9 Cenozoic Tectonics in Asia and North America: Linking Processes Across Scales (Posters)

This session invites research integrating geologic, geomorphic, and paleoclimate data to advance understanding on orogenesis, seismic hazards, and global change.

Monday, 18 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Active Deformation of the Needles Monocline, Western Arizona

Paper number: 9-6
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

This study investigates the deformation history of the Needles monocline, a southwest-verging, potentially fault-related fold within the Needles fault system in the Colorado River Extensional Corridor (CREC) of western Arizona. The Needles fault system is one of few major structures in the CREC, and while faults in the Needles fault system are known to be active, the recent deformation history of the associated monocline remains unconstrained. Here, we help fill this data gap by completing tectonogeomorphic mapping, sedimentologic characterization of alluvial pre-growth and growth strata, measurements of vertical separation across the fold, and trishear forward modeling to quantify the deformation timing, rates, and subsurface geometry of the fault underlying the Needles monocline. 

Our analyses provide four key results: first, field mapping and stratigraphic descriptions of pre-growth and growth strata suggest deformation initiated on the Needles monocline during the deposition of the Bullhead Alluvium (ca. 4.5-3.5 Ma). Second, trishear forward models successfully reproduce the observed fold geometry and stratigraphic thicknesses, suggesting that the monocline is related to a subsurface fault. The Needles monocline is best modeled as the result of ~60 m of slip along a southwest-dipping, blind, high-angle (~70°) normal fault. Third, tectonogeomorphic mapping and topographic profile analyses confirm systematic folding and progressive deformation of alluvium across the Needles monocline that has continued into at least the Late Pleistocene to Early Holocene. Vertical separations are systematically larger in Late Pliocene-Early Pleistocene alluvial units compared to younger Pleistocene units, indicating progressive fold growth. Estimated ages of these deformed units indicate vertical uplift rates of ~0.01-0.39 m/ka. Fourth, elevation differences between Pleistocene-Holocene transition fluvial terraces and the active wash are consistently greater upfold of the fold axis than downfold, indicating active channels are responding to ongoing deformation. 

These results suggest the Needles monocline is an active structure that deforms Pliocene to Holocene alluvial and fluvial deposits and provide new constraints on the deformation history of the Needles monocline. These parameters are essential for understanding seismic hazards posed to infrastructure and communities in the Arizona-California-Nevada border region.

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

doi: 10.1130/abs/2026RM-410600

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