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published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

I determined the source parameters of 53 moderate-sized earthquakes in the region of the Eastern Himalayan Syntaxis through the joint inversion of regional and teleseismic distance long-period body waves. The average rates of deformation are determined by summing the moment tensors from both recent and historic earthquakes. Strike-slip movement on the Sagaing fault terminates in the north (just south of the syntaxis), where thrusting (northeast convergence) and crustal thickening are predominant. Slip vectors for thrust mechanisms in the Eastern Himalaya in general are not orthogonal to the Himalayan mountain front but show an oblique component of slip. A combination of thrust and strike-slip faulting (Molnar and Deng, 1984) for the great 1950 Assam earthquake is consistent with the rates of underthrusting in the entire Himalaya and the rate of spreading in Tibet (assuming that a 1950-type earthquake recurs every 400 years). An estimated 4-21 mm/yr of right-lateral motion between southeast Asia and the Burma subplate is absorbed within the zone of distributed shear between the Sagaing and Red River faults. A component of westward motion (3-7 mm/yr) of the western boundary of the distributed shear zone may cause some of the late Cenozoic compression and folding in the northern Indoburman Ranges. Distributed shear and clockwise rotation of blocks is also occurring in Yunnan north of the Red River Fault. The inversion of 130 regional distancewaveforms for average crustal thickness and upper mantle Pn velocity indicates an increase in Pn velocity, coincident with increase in crustal thickness, of about 0.20 km/s beneath the Tibetan Plateau. Impulsive Pn arrivals from paths that cross the Tibetan Plateau can be modeled with a positive upper mantle velocity gradient, indicating an upper mantle lid approximately 100-km-thick beneath southern Tibet. This "shield-like" structure supports a model in which Indian continental lithosphere has underthrust Tibet. The crustal shortening within Tibet 8 mm/yr is thus viewed as an upper crustal phenomenon in which the faults do not penetrate the deep crust or upper Mantle. The forces generated by the thick crust in Tibet may partly cause the strike-slip faulting and east-west convergence in Sichuan and the movement of upper crustal blocks in Yunnan.

Rates of latest Quaternary slip obtained from stream terraces and glacial moraines displaced by faults of the Alpine shear system vary with space and time. Field measurements yield displacement values for faulted geomorphic surfaces, while the rate of thickening of weathering rinds and changes in soil properties, calibrated at sites of known age, yield age estimates. Precisions are 5-20% from weathering rinds and 15-50% from soil data. The oldest surfaces examined have ages of 15-20 ka and right-lateral fault displacements up to 400-600 m. Latest Quaternary lateral-slip rates are 20-45 mm/yr across the Alpine fault in the Southern Alps. To the northeast slip is distributed across a system of faults in Marlborough. The main faults of this shear system--the Wairau, Awatere, Clarence, Hope, and Porters Pass--have latest Quaternary rates of 5-10, 7-10, 7-9, 20-40, and 4-5 mm/yr respectively. Each fault has undergone a substantial decrease in lateral slip in the last 3-5 ka. Long-term rates of horizontal slip across the Australian-Pacific plate boundary--the Alpine shear system in most of the South Island--are 35-50 mm/yr parallel and 8-25 mm/yr normal. Sums of fault-slip rates exceed these plate motions for the early-middle Holocene, but late Holocene fault-slip rates are less than half the long-term average. Rates of geodetic strain and seismic moment release over the last 50-100 yr approximate the long-period rates in Marlborough but are only half in the Southern Alps. The best explanations of these variabilities are that the proportion of plate-boundary motion accommodated by fault slip changes, or that the rate of motion across the plate boundary varies, perhaps over 5 ka intervals. The first hypothesis is not consistent with the early Holocene rates exceeding the long-term average, but the second hypothesis implies that the last 50-100 yr is a period of renewed high tectonic activity. The second hypothesis is more consistent with the data, and the last 15-20 ka may be the time interval necessary to average out shorter, 5 ka episodic variations in plate-boundary motions.

This is an integrated study of the stratigraphy, deformation, magmatism, and metamorphism in the vicinity of the Ida and Palmenhorst Domes, an area in the southwestern Central Zone of the Damara Orogen, Namibia. The principal aim is to understand the timing of tectonic events through high-precision U-Pb dating of structurally constrained intrusions and anatectic rocks, and link these tectonic events across the Damara Orogen and Pan-African Orogeny. A secondary aim is to compare the Central Zone and Damara Orogen to other collisional orogens. The stratigraphy of the study area is similar to that noted elsewhere in the Central Zone, but the mapped distribution of lithologies differs slightly from previous work. Specifically, Damara Supergroup rocks have been found infolded with the Abbabis Complex, and the stratigraphic positions of certain units in have been locally reclassified. The mapped distribution of lithologies suggests a Type-2 fold interference pattern across the study area. This Type-2 fold interference is confirmed by structural analysis. A D2 deformation event formed strongly S- to SE-verging km-scale recumbent to shallow NW-dipping folds with smaller-scale parasitic folds. The long limbs of these folds are extended, and a number of shear zones are found on these extending limbs, as well as near the contact between the Abbabis Complex and the Damara Supergroup. NE-SW extension is associated with the late stages of D2, and forms a conjugate set of shear bands and a shallow NE-plunging mineral stretching lineation. This D2 event was overprinted by upright to steeply WNW-dipping km-scale D3 folds to form the domes in the study area. Mesoscale fold interference structures are rare, but D2 structures are shown to be consistently reoriented by D3 structures. D3 deformation does not have a strong vergence, and mesoscale D3 folds are rare. D2 and D3 were preceded by a D1 fabric forming event locally observed as rootless isoclinal intrafolial folds, and followed by brittle deformation. The Ida Dome is a fairly simple domal structure formed by the km-scale interference between a shallow NNW-dipping D2 anticline and an upright to steeply WNW-dipping D3 anticline. East of the Ida Dome, NE-trending D3 structures predominate, but are seen to overprint earlier D2 structures. The Palmenhorst Dome is a larger area where Damara Supergroup rocks have been infolded into the Abbabis Complex during D2 deformation. These isoclinal, N- to NW-dipping D2 folds have been refolded by upright D3 folds to form a Type-2 fold interference pattern. D2 structures along the southern margin of the Palmenhorst Dome dip steeply towards the south, in contrast to D2 structures elsewhere. This is interpreted to be the result of a lower-intensity km-scale D2 fold. The orogen-parallel extension and orogen-perpendicular recumbent folding that took place during D2 cannot be explained by previous structural models for the Central Zone and a new model is suggested where these structures form as the result of coeval irrotational NE-SW extension and S- to SE-verging simple shear during extensional collapse of the orogen. A number of intrusive rock types are found in the study area and have been dated using SHRIMP U-Pb. Amphibolite dykes have a chemical affinity to mafic rocks of the Goas Suite, and are suggested to be either pre-Damaran or early Damaran intrusives as they cut the gneisses of the Abbabis Complex, and are affected by D2. They have been dated at 2026.9 ± 2.3 Ma (zircon) or 557.2 ± 7.4 Ma (zircon) with metamorphic overgrowths in this sample giving 520 ± 6.9 Ma. Red, potassic granites emplaced near the contact with the Abbabis Complex and Damara Supergroup contain a D2 gneissic fabric and give ages of 536 ± 7.2 Ma (monazite), and zircons have lower intercept ages of 539 ± 17 Ma and upper intercept ages of 1013 ± 21 Ma. Grey granites are abundant in the study area, and form a continuum from dark grey granites (which are tonalitic to dioritic in composition and contain hornblende and abundant biotite) to light grey granites (which are leucogranitic and contain abundant K-feldspar and minor biotite). These grey granites show a fractionation trend from dark to light varieties, and cross-cutting relationships indicate that the lighter variety is younger than the darker variety. The grey granites show syn-D2 structural relationships and contain a fabric subparallel to the S2 fabric, and which is more pronounced in the darker varieties. They show similarities with granites described by earlier workers, and two samples have been dated at 519.1 ± 4.2 Ma and 520.4 ± 4.2 Ma (zircon). A variety of sheeted granites are found – quartz-feldspar-magnetite pegmatitic granites are associated with grey granites, occur axial-planar to F2 folds, and have metamict zircons which are dated at 530-525 Ma. Garnet (± cordierite) granites are leucocratic, have garnet poikiloblasts, are emplaced axial planar to F2 folds and are also folded and boudinaged by D2. They are associated with pelitic units in the Damara Supergroup and are dated at 520.3 ± 4.6 Ma (zircon) and 514.1 ± 3.1 Ma (monazite). Uraniferous leucogranites found are similar to those widely described in the Central Zone, but metamict zircons give imprecise ages of between 515 and 506 Ma. Pink pegmatitic leucogranites comprise pink perthitic feldspar and milky quartz, are emplaced into more brittle structures and gives an age of 434.4 ± 2 Ma (zircon). Almost all granites analysed appear to be crustal-melt granitoids, with the exception of the darker grey granites, which show a calc-alkaline affinity. No Salem-type granites are found in the study area. In addition, SHRIMP U-Pb analyses of zircons from three Abbabis Complex gneisses give ages of 2056 +11/-10 Ma, 2044 +32/-27 Ma and 2044 +17/-14 Ma, and titanites from an amphibolite sample give ages of 493.4 ± 6.4 Ma. Two anatectic leucosomes from D2 shear zones and shear bands give zircon ages of 511 ± 18 Ma and 508.4 ± 8.7 Ma in spite of high-U zircons. Lu-Hf data on zircons from an Abbabis Complex gneiss gives model ages of ca. 3 Ga, whilst similar data for a grey granite gives a model age of ca. 2 Ga. Zircons from the Abbabis Complex gneiss have variable O-isotopic values, whilst the grey granite gives O-isotopic values of ca. 7‰. These geochonological and isotopic data show that the Abbabis Complex is part of the Congo Craton, and that some amphibolites are pre-Damaran, whilst others may be related to the Goas Intrusive Suite, and represent a phase of early Damaran magmatism. In contrast to the chronology previously presented for the Central Zone, M1 in the study area appears to have occurred at 535-540 Ma, with M2 coeval with D2 deformation at 510-520 Ma. Elsewhere in the Central Zone, NW-verging D2 deformation is dated at 540-560 Ma, and the Central Zone appears to have a diachronous tectonometamorphic evolution along strike. It is suggested here that this represents the preservation of two separate tectonic events in the Central Zone at different crustal levels, one at 540-560 Ma and the other at 520-510 Ma. D3 deformation is suggested to have taken place at 508 Ma, immediately after D2 extension. The Central Zone began to cool following D2, and the 495 Ma titanite age reflects this cooling. Isotopic evidence from this and other studies shows that Damaran granitoids (with 1.5-2.2 Ga model ages) cannot be derived from the Abbabis Complex (with 3 Ga model ages) but must come from an alternative source, suggested here to be Kalahari Craton material subducted below the Congo Craton. Textural studies of a number of pelitic samples indicate syn-D2 low-pressure, high-temperature metamorphism. Differences in observed assemblages between various sample types are due to compositional differences, and samples appear to have reached similar conditions across the study area. Mineral compositional profiles show no prograde zoning, indicating mineral re-equilibration. Orthopyroxene is locally observed, suggesting lower-granulite conditions. This is confirmed by pseudosection modelling of a number of samples, which gives peak conditions of 750-850 °C and 4.5-5 kbar. This modelling shows lower-granulite facies conditions with higher temperatures than previous estimates based on mineral compositional geothermometers, which are affected by re-equilibration. These conditions are sufficiently high for fluid-absent biotite breakdown to form the voluminous anatectic leucosomes and granitoids in the southwestern Central Zone. Pseudosection modelling and phase relationships indicates a low-pressure (ca. 4 kbar) clockwise heating path, with slight decompression at the thermal peak. All metamorphism noted is 520-510 Ma M2 metamorphism, and no petrographic evidence exists for earlier 540-535 M1 metamorphism. This cryptic M1 is suggested to be related to the emplacement of the Goas Intrusive Suite and Salem-type granites early in the orogenic history, whilst M2 may be related to thermal relaxation following crustal thickening early in the orogenic history, but requires an additional heat source. The difference in ages for deformation and metamorphism between the study area and elsewhere in the lower grade portions of the Central Zone is suggested to be related to the preservation of different portions of the orogenic history in different areas. The results of this study together with previous work details a multi-stage evolution for the Central Zone involving subduction, continent-continent collision, crustal thickening, slab breakoff, magmatism, granulite-facies metamorphism and exhumation of the mid-crust. This multistage evolution explains the multiple ages for deformation and metamorphism in the Central Zone. NW-folding and thrusting documented in the Karibib area at 560-540 Ma is related to an early phase of crustal thickening owing to continent-continent collision following a brief period of subduction. Slab breakoff led to asthenospheric upwelling and heating of the lower crust, and produced the Goas Intrusive Suite and Salem-type granites, as well as providing heat for 540-535 Ma M1 metamorphism and the melting of the crust to produce anatectic red granites. SE-verging deformation, extension and granulite facies metamorphism recorded in this study is related to orogenic collapse following crustal thickening, and the heat source for low-P, high-T metamorphism may be highly radiogenic crust that was thickened , which is suggested to be either burial of crust enriched in heat-producing elements, or asthenospheric upwelling owing to delamination of the Congo Craton lithospheric mantle or asthenospheric upwelling owing to the position of the southwestern Central Zone on a major orocline. The events recorded for the Central Zone have been correlated across the entire Damara Orogen, and the timing of events can be correlated along strike into the Zambezi Belt. Events in the Kaoko Belt appear to predate those in the Damara Belt, which appears to also show a similar collisional timing to the Gariep Belt. It is therefore proposed that the Gariep and Damara Belts formed part of a younger orogenic episode to that which formed the Kaoko and Dom Feliciano orogenic belts. The Damara Belt shows similarities to both Alpine-style and Himalayan-style orogens. An evaluation is provided of a channel flow model for the Central Zone, but there are currently insufficient data for the Damara Belt to confirm or repudiate this model. Nonetheless, this study has identified a more complex tectonic history for the Central Zone than previously, with chronological and lithogeochemical evidence for two episodes of deformation and metamorphism that have been linked to the collisional history of the entire Damara Belt and have been correlated with events in other Pan-African belts.

The Australian-Pacific plate boundary is an uncomplicated structure along most of its length in the South Island, New Zealand. In South Westland, south of the Arawata River, however, several terranes converge onto the Alpine fault. Inherent anisotropies arising from the position of pre-existing fault structures, lithological contacts and rheological heterogeneities within these give rise to an atypically diffuse and complex zone, the overall geometry of which resembles a regional scale transpressive flower structure. The flower structure is a broad deformation zone 60 km in length extending approximately 7 km from the Alpine fault to its eastern limit, the Dun Mountain Ophiolite Belt. Integral parts of the structure are the Hollyford Fault System and the Livingstone Fault System. The area is characterised by an array of left-stepping, subparallel faults with an average 060� strike linked by 020� striking structures. All fault traces offset Quaternary features. Fractions of the total interplate slip are partitioned across the reactivated structures. Additionally, kinematic indicators reveal partitioning of strike-slip and oblique/dip-slip deformation across the related secondary fault zones. The behaviour of the plate boundary zone in South Westland is fundamentally controlled by reactivation of the Hollyford Fault System and the Livingstone Fault System which partition slip away from the Alpine fault. As a consequence, the eastward transferral of slip onto the curved geometry of the converging fault systems has ultimately created a left-stepping contractional regime, the equivalent of a restraining bend in the plate boundary zone. The competent Dun Mountain Ophiolite Belt controls the geometry and evolution of the reactivated structures. It also acts as an indenter and imposes additional boundary conditions adding to the shortening component in the region and the onset of complex transpressional strain patterns. The geometry and kinematics of the flower structure in the upper crust is mimicked in the ductile mid to lower crust. Upper greenschist facies mylonites reveal a complex fold pattern developed in response to contemporaneous non-coaxial and coaxial deformation. The folding formed during a continuation of deformation associated with mylonitisation at depths within the fault system. The fact that strain localisation and transpressive strain patterns in the brittle crust continue into the ductile zones suggests there is a feedback relationship between the two regimes. The reactivation of pre-existing structures and the influence of rheological factors are considered as first order factors controlling strain partitioning in the plate boundary zone. Recognition of local strain partitioning is important for assessing slip rates and earthquake recurrence. Similarly, the faults extend down below the seismogenic zone so that interaction of the different structures with each other may produce changes in fault behaviour which affects earthquake nucleation. Although the Alpine fault is a major structure in the South Island of New Zealand with over 400 km of dextral movement, the reactivated structures still exert a degree of control locally on the structure and kinematics of the plate boundary zone. Reactivation of inherent fault structures has important implications for the initiation of plate boundary faults and the alteration of the plate boundary geometry with evolving deformation.

Graduation date: 1985

High seismic activity occurs along the Caribbean and the North American Plate boundary beneath the eastern part of Hispaniola. A large number of intermediate to deep earthquakes are clustered between the Puerto Rico Trench to the north and the Muertos Trench to the south suggesting the possibility of concurrent subduction from both north and south. The body wave inversion technique was used to analyze nine earthquakes, the largest teleseismically recorded events since the establishment of WWSSN (World Wide Standardized Seismograph Network) in 1963 in the geographic region between 72°W and 66°W latitude and 16°N and 21°N longitude. Their body-wave magnitude ranges from 5.6 to 6.1. Each event was inverted for strike, dip and slip of the two possible fault planes, as well as for the centroid depth, the total seismic moment and the source time function. In order to optimize the crustal structure parameters used in the body wave inversion method, a two-dimensional geophysical cross-section across Hispaniola was constructed by forward modeling of gravity and magnetics data. The inversion results are consistent and can be divided into groups according to the depth and the epicentral location of the events. The shallow events, with depths of 6 to 12 km, represent crustal deformation and show thrust mechanisms with large strike-slip component. The intermediate depth events range from 42 to 107 km in depth and occur to the south of eastern Hispaniola. They show clear thrust mechanisms with a consistent dip of the compressional P-axis at about 30° to the north and approximately north-south P-axis strike. The deep earthquakes occur between 110 and 177 km depth, have a steep-dipping tensional T-axis, and define another slab, possibly originating at the Puerto Rico Trench to the north. One m[subscript b]=6.l event, which occurred on 6/24/84, shows opposite orientations of the P- and T-axes from the surrounding intermediate events. It is interpreted as an interface event in the upper mantle. The southern subduction zone is well defined and indicates that the Muertos Trench is active, with the subducting plate dipping to the north beneath eastern Hispaniola. At a depth of about 110 km, the northward dipping slab collides with the almost vertical segment of the other slab. This deep vertical slab segment, extending to at least 200 km in depth, may be a remnant of an earlier subduction zone associated with the Puerto Rico Trench. Alternatively, it may be connected with a more gently dipping part of the slab towards the north or, even in some way, with subduction from the south. / Graduation date: 1990

Thesis (Ph. D.)--Open University. BLDSC no. DXN039344.