Magmatic structures in the Tuolumne Intrusive Suite, California: a new model for the formation and deformation of ladder dikes

Abstract

Understanding the growth and differentiation of silicic magma chambers is a central issue in volcanology. We use field observations of meter-scale “ladder dikes” from the Tuolumne Intrusive Suite (TIS), California, USA, together with analogue experiments and scaling theory to demonstrate that prior to solidification, these features are deformed and broken by shearing motions in the magma chamber. From field observations, the majority of the ladder dikes are: (1) exposed perpendicular to gravity in a nearly horizontal slice through a solidified magma chamber, (2) located near a contact (specifically where they occur in large clusters) with either host rock or another intrusive unit in the TIS, and (3) oriented roughly perpendicular to that contact. The ladder dikes show evidence of breakup at the meter-scale as well as at the centimeter-scale (mafic-felsic banding). We hypothesize that the ladder dikes are buoyant plumes in a stirred magma chamber. In particular, their orientation and deformation are related to their rise (or descent) velocity, the stirring velocity within the chamber, and the rheological contrast between the plume and ambient material. The comparison between field data and laboratory experiments suggests that most of the ladder dikes deformed in a regime dominated by breakup from yielding under tension.