Nancye H. Dawers, Tulane University
The aim of this ongoing study is to unravel the changes in fluvial channel geometry that arise from the evolution of topography associated with extensional (normal) faulting. Previous studies of rift basin evolution and the evolution of fault populations, in general, have shown that channel networks tend to flow down tilted topography between overlapping normal fault segments. Such channels offer opportunities to relate channel morphology to the detailed structural geology of the associated fault population. Thus the practical applications of this project are to utilize the scale and patterns of both the faults and the channels to predict sedimentary distribution pathways and deposits within normal faulted reservoirs.
The study area is located in northern Owens Valley, California. Owens Valley is a transtensional basin within the Eastern California Shear Zone, a zone of dextral shear created by differential motion of North America relative to the Pacific plate. North-central Owens Valley is defined, in part, by the Volcanic Tableland, which is the upper surface of the Bishop Tuff. The Bishop Tuff is a rhyolitic ash-flow tuff emplaced during an eruption at Long Valley Caldera ~760,000 years ago.
The most prominent geological features on the Tableland are a population of north-south trending normal faults and a fluvial channel network. Though the channels are not actively flowing today, the fluvial network initiated in the Late Pleistocene, essentially coevally with the fault population. This area offers a unique opportunity to investigate interaction between channel geometry and normal faulting. In addition, the scale of the study area is comparable to many hydrocarbon fields, with fault lengths up to several tens of kilometers; the typical scale of the paleo-channels would be essentially “subseismic” in industry 3d seismic volumes.
We mapped channels and faults in the field, utilizing a high-resolution DGPS with a post-processing vertical accuracy of 10 cm. Displacement patterns along fault strike are derived from the DGPS data points for the upthrown and down-dropped sides of the various normal faults. Based on the displacement patterns, the geometry of en echelon stepovers and whether linking faults are present in the fault stepovers, we define a spectrum of en echelon fault evolution – from unlinked segments to linked segments with a linking fault breaching the stepover zone. By identifying this spectrum of fault interaction and linkage, we are able to relate the morphology of incised bedrock channels to the tectonic processess that are driving the topographic changes.
Channel profile and channel slope plots are generated directly from field-surveyed data. Channel width to depth (W/D) ratio and channel bed shear stress plots are generated by HEC-RAS models. HEC-RAS is a one-dimensional flow model developed by the U.S. Army Corps of Engineers that allows for the input of real channel geometry (in the form of cross-sections). Because the Tableland channels are no longer active we utilize the flow model to extract channel width, depth and bed shear stress for a specified discharge. Once channel geometry is input a flow regime can be specified (i.e. discharge). After we specify a flow regime, we run the model and collect the width, depth and shear stress data.
Examples of our field data and results are illustrated in the figure shown on the summary “nugget” attached to this report. For the unlinked en cchelon fault scenario, we do not observe significant channel slope variations in the channels that are sourced from the fault overlap zone. The W/D ratio and shear stress trends also exhibit no significant transitions.
More strongly interacting fault segments, with partially breached zones of overlap, are associated with incised channels located in the fault stepover. These channels show significant variations in channel slope characterized by channel profile convexities. Areas of low W/D ratio and elevated shear stress show that the channels progressively incise more deeply through time, which indicates accelerating displacement rate. Anomalies in channel slope, shear stress and W/D ratio are all indicate active incision that has not progressed through the entire channel reaches.
In the case of fully linked overlapping segments, the channel profile data show little evidence of elevated channel slope, except for a convexity related to displacement on the linking fault. The channel profile appears well adjusted, however W/D ratio and shear values are highly variable. Together these observations suggest that channel slope may adjust more rapidly than width and
1. Displacement acceleration, related to fault interaction as fault segments grow longer, drives channel incision; the incision appears to begin well before fault linkage. We see evidence of fault interaction (steep displacement gradients and asymmetrical displacement profiles) at unlinked, but overlapping, fault segments. This scenario is coupled with higher channel slope, low W/D ratio, and elevated bed shear stress in channels that flow through the zone of overlapping normal faults. These trends suggest that the channels are incising in response to accelerating displacement rates. Importantly, this acceleration clearly occurs well before the fault linkage; this result has been predicted in some fault growth models, but has rarely been confirmed in field studies.
2. Channel slope and W/D ratio do not always respond together. We interpret this decoupling to mean that channel slope may respond more rapidly than width and depth, or that width and depth could respond independently of slope. However, the highly variable nature of W/D ratios in fault overlap zones with linking faults lends support to the idea that width and depth changes may take longer to erode through the system than channel slope anomalies.
3. Our observations of channel incision during fault linkage suggests that the development of sediment deposits in the channels and at the bases of the ramps (i.e. the overlap zone) may be favored during the latter stages of the fault linkage process.