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.

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.…”

“…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].…”

Tectono-Magmatic Precursors for Porphyry Cu-(Mo-Au) Deposit Formation

“…5;Brown, 1994). The literature contains numerous examples of the localization of plutons, volcanoes, and related ore deposits in transpressional settings within arcs (e.g., Bussell, 1976;Aydin et al, 1990;Glazner, 1991;D'Lemos et al, 1992;Tikoff and Teyssier, 1992;Bellier and Sébrier, 1994;Tommasi et al, 1994;Tobisch and Cruden, 1995;Román-Berdiel et al, 1997;Acocella et al, 1999;Brown and Solar, 1999;Benn et al, 2000;García-Palomo et al, 2000;Adiyaman et al, 2001;Gleizes et al, 2001;Hildenbrand et al, 2001;Richards et al, 2001;Chernicoff et al, 2002). Additionally, numerous studies have shown how regional stress fields and resultant crustal strain influence the orientation and structure of volcanic-plutonic systems (Robson and Barr, 1964;Pollard and Muller, 1976;Nakamura, 1977;Weaver et al, 1987;Gudmundsson, 1988;Takada, 1994;Alaniz-Alvarez et al, 1998;Román-Berdiel, 1999;Mériaux and Lister, 2002).…”

Extensional and compressional strain in lava flows and the formation of fractures in surface crust

“…Silicone is commonly used to simulate fl uid fl ow during geological processes (e.g., Dixon, 1974;Brun and Merle, 1985;Merle, 1986;Dixon and Simpson, 1987;Vendeville et al, 1987;Davy and Cobbald, 1988;Dauteuil and Brun, 1993;van Wyk de Vries and Merle, 1998;Benn et al, 2000;Acocella and Mulugeta, 2002;Buisson and Merle, 2002). It has a high viscosity (~10 4 Pa s for the transparent silicone used in these experiments) and behaves as a true Newtonian fl uid.…”