Microfracture pattern compared to core-scale fractures in the borehole of Soultz-sous-Forets granite, Rhine graben, France

Microfractures appearing in thin section as fluid inclusion trails in quartz crystals were studied in four core samples of Soultz-sous-Forets granite. Their orientations in four series of three mutually perpendicular thin sections were estimated using a previously described apparent dip method and a new method involving measurements of strike and apparent dips. Three samples display three microfracture sets and one sample displays two sets. In all samples, one set is nearly vertical and strikes N-S. In two samples, one and two sets are nearly vertical and strike E-W, In two samples, two sets strike NW-SE: one is vertical, the other dips gently to the NE (or SW), Comparing microfracture and mesofractures sets in the same cores shows that (1) the N-S microfracture set is always dominant at both scales and (2) all other microfracture sets have no mesoscopic counterpart. The N-S microfracture sets could have been created during E-W extension of earliest Cenozoic age (Rhine Graben rifting). Differences between the two scales are explained by a sigma(1)/sigma(2) switching which occurred at the crystal scale and generated mutually perpendicular cracks. (C) 2000 Elsevier Science Ltd. All rights reserved.