Abstract: Challenge for Material Balance in 3-D: A Fold Adapts to Varying Boundary Conditions Along Strike

Hans Laubscher

Preexisting, basement-rooted structures profoundly influence the development of frontal fold and thrust belts as has been verified by both digital and scale models. This influence, which may be of commercial interest in some thrust belts, can be studied in great detail in the Jurassic south of Basel. I have in the last few years re-mapped the area and analyzed some of its structures by means of closely spaced balanced sections.

The front of the Jurassic fold and thrust belt south of Basel (Switzerland) has been molded by the presence of a series of basement-rooted older structures that modified the geometry of the basal decollement plane in the Triassic evaporites. A series of balanced cross sections through the Homberg-Wisig pair of folds shows, from west to east, the following segments:

(1) At the western end two transfer zones of opposite sign initiate a west-east-trending pair of folds. A weak dextral transfer zone moves the western thrust front back to the south in the form of a series of very small folds that coalesce to the east to form a larger fold. The site of this transfer zone is a small, preexisting, north-south-running, west-dipping flexure. It interferes with an equally small, preexisting east-southeast-trending series of flexures, which became the site of a frontal triangle (back-thrust) fault-propagation fold on the east side of the transfer zone.

(2) Immediately east of the old north-south-trending flexure there are a series of old north-northeast-south-southwest-running normal faults. They were reactivated as a sinistral transfer zone for an out-of-sequence thrust. This transfer zone carried the thrust from within the Jurassic northward close to the frontal triangle. At its tip it first created an oblique triangle (underthrust) of its own. Thereafter, for a segment length of 1 km to the east, the front of the decollement thrust becomes an upright, tight fold. This fold ends against another old fault that became the site of a second sinistral transfer zone. It both enhanced the amount of shortening by adding further out-of-sequence thrusts and transformed the upright frontal fold into a ramp-and-flat thrust. This thrust moved o the north into an area that later was encroached by the frontal blind Wisig triangle thrust and therefore was subsequently folded--a real challenge for balanced kinematics. The thrust then continues for some kilometers to the east, and acquires an unexpected feature, inasmuch as in this segment other preexisting faults were hardly reactivated at all but cut by the thrust as if they had not existed. In the upper plate, the cut-off preexisting horst-and-graben tectonics of the lower plate may be recognized, topped by a middle Miocene unconformity.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers