17 Structural Inheritance in the Rocky Mountain Region

The Rocky Mountain region preserves a complex tectonic history shaped by multiple contractional and extensional events from the Proterozoic to present. This session examines the role of structural inheritance in guiding deformation across superposed orogens.

Monday, 18 May 2026
4:10 PM - 5:30 PM
Alvarado F

The Potential for Pre-Laramide Fault Inheritance in the Livingston Fault, Boulder County, CO

Paper number: 17-3
Program: Technical Sessions
Time: 4:55 PM – 5:15 PM

Authors

Abstract

Reactivation of inherited structures is an important aspect of orogenic systems given the complexities of fault zone rheology, fluid flow in the crust, and interpretation of tectonic histories. In the Rocky Mountain region, previous studies have demonstrated that a number of faults active during the Laramide orogeny (70-50 Ma) were reactivated structures that date to the Ancestral Rockies or earlier in the Neoproterozoic. The Livingston fault is one of many northwest-striking faults in the central Front Range of Colorado known to have been active during the Laramide orogeny, but no in-depth mapping has been done since the 1990s. 

 

We focused on a segment of the Livingston fault that crops out in the Walker Ranch open space area and is entirely hosted in the 1.72 Ga Boulder Creek Granodiorite. A combination of fieldwork and lab analyses were used to map and assess brittle features attributed to the Laramide as well as potentially older features along the fault. Fieldwork included mapping the extent of brittle deformation, measuring brittle and ductile features, recording kinematic indicators, and collecting samples. Several samples were prepared into thin sections for petrographic observation and microstructural analysis. Multiple outcrops in the study area exhibit mylonitic fabric. Several of these mylonite zones occur directly along a northwest-striking segment of the Livingston fault and are oriented parallel to cataclasite and fault surfaces. Plastic deformation during mylonitization would have required temperatures of at least 300 ºC, while previous thermochronology studies have shown that the present level of exposure experienced temperatures of only about 100 ºC during the time of the Laramide orogeny. One nearby outcrop with greenschist-grade mineral assemblages, similar to some of the mylonites in the field area, yields a 1.4 Ga U-Pb titanite date. The results from this study, along with recent observations of Neoproterozoic Tava sandstone injectites in an adjacent subparallel fault, offer compelling evidence for pre-Laramide fault inheritance along the Livingston fault and suggest that other faults in the Front Range should be revisited in this context.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410529

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