ICTJA PhD presentation award 2018 - Angel Valverde (Poster Presentation)

Everyone is familiar with the popular theory of plate tectonics.  It establishes the bases of the movement of the plates over a partially fluid layer called asthenosphere at around 150 km depth.  These movements generate different boundaries between the various plates. At the converging boundaries, the two plates collide and, one subducts under the other. For several reasons, the subduction is related to the composition of each plate that strikes and-or the convergence velocity.  Usually, this generates mountain chains along the subduction zone and if these plates transport continents, we have a continental-continental collision scenario.  That happened in the Himalayan area or without going far from the Iberian Peninsula, in the chain of the Pyrenees.

modified from :http://geophile.net/Lessons/PlateTectonics/PlateTectonics2_06.html

However, whether an orogen is of a certain size, or have different geological structures depends on many factors encompassed in the heterogeneity of the materials that set up the lithospheric plates during the collision.

The heterogeneity of the plates has been studied in these geological processes of a continental collision for years to better understand the characteristics of the materials that generate a type of deformation or orogen style.  Several scientist have used numerical models for these studies with different computational codes that solve the physicochemical relationships between materials. Over the years, these models have been improving thanks to the progress of the technology and the calculation capacity of computers to solve complex problems and equations Also, the level of detail or resolution for models has been improved allowing understand better the mechanism under this tectonic process. On the other hand, in laboratories scientist try to model this processes scaling different materials. These are known as analogous models. Some analogous models have been used to understand the influence of the area where both plates collide and one begins to subduct under the other.

However, we have seen that not all deformation styles that occur in numerical models and analogue models have their reflection in reality, and studies are still being carried out to clarify the factor that most influences one evolution or another. For example, some researchers argue that the folding of the entire lithosphere is a normal response to tectonic compression. Although, other researches suggest that there are levels of detachment between the different layers where stress is transmitted over long distances generating differences in shortening between the lithospheric layers.

We will focus on a model of a continental collision without taking into account the role played by the temperature (mechanical models) to try to know what is the controlling factor plus a mode of deformation or another of the lithosphere.

We consider two deformation modes; the first is known as the double-divergent orogen, where the boundaries of the mountain range are the main zones where strain rate is localised, generating structures in V called pop -UPS, which rise the ground near the suture area. The second type of deformation is the crustal folding. In this case, folds appear in both sides of the suture zone that can be very far from the subduction zone. Those folds can generate mountain chains equispaced several kilometres.

First, with a reference model and compressing the lithosphere 100 km, we verified that the initial subduction angle and changing the upper crust density were not the main factors that controlled the mode of deformation. We employ  7 different models in our study. Our results show that the main factor controlling the style of deformation is the viscosity contrast between crustal layers. This viscosity contrast can be changed directly in the lower crust (LC) or with the plastic criteria changing the cohesion.

Models M1, M2 and M7 present double vergent style of deformation. In M3 to M6 predominates crustal foldinf style of deformation. In particular, when the is a contrast higher than 100 of magnitude, the lower crust acts as a takeoff level, allowing cortical folding. The subduction of the plate is favoured reaching greater depths. And the deformation is concentrated in the crust.

But what if there was already a detachment level between the lithospheric plates that collide? We have to take into account that these processes take millions of years and the uncertainties increment when we try to look too much in the past. So, rheologies or geological structures are factors that we are not secure at all how where they in the past. 

What happens if we employ the same setup as model 2, a double vergent type of orogen, and we include two detachment levels at different depths? We will generate two different crustal folds correlated to the depth. Depending on the dimensions and position of the detachment level found in the middle crust, we have different folds near the suture zone.

If we compare the B model with the structure that the Pyrenees have, we can see some similar characteristics such as the subduction of the crust, two main faults of double V type orogen and others generated by the tectonic compression.

These detachment levels can be old faults reactivated or differences between rheology that evolve in different ways depending on the stress or strain rates deformation that affect them.

Conclusions

·         Folding appears when viscosity contrast between crustal layers is > 10².

·         Decoupling the crust from mantle favours crustal folding with lower crust acting as a decollement, localising the strain rate. Upper mantle in the overriding lithosphere remains undeformed.

·         Strong coupling between crust and mantle favours subduction. Strain rate localises in the upper mantle of the overriding lithosphere favouring upper mantle folding.

·         Plasticity is the mechanism that most controls the crustal mode of deformation, localising the strain rate favouring the subduction process.

·         Not all the lower crust acts as decollement.  

·         Mean topography is higher in the pro lithosphere than in the retro lithosphere.

·         Upper crust folding is related to decollement depth.

·         Decollement size influences in orogen deformation. If detachment level overpasses the subduction zone a main retro shear appears, and pop-up structures are likely to happen.