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The AAPG/Datapages Combined Publications Database
Montana Geological Society
1997 Bighorn Basin Symposium Guidebook
Pseudo Tear Faults at Red Lodge, MT: Clues to Late-stage Basin Understuffing and Tectonics of the Greater Beartooth Block
The Red Lodge Corner of the Beartooth Uplift is marked by two large faults cutting frontal thrust structures with apparent strike-slip displacements of 1/3 and 2 1/3 miles. These have long been interpreted as tear faults even though they cannot be traced for any significant distance into the range. Exposures of the western or Willow Creek Fault reveal a dominance of minor slickenline motions indicative of dip-slip normal fault displacement rather than the expectable strike-slip motion. Deeper geometry of the corner is constrained by the AMOCO #1 Piney Dell well in basement 1.4 miles behind the mountain front. This well passed from basement through the frontal thrust at a depth of 8400 feet (2100 feet below s.l..) into overturned and then right-side-up limbs of a major northwest-trending syncline. The overall corner pattern is that of a map view, keystone shaped wedge being uplifted along pivotal extension faults late in the range history at about the same time that other Beartooth frontal structures were being deformed by strike slip motion along either side of the corner. These include two large, late-stage, strike-slip disturbed zones, one dextral at Line Creek and the other sinistral, at Burnt Mtn., about 10 miles S and NW of the corner, respectively. A mechanism is proposed whereby enough structural relief was created by early stages of thrusting that ductile basement under the range was brought into juxtaposition with brittle basement under the adjacent basin. As the basin was understuffed NNE-SSW beneath the range, the corner was stretched and uplifted as a fault bounded flap while strike-slip motion was initiated in frontal structures on either side of the corner. The reverse problem of a failed attempt to understuff the range beneath the dense Stillwater Lopolith caused major change in range asymmetry along the main Beartooth N60W fault zone. Early NNE compression downwarped downwarped the paleo-axis of the Bighorn Basin and initiated thrusting along its peripheral upwarps as the early Beartooth thrust fault and the Gardiner - Cody disturbed zone bounding the greater block on the SW. Resistance of the Stillwater to uplift caused the range to upbuckle the edge of the complex as well as initiate a new phase of eastward directed escape tectonics. Eastward components of the block motion initiated the east flank of the Beartooth Block and the blind Line Creek thrust as well and the sinistral transpressional motion of the Mill Creek - Stillwater fault zone. On a broader scale the Nye-Bowler sinistral fault zone was established and the eastward pullaway of the main block created the Yellowstone Valley half-graben, separating the Gallatin Block as an independent range. The overall effect was to isolate the former axis of the basin as the Cooke City Sag Zone and leave the present Bighorn Basin axis to butt into the Beartooth Front in the vicinity of the Clarks Fork Canyon. The final phases of deformation involved basin understuffing to create volume problems beneath local parts of the range fronts and pop up corner flaps as at Red Lodge and isolated block corners such as that at the Clarks Fork Canyon. Similar late stage understuffing mechanisms coupled with basement anisotropy and cross-linking of areas of differential strain shortening probably operated to create structural details of many other ranges of the Middle Rockies.
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