Intracontinental Neotectonics: Case Studies from the Tian Shan Orogen and Kuqa Fold-Thrust Belt

This dissertation focuses on the kinematic properties of intracontinental deformation during short geologic time scales. Using three case studies this work characterizes active deformation at varying spatial scales within the continental interior of Eurasia. The far-field effect of the Cenozoic Indo-Asia collision is the driving force controlling deformation within the Eurasian continental interior. Active deformation across the intracontinental Tian Shan range challenges the plate tectonic model that proposes crustal deformation is concentrated along plate boundaries. This work further constrains the active kinematics of intracontinental motion that is necessary to understand the dynamics of the Eurasian intracontinental system. The two standing hypothesis that explain intracontinental deformation include the (A) discrete or (B) diffuse deformation models. While the discrete model suggests deformation occurs primarily along major faults between crustal blocks, the diffuse model suggests motion is distributed throughout the continental interior. On a smaller scale, I examine active deformation of sub-aerial salt bodies. Ephemeral subaerial salt exposure during the evolution of a salt structure can greatly impact the subsequent development and deformation of its tectonic setting. InSAR time series analysis and inspection of individual interferograms confirm that the majority of the salt bodies in western Kuqa are active, with significant InSAR observable displacements at 3 of 4 structures studied in the region. Decoupling between surface salt motion and climatic conditions suggests that the regional tectonic regime controls surface salt displacement rates. Lastly, on a more local scale, this work examines the characteristics of anthropogenic deformation. Unnatural, rapid rates of subsidence and/or uplift have extreme hazard potential because it may lead to infrastructure damage and increased flood potential. Surface subsidence resulting from hydrocarbon extraction has been widely observed across the globe. However, the occurrence of surface uplift caused by fluid injection has only recently been noted and is less documented. An unusual surface displacement distribution at the Dawanqi oil field in the Kuqa fold-thrust belt of northwestern China suggests that fluid extraction may not only cause widespread, irreversible subsidence but also facilitate local uplift

Constant slip‐rate on the Doruneh strike‐slip fault, Iran, averaged over Late Pleistocene, Holocene, and decadal timescales

Varying estimates of both present‐day strain accumulation and long‐term slip‐rate on the Doruneh left‐lateral strike‐slip fault, NE Iran, have led to suggestions that it exhibits large along‐strike and/or temporal changes in activity. In this paper, we make and compare estimates of slip‐rate measured using both geodesy and geomorphology, and spanning time periods ranging from decadal to 100 ka. To image the present‐day accumulation of strain we process seven years (2003‐2010) of data from six ENVISAT tracks covering the fault, with interferograms produced for 400 km‐long strips of data in order to image the long‐wavelength signals associated with interseismic strain accumulation across the locked fault. Our analysis shows that less than 4 mm/yr – and likely only 1‐3 mm/yr ‐ of slip accumulates across the fault. Using high‐resolution optical satellite imagery we make reconstructions of displacement across six alluvial fans whose surfaces cross the fault, in four separate river catchments. We determine the ages of these fans using infra‐red‐stimulated luminescence dating combined with U‐series dating of pedogenic carbonates. The six fans vary in age from ∼10‐100 kyr, and a regression line fitted to four of these yields a slip rate of 2.5 ± 0.3 mm/yr. We conclude that within the uncertainty of our measurements the slip‐rate has remained constant over the last ∼100 ka and is representative of the strain accumulation at the present‐day. The slip‐rate that we measure is consistent with the E‐W left‐lateral Doruneh fault accommodating N‐S right‐lateral faulting by 'bookshelf' faulting, with clockwise rotation about a vertical axis

Modelling co- and post-seismic displacements revealed by InSAR, and their implications for fault behaviour

The ultimate goal of seismology is to estimate the timing, magnitude and potential spatial extent of future seismic events along pre-existing faults. Based on the rate-state friction law, several theoretical physical earthquake models have been proposed towards this goal. Tectonic loading rate and frictional properties of faults are required in these models. Modern geodetic observations, e.g. GPS and InSAR, have provided unprecedented near-field observations following large earthquakes. In theory, according to the frictional rate and state asperity earthquake model, velocity-weakening regions holding seismic motions on faults should be separated with velocity-strengthening regions within which faults slip only aseismically. However, early afterslip following the 2011 MW 9.1 Tohoku-Oki earthquake revealed from GPS measurements was largely overlaid on the historical rupture zones, which challenged the velocity weakening asperity model. Therefore, the performance of the laboratory based friction law in the natural events needs further investigation, and the factors that may affect the estimates of slip models through geodetic modelling should also be discussed systematically. In this thesis, several moderate-strong events were investigated in order to address this important issue. The best-fit co- and post-seismic slip models following the 2009 MW 6.3 Haixi, Qinghai thrust-slip earthquake determined by InSAR deformation time-series suggest that the maximum afterslip is concentrated in the same area as the coseismic slip model, which is similar to the patterns observed in the 2011 Japan earthquake. In this case, complex geometric asperity may play a vital role in the coseismic nucleation and postseismic faulting. The major early afterslip after the 2011 MW 7.1 Van mainshock, which was revealed by one COSMO-SkyMed postseismic interferogram, is found just above the coseismic slip pattern. In this event, a postseismic modelling that did not allow slip across the coseismic asperity was also tested, suggesting that the slip model without slip in the asperities can explain the postseismic observations as well as the afterslip model without constraints on slip in the asperities. In the 2011 MW 9.1 Tohoku-Oki earthquake, a joint inversion with the GRACE coseismic gravity changes and inland coseismic GPS observations was conducted to re-investigate the coseismic slip model of the mainshock. A comparison of slip models from these different datasets suggests that significant variations of slip models can be observed, particularly the locations of the maximum slips. The joint slip model shows that the maximum slip of ~42 m appears near the seafloor surface close to the Japan Trench. Meanwhile, the accumulative afterslip patterns (slip >2 m) determined in previous studies appear in spatial correlation with the Coulomb stress changes generated using the joint slip model. As a strike-slip faulting event, the 2011 MW 6.8 Yushu earthquake was also investigated through co- and post-seismic modelling with more SAR data than was used in previous study. Best slip models suggest that the major afterslip is concentrated in shallow parts of the faults and between the two major coseismic slip patterns, suggesting that the performance of the rate and state frictional asperity model is appropriate in this event. Other postseismic physical mechanisms, pore-elastic rebound and viscoelastic relaxation have also been examined, which cannot significantly affect the estimate of the shallow afterslip model in this study. It is believed that the shallow afterslip predominantly controlled the postseismic behaviour after the mainshock in this case. In comparison to another 21 earthquakes investigated using geodetic data from other studies, complementary spatial extents between co- and post-seismic slip models can be identified. The 2009 MW 6.3 Qinghai earthquake is an exceptional case, in which the faulting behaviours might be dominated by the fault structure (e.g. fault bending). In conclusion, the major contributions from this thesis include: 1) the friction law gives a first order fit in most of natural events examined in this thesis; 2) geometric asperities may play an important role in faulting during earthquake cycles; 3) significant uncertainties in co- and post-seismic slip models can appreciably bias the estimation of fault frictional properties; 4) new insights derived from each earthquake regarding their fault structures and complex faulting behaviours have been observed in this thesis; and (5) a novel package for geodetic earthquake modelling has been developed, which can handle multiple datasets including InSAR, GPS and land/space based gravity changes

Active tectonics and palaeoseismicity of the Northern Tien Shan and Dzhungaria

This thesis focuses on the active tectonics and the palaeoseismicity around the Dzhungarian Basin. The study of surface ruptures is crucial to comprehending the earthquake occurrences of faults. I investigate geomorphic displacements along the boundary strike-slip Dzhungarian Fault using high-resolution drone and Pléiades satellite imagery. The results reveal possible single-event fault slip along the Dzhungarian Fault in the most recent earthquake. I suggest this earthquake is likely linked with a previously identified palaeo-earthquake rupture on the Lepsy Fault. With a joint rupture of the two faults, it could generate an earthquake with a magnitude up to Mw 8.4, which would be amongst the largest magnitude inferred for a continental earthquake. I further use Quaternary dating techniques and InSAR time-series analysis to determine the geological and geodetic slip rates of the Dzhungarian Fault. The results show that the northern Dzhungarian Fault has a long-term uplift rate of 0.6 ± 0.2 mm/yr, whilst the southern Dzhungarian Fault has geological and geodetic strike-slip rates consistent with a range of 2.1 – 4.7 mm/yr. I also re-investigate three historical earthquakes with magnitudes greater than Mw 7.0: the 1812 Nilke, the 1906 Manas and the 1944 Xinyuan Earthquakes in the Borohoro Shan. By re-analysing source parameters and integrating published data, seismological analysis results, and remote-sensing mapping, the study demonstrates the significance of both reverse and strike-slip faulting in the regional seismotectonics, which also indicates the deformation kinematics of the Borohoro Shan as being in a transpressional zone. I collate my results with those from the literature to propose updated earthquake scaling relationships of intra-continental earthquakes. Finally, this study suggests that the Dzhungarian Basin and its surrounding tectonic units are rotating anticlockwise to accommodate both the N-S crustal shortening and the left-lateral shearing within a large-scale zone from the SW Tien Shan to the Altay Mountains