Insights into the mechanism of rift and transform zones for resource exploration

Whether at a hotspot or not, any extensional plate boundary consists of rift segments, transform faults connecting these segments, and transcurrent faults, i.e., the extension of the transform faults beyond rift segments. Rift and transform zones act together during the lifetime of a basin. While the features of the rift-parallel normal faults are well known, those of the transform zones have received less attention, partly because few outcrops exist for detailed studies, and partly because their fracture sets are more subtle and thus often undetected on seismic, gravimetry, and magnetic data. For these reasons, the role of transform faulting is currently undervalued in rift models. Equally under-investigated is the instability and re-location of the rift systems (rift jumps) in time. Selective field-based results from Iceland, backed up by other rifts worldwide, illustrate the importance and complexity of the rift-transform combination.

Iceland lacks major sedimentary basins due its high position above sea level, but its tectono- magmatic processes, crustal structures and seismicity are abasis for modelling the rift and transform mechanisms. Two active rift and two transform segments exist at various stages of evolution, ranging in age from 4-6 Ma to present. Due to successive rift-jumps over the last 24 Ma., a series of extinct rift and transform segments also stretch from central Iceland, with an increasing age, towards West-Northwest.

The island is on the Iceland-Faroe-Greenland Ridge where NW dykes dominate. However, under a WNW spreading direction, Iceland undergoes rifting along NNE and NS normal faults and eruptive fissures. The associated central volcanoes are loci of high temperature geothermal fields, severe alteration, and earthquakes related to magma intrusions. The fracture pattern of the transform zones consists of Riedel shears (strike- and/or oblique-slips), and periodical earthquakes occur along individual faults that increase the fracture permeability but also cause landslides. Rift-parallel fractures represent 1/3 of young fracture populations where rift and transform segments are superimposed or at oblique-rifts. Initially, the fracture density is high, with some major and a great number of broken zones of fine short fractures, which coalesce in time. While eruptions occur along eruptive rift fissures and dykes, sills are frequent, and magma injects into both the rift normal faults and the strike- and oblique-slips of the transform zones. Fracture reactivation is also seen in their cumulative displacements, or when the young fractures of the active plate boundaries extend into rift shoulders or into the micro-plates where they connect to older faults and dykes.

Where rift and transform zones are combined, Riedel shears control the gravimetry, aeromagnetic response, and resistivity. The highly deformed crust is buried and the fracture reactivates upwards in the younger crust. When the rift system re-locates, the new spreading site undergoes the same fracturing, compartmentalisation and processes.

Rift-transform interactions are critical in controlling oceanic break-up and widening, rift propagation, basin configuration and, hence, petroleum systems. Observations in Iceland, where such interactions can be studied in detail onshore, can elucidate these processes and be applied to petroleum resource exploration in basins with lower data density.