Factors controlling the style of back-arc extension
From Eötvös Loránd University, Budapest
Hosted by Ágnes Király
Both divergent and convergent plate boundaries had been studied extensively throughout the last five decades. Among a host of other aspects came the realization, that given the right circumstances, a broad extensional basin termed as back-arc basin can form behind a convergent plate boundary. The exact mechanisms shaping the style of back-arc extension and why it is often episodic is still debated. The absolute and relative velocities of the plates, the age (and hence buoyancy) of the subducting oceanic plate and the inherited rheological properties of the back-arc lithosphere are all thought to be key players shaping the extending overriding plate. Moreover, the presence of microcontinental terrains embedded in the subducting plate can further complicate the dynamics of the fore-arc - back-arc systems.
Here we use 2D mantle scale plane-strain thermo-mechanical model experiments to investigate how the lithospheric strength of the overriding plate and the accretion of small continental crustal terrains affect the dynamics of the subducting slab and the deformation of the back-arc region.
Our results demonstrate that there is more than one way to achieve back-arc extension and with the variation of just a few parameters can result in a wide range of deformation styles.
A rheologically weak back-arc mantle-lithosphere can result in either narrow, localized, or wide, distributed break-up depending on the evolving balance between the strength of the overriding plate and the build-up of the slab-pull force. In a wide rifted setting rocks from high-temperature metamorphic conditions can be exposed on the surface of the back-arc through core-complex style extension.
The accretion of microcontinental terrains can also trigger back-arc extension. Moreover, this setup can also produce narrower or wider features depending on the rate of convergence and the number of terrains involved. One accreting terrain, and a slow convergence velocity results in a rather localized back-arc rifting. With the sequential accretions of two microcontinental terrains, the style of extension is wider, but also episodic with short extension/contraction/quiescence sequences. These periods are connected to slab break-off events, variations in slab-pull due to varying slab thickness and the buoyancy force acting on the accreted terrains.
During the accretion and subsequent extension of these terrains some rock-samples can go through a phase of deep burial followed by rapid exhumation to the surface, producing p-T paths reminiscent to those produced in the Mediterranean back-arc regions.
Finally, the accreted terrain models also preserve/exhume oceanic crustal material from the same oceanic basin embedded in the back-arc region on several distinct locations, often hundreds of kms apart from each other. If we would observe such a surface record along a transect in nature it would be difficult to decipher the original distribution of terrains and oceanic domains without understanding the deeper structures and the enclosing geodynamic situation.