Interaction between magmatic and tectonic stresses during dyke intrusion.

Abstract

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 intrusive 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 (u m ) 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. RESUMEN. Interacción entre esfuerzos magmáticos y tectónicos durante la intrusión de diques. Las rocas cataclásticas y miloníticas expuestas en la parte suroeste de la Península de Mejillones del norte de Chile están intruidas por un sistema de diques basálticos de alta inclinación (±70°). Estos muestran contactos intrusivos netos, grietas de tensión e internamente una orientación sigmoidal definida por un alineamiento de fenocristales de piroxeno, los que decrecen en tamaño y concentración desde el centro hacia los bordes de los diques. A partir de la geometría de la orientación preferencial se establecen los sentidos de desplazamiento de las rocas de caja, los que corresponden a sistemas de cizalle conjugados, con asimetrías en s y z respectivamente. La participación relativa de los esfuerzos fue segregada en una componente interna o magmática y una externa o tectónica. La zonación y concentración de los fenocristales están relacionadas con el flujo magmático (u m ), mientras que la orientación sigmoidal preferente con los esfuerzos tec...

Structure and tectonic setting of the SE Sardinia mafic dyke swarm. Insights for the stress state during magma emplacement in the upper crust

“…This is because most dykes propagate as extension fractures that form perpendicular to the minimum principal compressive stress (σ3) and because their geometric features are useful to constrain the tectonics of magma emplacement (Babiker and Gudmundsson, 2004;Le Gall et al, 2005;Skarmeta, 2011;Ni et al, 2016). The regional stress field and tectonic regime play an important role in dyking, but other factors like magmatic pressure and the interaction with rock layering or with pre-existing fractures also have to be taken into account (Baer et al, 1994;Rubin, 1995;Rivalta et al 2015).…”

40Ar–39Ar geochronology and palaeostress analysis using lamprophyre dikes and quartz veins in the Sizhuang gold deposit: new implications for Early Cretaceous stress regime in the Jiaodong Peninsula, North China Craton

“…Lamprophyre dikes and hydrothermal quartz veins, usually showing tabular geometric features, provide vital information on the palaeostresses under which they formed (Mazzarini & Musumeci, 2008; Skarmeta, 2011; Martinez-Poza & Druguet, 2016). In particular, they have been considered to be normal to the regional least principal compressive stress ( ) direction (Babiker & Gudmundsson, 2004).…”

Simultaneous Free Flow and Forcefully Driven Movement of Magma in Lamprophyre Dykes as Indicated by Magnetic Anisotropy: Case Study from the Central Bohemian Dyke Swarm, Czech Republic

“…For example, there are dykes that originated through the almost free flow of magma infilling more or less open fractures in host rocks (close to the surface) as well as dykes that originated through a forceful magma intrusion of the host rocks with no open fractures (in deeper portions of the crust) (see [1]). The structural aspects of these processes have been investigated both from the theoretical and practical points of views (see, for example, [2,3] and references therein). These investigations used various indicators of magma flow, which are unfortunately scarce, and the anisotropy of magnetic susceptibility (AMS), which can be easily measured on almost all types of dyke rocks (e.g., [4][5][6][7][8][9][10][11][12][13][14]).…”