Experimental investigation on granite emplacement during shortening

Abstract

We present analogue experiments performed to investigate the emplacement of granitic plutons in a shortening upper crust. The models were made of quartz–sand to simulate the brittle crust and a low-viscosity mixture of silicone and oleic acid to reproduce granitic magmas. Shortening of the models was obtained by a moving wall while a special injection apparatus allowed syn-kinematic magma intrusion from the base of the models. Experimental results show that: (1) space for intrusions is achieved during the movement along thrust faults and mostly coincides with low-pressure areas developed into the thrust–anticlines; (2) intrusion shapes are strictly dependent upon the competition between shortening rate (Sh) and injection rate (Inj). For high Sh/Inj values, plutons were elongated with the long axis parallel to the thrust surfaces; (3) magma migrates horizontally away from the injection point and towards the external sector in the direction of tectonic transport a longer distance for high Sh/Inj values; (4) syn-kinematic emplacement is also controlled by model thickness; an increase in this parameter results in an increase in the pluton plan-view aspect ratio. These results support that the final shape of orogenic plutons emplaced at shallow crustal levels may be strongly controlled by deformational features.

Introduction

Magma upraising during compression is generally believed to be unfavoured, because the least principal stress in these tectonic settings is vertical and consequently hydraulic fractures are expected to be horizontal, thus contrasting the upward transfer of melts (e.g., Galland et al., 2007a and references therein). Despite these theoretical suggestions, recent field studies have documented the presence of volcanoes and plutons coeval to horizontal shortening (e.g. Hutton, 1997, Kalakay et al., 2001, Lageson et al., 2001, Tibaldi, 2005, Musumeci et al., 2005, Tornos and Casquet, 2005, Galland et al., 2007b, Tibaldi, 2008). However, the modalities of magma migration and emplacement and their mutual relationships with structures in contractional settings still remain unclear. To the complexity of the process parameters such as emplacement dynamics (including rheology – e.g., mechanical layering – emplacement and cooling rates), deformation rates and fault kinematics concurred.

It is well documented that the final shape of plutons emplaced at rather shallow crustal levels is strongly controlled by deformation (e.g., Castro, 1987, Hutton, 1988, Pitcher, 1992, Vigneresse, 1995, Vigneresse, 1999, Vigneresse and Clemens, 2000). Field-based analysis of the relations between compressional deformation and magma emplacement suffers from the scarcity of documented natural examples of plutons genetically associated with pure contractional (dip-slip) structures as well as from the considerable variations of the spatial relationships between plutons and faults depending on the level of exposure (Benn et al., 2000). To implement field studies, analogue modelling has been proven to represent a valuable tool for investigating the dynamics of syn-tectonic magma emplacement. Analogue models have investigated this process during strike-slip (Román-Berdiel et al., 1997, Corti et al., 2005), extensional (Román-Berdiel, 1999, Román-Berdiel et al., 2000, Corti et al., 2003) and compressional (Benn et al., 1998, Benn et al., 2000, Galland et al., 2003, Musumeci et al., 2005, Galland et al., 2006, Galland et al., 2007a, Galland et al., 2007b) deformation. Previous experimental studies in compressional settings have investigated (1) the emplacement of high-viscosity magmas during oblique convergence, with results applicable to transpressional orogenic plutons (Benn et al., 1998, Benn et al., 2000), (2) the emplacement of high-viscosity magmas during pure convergence in specific boundary conditions reproducing local pluton geometries rather than general cases (Musumeci et al., 2005), and (3) the emplacement of low-viscosity magmas in pure compression, with results mostly applicable to compressional volcanoes (Galland et al., 2003, Galland et al., 2006, Galland et al., 2007a). The final geometry of these latter experiments depended on the shortening rate to injection rate kinematic ratio R: higher values of R, i.e. high shortening rate with respect to injection rate, led to smaller intrusions. In this paper we use analogue models to explore in general terms the process of emplacement of a high-viscosity magma in the brittle crust during pure compression, focusing on how the variations in the velocity of magma intrusion as well as in deformation rate and overburden thickness affect magma migration, emplacement and the final pluton shape at shallow crustal levels. The presented models were scaled for magma viscosity to better address the conditions of crystallizing magma emplaced at shallow (low-temperature) crustal levels (e.g. Benn et al., 1998, Román-Berdiel et al., 2000).

Our experimental set-up simulated the emplacement of magma with significantly higher viscosity than that employed in the models performed by Galland et al. (2007a). Consequently these two experimental approaches are complementary, allowing us to better understand the modalities and geometries of magma emplacement during thrusting in a wide range of natural viscosities.