On inclined features in the Iceland Research Drilling Project core

Abstract

Precise dip and direction of dip measurements are reported for 148 dike contacts, 126 bedding features in volcaniclastics, and 30 planar, upper surfaces of secondary mineral infillings in vesicles in flows from a 1919‐m drill core from Reydarfjordur, eastern Iceland. The drill core extends from about 1.6 to 3.6 km beneath the original surface of the crustal section. Lava flow dips and directions of dips are reported for a 750‐m exposed section situated directly above and in continuity with the drill core section. While dips and within‐unit differences in dip direction appear to be accurately represented by the measurements, absolute directions are only known inaccurately. This is a consequence of the use of the paleomagnetic method of obtaining dip direction for which both high site latitude and tectonic rotation contribute to poorly constrained results. The main results of the study are the commonplace irregular form of dike contacts, such that the drill hole often entered and exited from the same side of a dike, evidence for the presence of only one population of minor intrusions and the ability to recognize where several sections of intrusives in the drill core belong to the same dike. The irregular form of the dikes prevents a reliable estimate of change of dilation with depth from being obtained. It is clear, however, that a sheeted dike complex does not occur in the upper 4 km of the crust in the area. Measurement of bedding features in the volcaniclastics shows them to be considerably dispersed, particularly in dip direction. Tectonic tilt in the crustal section increases from 11° at sea level to an extrapolated value of close to 17° at 4 km crustal depth. Corresponding lava deposition rates increase from 1620 to 2580 m/m.y., respectively. Tectonic tilting was probably complete by the time of deposition of the last components of the secondary mineralization event in the vesicles of lava flows. The optimum conditions for this type of tectonic investigation where measurements made on vertical drill core are used are high‐speed diamond drilling and original magnetization at middle to low but not equatorial latitudes.