ACS PRF | ACS
All e-Annual Reports
41960-AC8
Illuminating the Evolution of Active Thrust-Fault Systems Using Deformed Geomorphic Markers
A quantitative understanding of thrust fault growth remains fundamental to our knowledge of how active collisional mountain belts evolve in response to ongoing deformation and crustal shortening. By controlling spatial patterns of rock uplift at the earth's surface, thrust fault systems exert a primary influence over landscape dynamics in areas undergoing active tectonic compression. Frequent, large, and damaging earthquakes that often accompany the growth of such structures underscore the societal importance of understanding thrust development, whereas the influence of active thrust systems on the evolution of hydrocarbon traps attests to their economic significance. Actual kinematic constraints on the growth of these structures, however, are surprisingly scarce.
We delineate patterns and processes of fault growth for a series of Quaternary thrusts in southern New Zealand through a focus on interactions with the geomorphic system. The bulk of this work centers on the Ostler fault zone, where a spectacular suite of deformed geomorphic makers, e.g. fluvial terraces and outwash surfaces, provides an ideal natural laboratory for examining the kinematics of active thrusting. Development of a new geometric model that relates spatial patterns of terrace deformation to slip along a listric, or curviplanar, thrust fault accounts for the progressive tilting of terrace treads and apparent backlimb rotation observed along this structure. Constraints on the model from topographic surveying, digital elevation data, ground-penetrating radar, and luminescence dating of terrace fills enable quantification of the rates and styles of hanging-wall deformation in the absence of seismic control.
Integration of survey measurements from >30 terrace profiles and >100 fault scarps from nine transverse drainages across Ostler fault also allows examination of along-strike deformation patterns and evaluation of fault interactions at the scale of individual scarps to the entire fault zone. Estimated slip rates are highest near the center of the fault array (1.3–1.9 mm/yr) with increased displacement-rate gradients in zones of fault segment overlap. Displacement profiles for each major segment of the Ostler fault zone exhibit remarkably similar spatial distributions of slip since ~100 ka, implying growth over this period at a nearly constant fault length without significant lateral propagation. As such, apparent along-strike patterns of northward drainage diversion along the Ostler fault represent the product of sustained displacement gradients, rather than fault lengthening, as is commonly inferred from such patterns. The punctuated nature of the climatic changes that control terrace formation and incision also contribute to this pattern of progressive wind-gap development.
Such well-characterized patterns of thrust growth also shed light on interactions between the fluvial system and surface deformation associated with active thrusts. Detailed surveying and stream-power modeling of formerly transverse rivers across growing folds suggest that dynamic channel narrowing represents the initial mechanism by which channels forced to cope with differential uplift bolster their erosive capability. This result contrasts with previous notions that channel slope serves as the dominant adjustable feature of rivers or that both channel slope and width adjust simultaneously. Using a unit stream-power model as a proxy for fluvial incision, we also demonstrate that observed limits on the magnitude of channel narrowing suggest that, above a threshold amount of differential uplift, continued incision does require steepening of the channel gradient. As such, we shed light on inherent linkages between these two classes of geomorphic responses and emphasize the need for explicit consideration of channel morphology in assessing river networks within actively deforming mountain belts.
Funding from the ACS Petroleum Research Fund has supported the graduate research of Colin Amos, who recently completed his Ph.D. at UCSB. In addition to financing his field research in New Zealand, this project has allowed him to draw upon his diverse interests and background in applying geomorphic techniques to structural and neotectonic problems, as well as to broader questions pertinent to landscape evolution in actively deforming mountain belts.
