Palaeomagnetic analysis of fold closure growth and volumetrics

Abstract

Abstract Fold closures (four-way dip closures) occur in several tectonic environments. These closures must attain lengths and widths of several kilometres before they can contain economically significant amounts of hydrocarbons. The growth kinematics of such large structures differ in some respects from those of geometrically similar, but smaller structures. Key aspects of the kinematics of large-scale fold structures are described, illustrated by new palaeomagnetic data from an example in the Salt Range of Pakistan. A numerical model describing the geometry of four-way dip closure is developed, relating the bulk volume to tangible geological parameters (limb dip, fold amplitude). An understanding of growth kinematics enables the application of a ‘fold test’, which provides a restoration of the palaeomagnetic data and host beds back to their attitudes at the time of magnetization. Comparison of the restored data with magnetic directions of known ages reveals the time of magnetization, so the geometry of the structure, including bulk volume at a known time, can be estimated. These data enable a curve describing the evolution of bulk volume through time to be calculated, which can be compared with the burial history of local source rocks — hence the trap volume available at the time of hydrocarbon generation can be estimated.