to uplift and exposure of rocks that were once located tens of kilometers below the surface. These rocks preserve a record of deformation that occurred at depth as the mountain belt was forming. My research focuses on unraveling this record of deformation at the macroscale in mountain-sides and cliff exposures and at the microscale in hand samples and crystal lattices. In particular, I look at the temperature and depth at which different rocks and minerals deform in order to determine how spatial and temporal variations in the mechanical properties of the crust control the formation of mountain belts.

How does your research contribute to a new understanding or synthesis?

The Himalaya comprises domains of rocks deformed at high temperatures deep within the crust, juxtaposed against domains rocks deformed at lower temperature and shallower depths. In our recent study we investigate rocks of both types, looking particularly at how the preserved record of deformation changed within and between these domains. By understanding how different minerals deform at different depths and temperature we were able to show how different parts of the Himalaya deformed under different conditions at different times. We found that deformation preserved in the high temperature domains matched the predictions of the channel flow model, whilst deformation preserved in the lower temperature domains matched the predictions of duplex formation.

We also saw evidence of lower temperature deformation overprinting higher temperature deformation. This led us to the understanding that as the mid-crustal rocks cooled, they strengthened and transformed from a weak crustal flow to a strong crustal block. Thus, the channel flow model was applicable to the early high temperature evolution of rocks at mid-crustal levels and duplex formation applied to the lower temperature evolution of rocks at upper-crustal levels. Importantly, the overprinting relationship between different types of deformation corresponds to changes in pressures and temperature felt by rocks from mid-crustal levels as they were uplifted to the surface.

What are the implications for better understanding mountain forming processes in other regions?

Despite its complexities, the Himalaya has a relatively simple geological history. As such, it provides a unique opportunity to determine the physical properties that control the formation of mountain belts and how these properties are interrelated. Such studies provide an invaluable modern day analogue for studying ancient, eroded, and now inactive mountain belts of which the geological record presents only a muddled and incomplete snapshot. Our study contributes to a growing understanding that the development of mountain belts is controlled primarily by its mechanical properties and that these properties change over space and time.

What are the major unsolved or unresolved questions in this field and where are additional data or modeling efforts needed?

Conceptual and numerical models are always an approximation of reality. Numerical models provide valuable insight into the boundary conditions that control tectonic process. In the broadest sense, such models have demonstrated that deformation of the lithosphere may be defined by a generalized mechanical stratigraphy typically corresponding to the upper and lower crust and lithospheric mantle. The ability of these models to reproduce observations reported from the geological record is testament to their validity and importance. Despite this, we know from geological observations that the mechanical properties of the lithosphere are highly variable and such variabilities have a first order control on the distribution of deformation from scales of microns to kilometers.

Our study and others also demonstrate how mechanical properties of rocks change drastically over time and space as they are pushed and pulled through different parts of a mountain chain. Bringing the capabilities of numerical simulation of lithospheric deformation closer to reality is one of the key efforts needed in the field of tectonics. At the same time, it is vital that researchers studying the geological record understand the boundary conditions that govern tectonic processes and how changes in these boundary conditions are reflected in the rocks that lie before them.

—Andy Parsons, Postdoctoral Researcher, Geological Survey of Canada; email: [email protected]