Wall Fracturing Versus Mechanical Instability as Competing Intrusion Mechanisms of Dikes: Insights From Laboratory Experiments

Abstract

Igneous dike intrusion is a primary crust‐forming process at the Earth's plate boundaries. Understanding its mechanism is thus crucially important in lithospheric studies. Our present article combines experimental and field observations to investigate the problem of dike emplacement from a mechanical perspective. Scaled laboratory experiments were conducted by injecting immiscible liquids into visco‐elastic and visco‐elasto‐plastic host materials at varying volumetric flow rates (VFR = 0.100 to 1.670 ml s−1). Another set of experiments used different injecting liquid–host combinations to set their viscosity ratios (η*) at low (105), moderate (106), and high (109) values. These two lines of experiments allow us to recognize three principal mechanisms of liquid pathways: tensile fracturing of the host, wave instability at the liquid‐host interface, and coupled fracturing‐wave instability process. The three mechanisms give rise to a wide variation in intrusion geometry, ranging from planar structures with elliptical outlines to typical bulbous geometry, with intermediate patterns characterized by in‐plane and off‐plane wavy interfaces with the host. This study uses the experimental data to constrain the VFR conditions that determine the fracturing versus wave instability‐controlled mechanisms. It is also shown from the experiments that η* can significantly influence the evolution of three‐dimensional intrusive geometry. The Chotonagpur Granite Gneiss Complex in eastern India is chosen as a field study area to validate our laboratory findings using a 2D shape analysis of the intrusive boundaries in terms of their fractal dimensions (D) and skewness‐kurtosis estimates.