Analogue scale models of pluton emplacement during transpression in brittle and ductile crust

Abstract

Analogue experiments were used to investigate pluton emplacement during transpression in a layered crust. Models consisted of (1) a silicone gum-PbO suspension as analogue magma, (2) a silicone gum-Pb suspension as a basal ductile layer, and (3) an overlying sand pack representing brittle crust. The models were transpressed at 3 mm/hr causing the extrusion of the analogue magma from a progressively closing slot, and its emplacement into the ductile layer. The thicknesses of the layers were critical in controlling the shapes of intrusions and the structures that developed in the brittle overburden. Thicker sand packs led to flattened, symmetrical laccolith-shaped intrusions and the nucleation of one oblique thrust in the sand pack above the extremity of the intrusion. Thinner sand packs led to thicker, asymmetrical laccolith-like intrusions with uplift of the overburden on an oblique thrust, and the formation of a shallow graben in the extrados of a bending fold. Reducing the thickness of the basal ductile layer resulted in a larger number of shear zones in the sand pack, and structural geometries approaching those produced in experiments involving only a brittle analogue crust and no ductile layer. Shear zones in the sand pack were localised by intrusions, and also played a key role in displacing analogue brittle crust to make space for intrusions. The results suggest that tectonic forces may play an important role in displacing blocks of crust during pluton emplacement in transpressional belts. They also suggest that pluton shapes, and the geometries and kinematics of emplacement-related shear zones and faults, may depend on the depth of emplacement. In nature, depending on the structural level exposed in the map plane, faults and shear zones that helped make space for emplacement may not appear to be spatially associated with the pluton.

Contrasted granite emplacement modes within an oblique crustal section: the Closepet Granite, South India

“…Shear zones may have accomodated space creation for the plutons are situated at some distance from them ( Fig. 1), which is compatible with the conclusions drawn from analogical modelling by Benn et al (1999). On the other hand, as pointed by Cruden (2000, and this volume), the extraction of large amounts of magmas from the lower crust can create vacant space in the upper levels, by downward displacement of the crust below the growing intrusions.…”

Magma emplacement in a thrust ramp anticline: The Gavorrano Granite (northern Apennines, Italy)

“…The analog intrusion was characterized by an elongation parallel to the thrust trend and to the anticline axis, with length/ width aspect ratio of $1.7 (see inset in Figure 10). Similar results were obtained by Benn et al [2000] in analog models of magma emplacement during transpression. Such an elongation testifies to a lateral magma migration from the injection point along the strike of the major thrust systems.…”

“…[64] Notably, model results suggest that, due to the structural complexity, the spatial relationships between emplacement-related structures and the intrusion vary according to the level of exposure [Benn et al, 2000]. For instance, the forethrusts and back thrusts accomplishing space creation for the pluton are expected not to be preserved if erosion acted sufficiently to expose the deepest portions of granites at the surface.…”

“…Starting from the geometrical scaling ratio and the reduction factors of density (r* = r mod /r nat $ 0.5) and gravity (g* = g mod /g nat = 1), the corresponding model to nature ratio of stresses was of the order of $5 Â 10 À6 . The scaling ratio of viscosity was $6 Â 10 À15 implying a natural viscosity for the décollement of $5 Â 10 18 Pa s [e.g., Bonini and Sani, 2002] and scaled magma viscosity of $10 17 Pa s. Thus the silicone-Oleic Acid mixture was only able to simulate a low-viscosity crystal-rich magma [e.g., Benn et al, 1998Benn et al, , 2000. However, this mixture was still suitable to model a significant strength contrast between the low-viscosity melt fraction and the host rocks [Cruden et al, 1995].…”

“…This processes may alter the statistical associations between faults and plutons [e.g., Paterson and Schmidt, 1999] where a close spatial relationship between shortening-related structures and syntectonic intrusions might not be evident following significant erosion. This would further complicate the interpretation of emplacement processes based on geological and/or geophysical data [e.g., Benn et al, 2000].…”