Abstract: Neogene Tectonic Modification of Laramide Thrust Faults, Northern Sangre de Cristo Mountains, Colorado

John C. Gries

Multiple Laramide thrust faults involving Pennsylvanian sediments and a variety of Precambrian intrusives occur north and east of Mt. Blanca in the Northern Sangre de Cristo Mountains. These features are found along the eastern boundary of the Neogene Rio Grande Rift adjoining the present day San Luis Valley. Partial sections of the Minturn Formation both overlie and underlie Precambrian rocks in thrust fault contact. Neogene high angle normal faults have separated the thrust fault surfaces along trends parallel to Rift tectonic alignments. Slivers of the Pennsylvanian units are near vertical in several high angle fault zones. Minor mineralization and alteration are present in some of the normal faults but is not found along the Laramide thrust surfaces. Minor structures ithin the fault zones show evidence of a strike-slip component on some of the normal fault surfaces.

Detailed mapping of thrust fault surfaces reveal folding of the thrust plates penecontemporaneous with both Laramide thrusting and later Neogene deformation. High angle faults trending northwest and northeast may represent reactivation of Laramide, Pennsylvanian, or Precambrian structural features. Neogene shear may have been a factor along these faults. It is unclear if this shear is related to the 25 km eastward offset in the Sangre de Cristo Mountains near the south border of the Mt. Blanca study area.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada