TECTONO-STRATIGRAPHIC EVOLUTION OF THE UPPER PLATE OF THE ANACONDA METAMORPHIC CORE COMPLEX, SW MONTANA

Hanging wall collapse in the Anaconda Metamorphic Core Complex (AMCC) during Eocene times was accomplished by successive formation and abandonment of small half-grabens, each of which developed its own stratigraphy. Mapping within the hanging wall at 1:24,000 reveals fault-bounded packages that contain variable thicknesses of fluvial sand and gravel interlayered with volcanic tuffs and flows. Angular unconformities within these packages indicate as much as 90° of tilting – and local overturning -- between depositional units. Faults dip between 11° and 90° in all directions, and fault-slip lineations have a wide range of trends. This is in stark contrast with published data from the mylonitic detachment zone which are dominated by lineations plunging 10-15° towards about 110°.

Conclusions that can be drawn from map relationships in the hangingwall of the AMCC include; 1) extension, volcanism, and fluvial sedimentation occurred simultaneously during exhumation of the metamorphic core; 2) simple layer-cake stratigraphic interpretations cannot be applied because not all units are present in all blocks and most layers are probably wedge-shaped; 3) even though ductile extension within the metamorphic core might have been unidirectional, brittle extension was not; 4) extension in the hanging wall was accomplished by initiation of steep normal faults which were subsequently rotated to shallow dips and then overprinted by younger, steeper normal faults that accommodated continued extension. Though the lack of relative and absolute age data for each block hinders upper plate reconstruction, it appears that brittle extension began in the west, nearest the exposed detachment, and migrated eastward with time.