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K.R. McClay Department of Geology, Royal Holloway and Bedford New College, University of London, Egham, Surrey, England TW20 OEX. Since the first Thrust and Nappe Tectonics Conference in London in 1979 (McClay & Price 1981), and the Toulouse Meeting on Thrusting and Deformation in 1984 (Platt et al. 1986) there have been considerable advances in the study of thrust systems incorporating new field observations, conceptual models, mechanical models, analogue and numerical simulations, together with geophysical studies of thrust belts. Thrust Tectonics 1990 was an International Conference convened by the editor and held at Royal Holloway and Bedford New College, University of London, Egham Surrey, from April 4th until April 7th 1990. There were one hundred and seventy participants from all continents except South America. The conference was generously sponsored by Brasoil U.K. Limited, BP Exploration, Chevron U.K. Limited, Clyde Petroleum, Enterprise Oil, Esso Exploration and Production UK Limited, and Shell U.K. Exploration and Production. One hundred and five contributions were presented at the meeting, - seventy six oral presentations (together with poster displays) and an additional twenty nine posters without oral presentation (McClay 1990, conference abstract volume).
Fault-related folding is a widely-observed process in the upper crust associated with growth of mountain ranges within active organic systems. The Tien Shan is one example of an active mountain range where fault-related folding is shaping the landscape. Several intermontane basins within this mountain belt exhibit active faulting and folding including the Jumgal Basin, which is the focus of this study. There are two fault zones determined to be active in the Jumgal Basin including the intra-basin structure and a basin-bounding reverse fault that splays basinward from a principal range-bounding fault. Placing constraints on the type of fault-related deformation is difficult using surface bedrock geometry alone. Studying the geometry of actively deforming fluvial terraces, which give incremental and temporal snapshots of deformation is useful for determining the geometry and kinematics of an active fault. The Jumgal Basin located in the Kyrgyz Tien Shan has a prominent anticline that exhibits well-preserved deformed fluvial terraces within several water gaps along strike. These actively deforming terraces were surveyed using a global positioning system (GPS) unit to quantify the progressive deformation of terraces emplaced over the mapped Neogene and Quaternary stratigraphic units. At the eastern end of the 20-km long project area the abandoned terraces exhibit predominantly limb lengthening, indicative of fault-bend folding with a discrete synclinal axial surface bounding the well-preserved terrace backlimbs. In the central and western portions of the study area the abandoned fluvial terraces exhibit progressive steepening of the older terraces. Progressive limb rotation at Aral is indicative of slip above a broadly curved fault. The zone of fault-related folding broadens westward along strike from less than 400 m at Chaek to roughly 1800 m at Aral. Tilting of terraces increases eastward along strike from roughly 2° at Aral to roughly 18° at Chaek. Slip along the active faults was calculated using kinematic models of fault-bend folding, listric faulting, and emergent thrust-faulting, as appropriate. Uncertainties of the observed bedrock and deformed terrace geometry were also propagated into calculations of dip-slip. Numerical ages of abandoned terraces in the Jumgal basin were correlated using a combination surveyed terrace profiles and 14C ages, which are
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Here we investigate fault-propagation fold kinematics in North Canterbury, New Zealand, addressing questions of how kinematic model parameters can be constrained and different models distinguished and how marine terrace uplift rates reflect fold kinematics. Kinematic models are powerful tools in the study of fault-related folding, but they are subject to problems of non-uniqueness and uncertainty. The North Canterbury fold and thrust belt provides a location where actively growing basement-involved fault-propagation folds can be studied, where uplifted marine terraces provide critical information on fold growth rates, and where the results of kinematic models can inform understanding of deformation in a seismically active and tectonically complex region. We begin by developing methods to fit trishear kinematic models to data and to estimate model uncertainty using Markov chain Monte Carlo (MCMC) methods. We then use amino acid racemization, infrared stimulated luminescence, and radiocarbon dating to provide new age dates for marine terraces uplifted by folding and faulting in North Canterbury, where ages were poorly known before. Using the new ages, we calculate uplift rates for the marine terraces, which reveal significant temporal and spatial variations. We use two anticlines along the North Canterbury coast as examples to show that marine terraces can be used to constrain fault-propagation fold kinematic models, both by serving as originally horizontal surfaces to be restored and by facilitating comparison of uplift rates at different structural positions. These approaches allow us to distinguish between trishear and kink-band kinematic models and to constrain the values of trishear parameters, eliminating models that are consistent with the geologic evidence but not the terrace uplift. By incorporating terrace uplift into MCMC simulations, we are also able to provide estimates of fault slip rate and age of folding. Ages are consistent with previous estimates, while fault slip rates are likely somewhat higher than previously thought. Finally, we test models for fault-propagation folding in North Canterbury that incorporate listric faults, we consider the implications of recent earthquake sequences and of the reactivation of inherited normal faults for understanding fault geometry at depth, and we construct a regional cross section to estimate shortening across the North Canterbury fold and thrust belt. We find that models of rigid basement block rotation on listric faults, although often used to explain basement-involved folding, are not consistent with the style of faulting and folding seen in North Canterbury. Instead, we develop a model combining trishear with simple shear on steep listric faults, which serves to explain the regional characteristics of faulting and folding in North Canterbury. We also compare this model to the simpler fault geometries tested previously and consider the possibility that not all faults in North Canterbury fit the same model. Depth to detachment is poorly constrained by our kinematic models, but a mid-crustal detachment as proposed by previous authors is consistent with our results. Total shortening estimated from our regional cross section is consistent with the low end of estimates from the geodetic shortening rate across the fold belt and the expected age at which folding began.