Nadine Friese

Dike-induced reverse faulting in a graben

Normal-fault slip of the boundary faults of a graben is commonly attributed to dike-induced stresses. Here, however, we report for the first time clear field evidence of a large dike-induced reverse-fault slip on a fault associated with a volcano-tectonic graben. The measured reverse faulting of at least 5 m occurs in the Holocene rift zone of northern Iceland. The reverse slip has apparently occurred on an existing normal fault as a result of over pressure of a nearby 6–13-m-thick 8 ka feeder dike. A numerical model of the dike-fault interaction supports our interpretation. The results indicate that dikes, particularly potential feeder dikes, may cause large reverse slips on nearby normal faults. This conclusion should improve the general understanding of geodetic deformation during volcanic unrest periods and may help forecast dike-fed eruptions.

Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength

Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.

Brittle tectonics of the Thingvellir and Hengill volcanic systems, Southwest Iceland: field studies and numerical modeling

This paper focuses on field studies and numerical models of fracture development in the area of the Hengill Central Volcano and its northern fissure swarm containing the Thingvellir Graben, in Southwest Iceland. Apart from additional field data on normal faults, a new detailed map of the Holocene fractures in the Thingvellir Graben is presented and used as a basis for numerical models on normal fault development. Field observations and models yield four basic results. First, hyaloclastite mountains in the area (“soft mechanical inclusions”) tend to deflect or arrest propagating tension fractures and normal faults. Second, fractures with an en echelon arrangement and an overlapping configuration develop shear stress shadows and tend to prevent the linking up into larger fault segments. Third, fractures with an en echelon arrangement and underlapping configuration develop shear stress concentrations between nearby tips resulting in development of transverse shear faults (transfer fault). Fourth, colinear normal fault segments concentrate tensile stresses at their tips and encourage tip-to-tip growth into larger segments.

Multi-stage development of Cambrian sedimentary dykes in the Paleoproterozoic granites from the Västervik area (SE Sweden): evidence from macro and microfabrics

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).