Records of continent-continent collisions in the Paleoproterozoic: exploring the effects of convergence obliquity and temperature on P-T-t paths

In the Paleoproterozoic era (2.5-1.6 Ga ago), the mode of the plate tectonics was shifting from Archean plume-lid tectonics to modern tectonics, with colder and deeper subduction due to a decreasing mantle potential temperature. Hence, the geodynamic regime was different as well; subduction was more episodic and characterised by frequent slab breakoffs, while weaker lithosphere resulted in wider and lower-relief orogens. Metamorphic rocks also recorded a fingerprint of these conditions, generally lacking evidence of UHP metamorphism and indicating higher temperatures in the lithosphere.

However, studying Paleoproterozoic orogens is challenging, as metamorphic rocks at the present-day erosional level often represent the middle-to-lower crustal orogenic interior. We aim to overcome this issue using pressure-temperature-time (P-T-t) paths extracted from generic, geodynamic continent-continent collision models and comparing them to P-T-t paths reconstructed from metamorphic minerals. The models are loosely based on Paleoproterozoic Svecofennian orogen, which formed the majority of the bedrock in southern Finland. It is well studied by number of geological and geophysical means, but physics-based geodynamical models are still lacking.

The models were run using the 3D thermo-mechanical, finite-element geodynamic modeling code DOUAR (Braun et al., 2008), which uses the PETSc version of the direct matrix equation solver MUMPS and the landscape evolution model FastScape. The work explored the effects of various continental collision obliquity angles, temperature conditions, and crustal thicknesses in a set of 13 different models. The spatial dimensions of the models are 1000×1000×70 km and crustal thickness values of 35 km and 45 km were used. In the Svecofennian orogeny, continent-continent collision was an event between colder and hotter continental blocks, which is implemented in the models by including a temperature difference of 100ºC along the model base at 70 km depth. Along this boundary, heat production is varied laterally to explore three different temperature scenarios. The convergence obliquity angle is also varied between 0º, 30º and 60º, while the subduction dip angle is constant at 45º.

With the thinner 35 km crust, the models do not show much difference in the dynamics between the temperature scenarios, as the crust is too thin to develop wide orogens, and eventual partitioning of strain due to oblique collision. Similarly, the P-T-t paths represent only straightforward retrograde metamorphism, due to simple model dynamics and the lack of large-scale internal orogenic heating. Increasing the crustal thickness to 45 km significantly affects the orogenic development. The Paleoproterozoic temperature scenario with a 45 km crust creates both wide and lower-relief orogens, also producing clear strain partitioning for the 60º obliquity angle. This difference in dynamics further results in more variation in the recorded P-T-t paths, suggesting potential for their use to explore Paleoproterozoic orogen dynamics. Ongoing work is exploring which stable mineral assemblages these P-T-t paths would correspond in metamorphic rocks.