Abstract
Cataclastic and mylonitic rocks exposed in the southwestern part of the Peninsula de Mejillones, northern Chile, are intruded at high angles of the foliation by younger, steeply inclined (±70°) basaltic dykes that resemble intru¬sive tension gashes with knife-edge contacts with the country rocks. These late dykes developed sigmoidaly-shaped, preferred orientation paths defined by oriented pyroxene phenocrysts that vary in size, aspect ratio, concentration and distribution across the width of an individual dyke. This banding has z and s asymmetries that indicate the sense of displacement of the country rock. The relative involvement of the coeval, internal and external stresses that caused the finite strains is estimated by using a partition analysis. The phenocryst location and size distribution are related to the internal magma flow velocity (um) stress component, whereas the sigmoid banding is linked to the external tectonic wall displacement velocity (±u). Dyke wall sliding with or against the magma flow induced the asymmetric shear strain distribution. The measured strain and displacements are analyzed using the deformation model of viscous laminar flow confined between two parallel plates moving parallel to each other with opposed motion. The shear stresses related to magma intrusion and frictional dyke-wall shear are quantified on the basis of magma flow displacements, cooling times and the temperature dependent viscosity of basalts in the linear rheology range. At the estimated depth where the intrusion and deformation occurred, the state of stress was close to being hydrostatic. This conclusion is in agreement with established models of active-collapsing volcanic centres, where bulk permeability is accommodated by means of a mesh of interconnected dykes and active faults. This interactivity tends to re-equilibrate, locally and transiently, any excess differential stress and redistributes excess magmatic pressures to create a uniform hydrostatic stress regime.
Highlights
Dyke intrusion is one of the most efficient mechanisms for transporting magma from deep reservoirs to shallow levels in the crust
The dyke swarm system exposed within the Peninsula de Mejillones intrudes mylonite and cataclasite of a foliated basement
These crystals indicate a rotational sense of movement compatible with that of conjugate normal faults with z and s shape-preferred orientation fabrics (SPOs) asymmetries shown in figure 1
Summary
Dyke intrusion is one of the most efficient mechanisms for transporting magma from deep reservoirs to shallow levels in the crust. Regardless of when the shear fractures were formed, any magma injected along them may cause reactivation and be subject to shearing along the fracture walls This departure from an ‘Andersonian’ state of stress (Anderson, 1951) will typically give rise to dykes with an en-echelon orientation (Pollard, 1987), offsets and curved geometries (Delaney and Pollard; 1981), lateral drag folds, (Rickwood, 1990; Smith, 1987; Baer, 1995), shape-preferred orientation fabrics (SPOs) developed by the re-orientation of rigid particles immersed in a viscous fluid (Arbaret et al, 1996), crystal tiling (Den Tex, 1969; Blumenfeld and Bouchez, 1988) and magma gashes. This shearing has resulted in the formation of internal asymmetric sigmoidal SPO (Figs. 1 and 2), and the coeval internal magmatic flow differentiations that can account for size and concentration variations of phenocrysts across the dyke width (Fig. 3)
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