Mechanisms of magma emplacement in the upper crust

Abstract

The mechanisms that control magma emplacement in the upper crust are characterised by brittle failure of the host rock and subject to the influence of magma. This thesis is a compilation of several case studies that address different aspects of magma emplacement by brittle mechanisms.Chapter 3 outlines the results of a field study of the Njardvik Sill in the Tertiary Dyrfjöll Volcano in Northeast Iceland. This small basaltic sill provides insight into the mode of its emplacement along the interface between two mechanically contrasting materials, its feeding relationships with several inclined sheets, as well as the circumstances of its growth by episodic magma supply. Once initiated, the Njardvik Sill furthermore influenced the stress field in its surroundings that resulted in the diversion of subsequently injected inclined sheets into units of the sill. The results of detailed field analyses served as the basis for a simple numerical model of the mechanical conditions for the emplacement of the first sill unit. The study of the Njardvik Sill thus provides insight into the mechanism of sill emplacement particularly with regards to feeding relations, mechanical conditions for sill initiation, the influence of the magma-supply rate, and the mechanical effect of a sill on its surroundings. Chapter 4 documents a series of scaled analogue experiments on the kinematics of ring faulting above a deflating magma reservoir processed using digital image correlation and strain-analysis techniques. The results show a sequential process of ring faulting characterised by initiation, propagation, and interaction of several sets of ring faults. The strain analysis gives detailed insight into the mode of initiation, propagation, and final deactivation of individual ring faults and displacement transfer between successive sets of ring faults. Based on the experimental results, an enhanced understanding of the process of underground ring faulting that preceded the caldera collapse in Miyakejima Volcano, Japan, in 2000 and documented by seismic monitoring can be gained. Chapter 5 describes the results of a detailed structural study of the emplacement of the Slaufrudalur Pluton, Southeast Iceland. As the largest granitic intrusion exposed in Iceland, the Slaufrudalur Pluton is exposed in three dimensions in five glacial valleys, which allowed a high-precision GPS mapping of its margins. Based on the GPS mapping, the three-dimensional shape of the pluton could be reconstructed. Chapter 5.1 deals with this new technique and discusses which implications can be derived from this approach about the mode of emplacement of a pluton. Chapter 5.2 outlines the results of a detailed field study of the contact between the Slaufrudalur Pluton and its basaltic host rocks in addition to the pluton s inner structure to deduce the mechanisms that controlled its emplacement. From this, it is demonstrated that the exposed 8 to 10 km3 of the pluton were emplaced by a combination of cauldron subsidence and magmatic stoping. However, the derived emplacement model shows that the mechanism of cauldron subsidence was modified by the structure of the host rock and regional tectonic forces. In addition, detailed insight into the mechanism of magmatic stoping could be gained by a study of the roof structures of the Slaufrudalur Pluton. Furthermore, using the three dimensional model of the pluton s shape and the knowledge of the roof structure, it was possible to balance the volumetric contribution of individual emplacement mechanisms.The electronic appendix (attached as CD) includes movies of experiments in Chapter 4 and a movie of the three dimensional model in Chapter 5.1, as well as the already published version of Chapter 3.