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Constraints on Late-Quaternary Fault Displacement and Tectonic Hazards in the Sacramento – San Joaquin Delta, Northern California, From Shallow Sediment Cores Across the Pittsburg – Kirby Hills Fault System

Description: 

Seismic sources and their associated hazards within the Sacramento-San Joaquin Delta region of north-central California (“the Delta”) are poorly characterized, due to slow slip rates that combine to accommodate no more than ~5 mm/yr of plate-boundary-parallel slip and that do not keep pace with the fluvial and tidal processes that dominate the geomorphology of the region. Local topographic highs in the western Delta suggest that some structures accommodate vertical displacement, but the age and rate of this deformation is not known. Tidal marshes preserve stratigraphy that can be used to constrain fault offsets and timing across active structures, providing insights to refine our understanding of Quaternary fault activity within the Delta.

The ~100-m-high Kirby Hills are at the core of a positive flower structure within the north-south trending Pittsburg – Kirby Hills Fault System (PKHFS), and are surrounded on three sides by tidal slough. The PKHFS originated in the Late Cretaceous as an extensional system associated with the Rio Vista forearc basin. In the modern tectonic regime, apparent reverse displacement of Eocene bedrock across the PKHFS suggests the possibility of active uplift or transpression across the structure, but the modern kinematics and rates of deformation on the PKHFS are not well constrained.

Shallow (<6 m depth) subsurface stratigraphy from 15 hand-auger cores in the Montezuma Slough tidal marsh, along strike of the PKHFS to the south of the Kirby Hills, provides insights into Quaternary deformation across the PKHFS. A peaty deposit overlies the PKHFS in the northern part of the marsh. This layer is thinner within the PKHFS (~1-2 m) than outside it (~3+ m), consistent with relative uplift within the fault zone and suggesting that the positive flower structure that forms the Kirby Hills is producing vertical displacement in the late Quaternary. Radiocarbon ages of subsurface peat samples within the fault zone are older than peat ages at comparable depths elsewhere in the Delta, suggesting relative uplift within the PKHFS during the Holocene.

Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Charles

Student Presenter: No

Invited Presentation: 

Authors