in situ stress data from the jura mountains — new results and interpretation

In situ stress data from the Jura Mountains Ð new results and interpretation Arnfried Becker Institut fu Èr Geophysik, ETH±Ho Ènggerberg, CH-8093 Zu Èrich, Switzerland Introduction The Jura Mountains in Switzerland and eastern France comprise one of the best known foreland fold-and- thrust belts in the world. One important question remains: are they still deform- ing actively as a foreland fold-and- thrust belt, as in the late Miocene, or did the style of deformation change? Because of the rare occurence of young sediments, which would provide direct evidence of ongoing Jura decollement tectonics, the answer to this question is sought by enhancing existing stress data by the addition of new borehole slotter data as a basis for the evaluation of the tectonic development of the Jura. The objective of this publication is to present the borehole slotter test results (Fig. 1) and compare them with other data related to the recent tectonic stress field in the Jura Mountains. A detailed description of the borehole slotter, including the recoverable strain sensor, the test procedure and the theoretical background, was given by Bock (1993) and in a previous volume of this journal (Becker and Werner, 1994). Two developments are important: (i) no adhesives are needed for a secure con- tact between sensor and rock surface, and (ii) the borehole slotter can be used in pre-existing boreholes, allowing the separation of drilling from measuring activities. Both developments reduce the time required to complete the measurements at a test site to c.75% of that using common overcoring techniques. The strain sensor measures the tangential strain component at the surface of a borehole wall which arises after tangential stress relief has been completed. This is achieved by a blade which cuts a slot parallel to the borehole axis into the borehole wall close to the sensor. At least three such slots are needed in three independent directions to allow a 2D calculation of principal axes orientations and magnitudes (S H and S h ), using independently deter- mined elastic parameters. The use of the borehole slotter is restricted to those parts of boreholes which are above the groundwater table, i.e. at depths less than 30 m, and with a dia- meter between 95 and 103 mm. Stress provinces in the Jura Mountains Based on S H orientations from the in situ stress data, five `provinces' can be distinguished: (i) the East Province, restricted to the easternmost tip of the Jura Mountains (LaÈ geren chain) and its immediate vicinity. Here a characteristic E±W to NW±SE direction of S H is developed, also seen in the Black Forest (Wolter, 1988) and the Molasse Basin to the south (Gysel, 1975). This pattern indicates a possible homogeneous E±W to NW±SE orientated tectonic stress field for the whole region; (ii) The Cen- tral Province, which is the most important in the Jura Mountains, reaches from the northeastern Jura Mountains to the southern central Jura and north- wards to the limit of the Jura Moun- tains. This exhibits a general NNW± SSE to a NNE±SSW orientation of S H , with only some local disturbances south and south-east of Basle. This stress field seems to be developed also in the Molasse Basin to the south and the southern Upper Rhine Graben (Greiner and Illies, 1977; Cornet and Burlet, 1992). (iii) The Northwest Pro- vince with a E±W to NW±SE direction around BesancËon; (iv) the Southwest Province with a N±S to NE±SW orientation of S H in the external folded Jura, the Ile CreÂmieu and the easternmost Bresse Depression; and, finally, (v) the Southeast Province with a NW±SE orientation of S H along the southern in- ternal Jura Mountains. This stress province seems to be developed also further to the east and south-east, as indicated by fault-plane solutions related to the M L 5.3 earthquake in the sedimentary cover rocks close to An- necy (Thouvenot et al., 1998) and flat- jack measurements in a motorway tun- nel in the ChaõÃnes Subalpines (Grellet et al., 1993). In virtually all of the test sites of the five provinces, S H is com- pressive whereas S h is frequently ten- sional, especially in the eastern part of the Jura Mountains. Results and comparisons in key- areas Details of direct comparisons between borehole slotter and doorstopper overcoring tests in the Jura Mountains have * C 1999 Blackwell Science Ltd 9 ABSTRACT Recently completed in situ stress measurements using the borehole slotter at 33 new test sites within the Swiss and French Jura Mountains are presented. When combined with previously published stress data, these new measurements allow a detailed description of the contemporary state of stress in this fold-and-thrust belt and the adjacent foreland. The data now allow the recognition of five stress provinces, within which two general and different orientations of maximum horizontal stress S H can be distinguished: the Central and the Southwest Provinces, with a NNW±SSE to NNE±SSW orientation, and the East, Northwest and Southeast Provinces, orientated E±W to NW±SE. Each province has its own characteristic orientation. Measurements in three study areas are presented for comparison with already published acoustic anisotropy measurements, borehole breakout data, overcoring tests and triangulation measurements. The results show a generally good agreement with the new borehole slotter measurements. The tectonic implications of these stress data are discussed in terms of a large-scale stress reorientation of the NW±SE orientated contemporary Central European stress field in the foreland of the western Alpine arc. Terra Nova, 11, 9±15, 1999 Ahed Bhed Ched Dhed Ref marker Fig marker Table marker Ref end Ref start *Correspondence: Tel: +41/1-633-2633; Fax: +41/ 1-633-1065; E-mail: [email protected] Paper 215 Disc

in situ stress data from the jura mountains — new results and interpretation

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