Download Triassic and Upper Cretaceous Paleo

Geoforum 2009
Particle Technologies applied to Geo-Science
Triassic and Upper Cretaceous Paleo-Stress Paths in the Harz
Mountains and surroundings (Germany)
Dr. Hans Joachim Franzke, Dr. Rainer Müller 1
1 Technische Universität Clausthal, Institut für Geologie und Paläontologie,
Leibnizstr. 10, D-38678 Clausthal-Zellerfeld, Germany, [email protected]
In the Harz Mountains and its forelands, an almost continuous pile of sedimentary rocks of Paleozoicand Mesozoic ages is exposed, so that the paths of the tectonic evolution are recordable under
stratigraphic and radiometric age control. This paper focuses to the reconstruction of paleo-stress
fields which operated in Mesozoic times (stages 4 and 5). Variscan stress fields are yet under
investigation. The authors determined paleo-stress axis from 140 exposures in hydrothermal
mineralised fracture zones which are framed by Paleozoic wall rocks in the Harz Mountains and from
Mesozoic rocks in the northern border fault zone and from inner areas of the Subhercynian basin.
Time controls of fault activations are based on radiometric data from vein structures and on the
stratigraphic record in Mesozoic rocks of the forelands. To determine the paleo-stress axis in single
outcrops synchronously activated fault sets and other structural elements like pressure solution
seams, mineral fibre growth, mineralised feather jogs, drag folds from the flanks of faults and other
kinematical increments were used. The pelitic and evaporitic rock intercalations, mainly from the basis
of the cover rocks, played an important role as geohydraulical and geomechanical barriers for the
fracture tectonics in Mesozoic times. They hindered fluid transfers and provided a geomechanical
decoupling of the Variscan basement from the overlying cover rocks. Accelerated erosion of the cover
rocks from the uplifting Harz block in Upper Mesozoic times resulted in an increase of fluid pressures
and in a decrease of effective pressures below the barrier. This situation favoured an increase in the
stress sensitivity of the fault sets resulting in several fault activations which can be controlled by the
precipitation of numerous mineralising stages (mineral assemblages) in the vein structures of the Harz
Mountains. Since the Upper Triassic/Lower Jurassic, the minimal horizontal stress axis fluctuated
around SW-NE. Now, faults striking in the sector around NW-SE were predestined to be activated.
This mechanism generated fluid circulation cells and the precipitation of quartz-polysulfidic
mineralisations (stage 4.1 Upper Triassic/Jurassic) below the evaporitic/pelitic barrier of the cover
rocks. Mineral precipitation took place in course of pressure and temperature decrease caused by fluid
migration and fault movements.
Mixing of fluids from below and above the barrier and the precipitation of carbonate-fluorite-barite
veins was possible since the primarily more than 2 Km thick pile of cover rocks was partly eroded,
allowing fault planes to crosscut the barrier (stage 4.2). Reverse faulting observed as well in
hydrothermal vein structures as in the flexure zone in front of the Harz border fault indicates a younger
convergent tectonic impulse (stage 5), possibly caused by the collision of the Alpine-Carpathian
orogene, which converted the signs of the stress field but not their direction (Late Upper Cretaceous).
Keywords:
Paleo-Stress-Paths, Harz Mountains
Akademie für Geowissenschaften und Geotechnologien e.V.
Postfach 1114, 31519 Neustadt /Hannover, Germany
17th June 2009
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