LARAMIDE VS. NEOGENE STRUCTURAL STYLE ACROSS THE NORTH PARK REGION OF NORTHERN COLORADO

Detailed geologic mapping in the North Park region of northern Colorado has identified a wide variety of structural features not previously described. Gentle folding, fault-propagation folding, and high- to low-angle reverse faulting deforms Proterozoic crystalline rocks through syn-orogenic Paleocene to Eocene deposits. These northerly striking Laramide structures generate dramatic juxtaposition of basement-cored surface uplifts against deep, syn-orogenic basins. Post-Laramide fluvial and volcanic deposits (Oligocene to Miocene) within the study area are variably deformed by a younger set of structures. These high-angle faults are characterized by a lack of folding outside of fault zones, generating gently tilted block domains. North-striking, large offset normal faults intersect west-northwest-striking oblique-slip faults. Sinistral offset can be demonstrated along some structures, but kinematics are poorly constrained. En echelon, northwesterly to west-northwesterly striking faults generate tight transpressional fold geometries (flower structures) locally. In most locations, however, the faulting is best characterized as oblique normal, creating linear, sediment-filled half-grabens and transtensional rhomboid grabens. These younger faults truncate the older Laramide folds and faults in the North Park basin at high to low angles. Where the west-northwest-striking Neogene faults approach north-south-trending Laramide structures, the younger, through-going fault traces deflect locally towards north-northwesterly strikes, generating transpressional fault bends. In these locations, the younger faults strike at very low angles to the older Laramide faults, potentially reoccupying older fault surfaces for short distances. These interactions represent near surface deflections and adjustments to planar fault systems. The true geometry and kinematics of Laramide faulting cannot be properly understood without understanding younger Neogene faulting.