Elliptical calderas in active tectonic settings: an experimental approach

Caldera volcanoes form due to collapse of a magma chamber roof into the underlying magma chamber. Many field, theoretical and experimental studies have postulated that calderas are delimited by reverse ring faults and are surrounded by peripheral concentric normal faults. In the simplest theoretical scenario, circular magma chambers produce circular calderas. Many calderas, however, are elliptical in shape, particularly those in extensional and compressive tectonic settings. Several factors may explain elliptical calderas. The first is the presence of an elliptical magma chamber, established by, for instance, preferential intrusion along pre-existing basement structures or differential spalling of the magma chamber walls. The second is the overlap (nesting) of several discrete calderas to form a single, larger elliptical structure. The third is asymmetric subsidence. The fourth is variable pre-collapse topography. A fifth possible factor is distortion of the caldera faults by the regional stress field during caldera formation. A sixth factor is the post-collapse distortion of the caldera structure due to continued regional deformation. To better understand relationships between caldera surface expression, reservoir geometry and regional tectonic stresses, we conducted scaled analogue experiments. These experiments examined the impact of regional stress and associated structures on calderas formed during evacuation of reservoirs (circular rubber balloons) of known dimensions and depths. The results show that, in principle, calderas produced in compression/extension experiments are elongated parallel to the direction of minimum horizontal compressive stress, despite the chamber beneath being circular in plan view. As a consequence, model ring fault orientation varied from steeply dipping where striking perpendicular to the minimum horizontal regional compressive stress, to shallower dips where striking parallel to the minimum horizontal regional compressive stress. This leads us to suggest that the influence of a regional stress field on caldera fault orientation during and/or after caldera formation may be significant in the development of elliptical calderas. In addition, such variation of caldera ring fault dip from steep to relatively shallow could influence location and behaviour of ring fissure eruptions

Pulses of carbon dioxide emissions from intracrustal faults following climatic warming

Carbon capture and geological storage represents a potential means of managing atmospheric carbon dioxide levels. Understanding the role of faults, as either barriers or conduits to the flow of carbon dioxide, is crucial for predicting the long-term integrity of geological storage sites. Of particular concern is the influence of geochemical reactions on the sealing behaviour of faults and the impact of seismicity and stress regime on fault stability. Here, we examine a 135,000-year palaeorecord of carbon dioxide leakage from a faulted, natural carbon dioxide reservoir in Utah. We assess the isotope and trace-element composition of U-Th-dated carbonate veins, deposited by carbon-dioxide-rich fluids. Temporal changes in vein geochemistry reveal pulses of carbon dioxide injection into the reservoir from deeper formations. Surface leakage rates increase by several orders of magnitude following these pulses. We show that each pulse occurs around 100-2,000 years after the onset of significant local climatic warming, at glacial to interglacial transitions. We suggest that carbon dioxide leakage rates increase as a result of fracture opening, potentially caused by changes in groundwater hydrology, the intermittent presence of a buoyant gas cap and postglacial crustal unloading of regions surrounding the fault