Michael Krumbholz

The Slaufrudalur pluton, southeast Iceland—An example of shallow magma emplacement by coupled cauldron subsidence and magmatic stoping

The Tertiary Slaufrudalur pluton is the largest granitic intrusion exposed in Iceland. Five glacial valleys cut through the uppermost 900 m of the pluton, exposing spectacular sections through its roof, walls, and interior. The wall contacts are subvertical and sharp. Only in the northeast and southwest is the wall contact characterized by brittle faulting. The pluton roof is smooth at map scale, so that the overall cross-sectional shape of the pluton and its internal layering indicate emplacement by incremental floor sinking through cauldron subsidence. A pronounced elongation of the pluton, parallel to the trend of regional fissure swarms, and its angular shape in map view indicate strong tectonic control on horizontal ring-fault propagation, whereas faulted wall contacts represent step-over structures between the earlier-formed ring faults. On outcrop scale, the roof contact exhibits numerous steps, faults, and apophyses associated with conjugate fracture sets that are parallel and perpendicular to the strike of the length of the pluton. These structures were presumably formed by sequential inflation and deflation of the pluton during episodic magma intrusion and therefore are closely coupled to cauldron subsidence. As a result of roof fracturing and magma injection along the fractures, roof material is found partly or completely detached within the granite. The Slaufrudalur pluton therefore provides new insight into the coupling of the emplacement mechanisms of cauldron subsidence and magmatic stoping in the upper crust.

Three-dimensional geometry of concentric intrusive sheet swarms in the Geitafell and the Dyrfjoll volcanoes, eastern Iceland

Sheet intrusions (inclined sheets and dykes) in the deeply eroded volcanoes of Geitafell and Dyrfjoll, eastern Iceland, were studied at the surface to identify the location, depth, and size of their magmatic source(s). For this purpose, the measured orientations of inclined sheets were projected in three dimensions to produce models of sheet swarm geometries. For the Geitafell Volcano, the majority of sheets converge toward a common focal area with a diameter of at least 4 to 7 km, the location of which coincides with several gabbro bodies exposed at the surface. Assuming that these gabbros represent part of the magma chamber feeding the inclined sheets, a source depth of 2 to 4 km below the paleoland surface is derived. A second, younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm. The source of this second swarm is estimated to be located to the SE of the source of Swarm 1, below the present-day level of exposure and deeper than the source of the first swarm. For the Dyrfjoll Volcano, we show that the sheets can be divided into seven different subsets, three of which can be interpreted as swarms. The most prominent swarm, the Njardvik Sheet Swarm, converges toward a several kilometers wide area in the Njardvik Valley at a depth of 1.5 to 4 km below the paleoland surface. Two additional magmatic sources are postulated to be located to the northeast and southwest of the main source. Crosscutting relationships indicate contemporaneous, as well as successive activity of different magma chambers, but without a resolvable spatial trend. The Dyrfjoll Volcano is thus part of a complex volcanic cluster that extends far beyond the study area and can serve as fossil analog for nested volcanoes such as Askja, whereas in Geitafell, the sheet swarms seem to have originated from a single focus at one time, thus defining a single central volcanic complex, such as Krafla Volcano.

Predictability of properties of a fractured geothermal reservoir: the opportunities and limitations of an outcrop analogue study

Minimizing exploration risk in deep geothermics is of great economic importance. Especially, knowledge about temperature and permeability of the reservoir is essential. We test the potential of an outcrop analogue study to minimize uncertainties in prediction of the rock properties of a fractured reservoir in the Upper Rhine Graben. Our results show that although mineralogical composition, clay content, grain size, and fabric type are basically comparable, porosity and quartz cementation are not. Young’s modulus, as observed in the outcrop closest to the reservoir is about twice as high (~xa064xa0GPa) as observed in the reservoir (~xa034xa0GPa). Most importantly, however, the parameters that describe the fracture system, which are essential to predict reservoir permeability, differ significantly. While the outcrops are dominated by perpendicular fracture sets (striking NE–SW and NW–SE), two different conjugate fracture sets (striking NW–SE and N–S) occur in the reservoir. Fracture apertures, as reported from the FMI, are one order of magnitude wider than in the outcrop. We conclude that our outcrop analogue study fails to predict important properties of the reservoir (such as permeability and porosity). This must be in part because of the tectonically complex setting of the reservoir. We propose that analogue studies are important, but they must be treated with care when attempting to predict the controlling parameters of a fractured reservoir.

Emplacement of the Slaufrudalur Pluton, southeast Iceland, deduced from field observations and its three-dimensional shape .

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).

Determination of recent stress directions and faults in the Leinetal-Graben; Germany – first experiences with the new NEMR-method

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).