Growth of vertically segmented normal faults

The geometry and evolution of vertically segmented normal faults, with dip separations of < ca 11.5 m have been studied in a coastal outcrop of finely bedded Cretaceous chalk at Flamborough Head, U.K. Fault trace segments are separated by both contractional and extensional offsets which have step, overlap or bend geometries. The location of fault trace offsets is strongly controlled by lithology occurring at either thin (ca 1 mm-8 cm) and mechanically weak marl layers or partings between chalk units. Fault segmentation occurred during either fault nucleation within, or propagation through, the strongly anisotropic lithological sequence. An inverse relationship between fault displacement and number of offsets per length of fault trace reflects the progressive destruction of offsets during fault growth. The preservation of fault offsets is therefore dependent on offset width and fault displacement. Fault rock, comprising gouge and chalk breccia, may vary in thickness by 1.5–2.0 orders of magnitude on individual fault traces. Strongly heterogeneous fault rock distributions are most common on small faults (< 10 cm displacement) and are produced mainly by destruction of fault offsets. Shearing of fault rock with increasing displacement gives rise to a more homogeneous fault rock distribution on large faults at the outcrop scale.     

Limitations of dimension and displacement data from single faults and the consequences for data analysis and interpretation

The relationship between the maximum cumulative displacement on a fault (D) and the maximum linear dimension of the fault surface (W) is given by the expression D = cWⁿ, where the value of c is determined by rock properties; proposed values for n range from 1.0 to 2.0. Published datasets of D vs W measurements, together with new data, are presented in a common format. Most datasets are derived from maps and so the measurements of displacement and length do not represent maximum values for each fault. This factor, together with more than an order of magnitude range of c, causes regression on D vs W plots to be unsafe unless the range of W values plotted is ca 5 orders of magnitude. This restriction means that individual datasets must be combined to achieve the required range of fault size. Data analysis shows that the value of n must exceed 1.0 but discrimination between values of 1.5 and 2.0 cannot be made on the basis of data analysis alone. A modified fault growth model in which the increase in dimension of a fault with each slip event is proportional to W^0.5 gives rise to a value for n of 1.5. As this model has a sound mechanical basis 1.5 is the preferred value for n. The value of n influences other aspects of fault geometry, including the displacement profile on a fault surface, the spacing of depth contours on faulted horizons and the displacement populations of single fault surfaces.     

Land reclamation in Singapore,Hong Kong and Macau

The spatial and temporal significance of land reclamation from the sea in relation to the development of the three SE Asian small coastal territories of Singapore, Hong Kong and Macau are examined and compared. In all three territories the rate of reclamation has been an index of the rate of economic development. Four phases of land reclamation are identified: (1) pre-1900: involved relatively unplanned and uncostly projects in shallow water and swamplands around the main focus of settlement; (2) 1900–1945: involved military projects outside the main settlement core; (3) 1945–1980: this phase of large-scale projects unconstrained by natural conditions because of advances in technology and high rate of reclamation was driven by rapid industrial and population growth; (4) 1980 to date: a period marked by large infrastructure projects designed to maintain and extend the status of the territories as world economic and commercial centres. Currently 10%, 5% and 33% of the total land surfaces of Singapore, Hong Kong and Macau respectively comprise land reclaimed from the sea and these proportions will continue to rise.     

Geochemistry and petrogenesis of the early Tertiary lava pile of the Isle of Mull,Scotland

Major and trace element analyses and strontium isotope ratios are presented for twenty-four samples of lavas and plugs from the early Tertiary lava pile in Mull. The samples were selected on the basis of petrographic freshness from a large collection from outside the hydrothermally altered zone of pneumatolysis which occupies the central region of the volcanic complex. Most of the analyses yield normative hypersthene and we argue that these are essentially unaltered magmatic compositions. The analytical data indicate that the samples may be divided into three groups on the basis of major element chemistry, initial ⁸⁷Sr/⁸⁶Sr ratios and correlations between lithophile element contents. Group I comprises an alkaline series (basalt-hawaiite-mugearite) with extremely low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7030) and generally low lithophile element contents. Apart from their alkalinity and high Sr and Zr contents these samples have affinities with abyssal tholeiites. Group II contains hypersthene normative basalts with more tholeiitic characteristics but (as in the case of the Skye Main Lava Series) the more evolved rocks are trachytes. This group is characterized by more normal levels of lithophile element concentrations and relatively high initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.7055. Group III is less clearly defined and contains basalts that are generally sparsely olivine-phyric and in most chemical respects fall between Group I and Group II-including initial ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7043). They may represent mixtures of Group I and Group II type sources or magmas. Groups I and II appear to be similar, respectively, to the relatively sodic iron-rich and the relatively potassic ironpoor silica enrichment trends distinguished in the Skye Main Lava Series. In the Group I magma series the behaviour of Y and Sr relative to other incompatible elements can only be explained by differential partial melting of a deep garnet-lherzolite mantle source. Fractional crystallization has undoubtedly occurred at some stage during the ascent of these magmas from the mantle, as indicated by the behaviour of Ni and Cr, but has not been a major factor in the production of evolved magma compositions. The Group II magmas appear to have originated from a source more enriched in lithophile elements, and a relatively shallow (< 50 km) plagioclase-lherzolite mantle source is suggested for these magmas because they have Sr/Ba ratios between one and two orders of magnitude lower than those characteristic of Group I. Rb-Sr systematics suggest that the vertical heterogeneity of the mantle which was largely responsible for the chemical differences between these three groups may have existed for a very long time prior to Tertiary magmatism.     

The nature and distribution of organic matter in the surface sediments of world oceans and seas

Existing data on the distribution of organic carbon and nitrogen in marine sediments have been analyzed in order to better understand the physical and chemical processes involved in this aspect of the global carbon cycle. Maps of the global distribution of organic carbon and nitrogen in the sediments of world oceans and seas are presented.Correlation analyses of the available information dealing with the distribution of marine sedimentary organic matter has revealed that in terms of bulk parameters (%C_Org and %N Kjeldahl), there is an apparent accumulation of organic matter on the continental slope (water column depth 200–2000 m). Specific interactions between clays and organic matter, although indicated in laboratory experiments, have not been detected by these analyses.     

The impact,control, and mitigation of off-road vehicle activity in Hungry Valley,California

Hungry Valley, and arid basin situated within the Transverse Ranges of Southern California, has been utilized by off-road vehicles (ORVs) since 1971. Loss of vegetation, severe soil erosion and gullying, alluvial fan formation, and increased sediment discharge are the direct results of ORV activity in the valley. These hydrologic-geologic effects have been documented and monitored utilizing sequential aerial photographs, ground surveys, and sediment transport measurements. Despite the increased usage of ORVs on public lands, and the well-documented short-term physical environmental impacts, little data exists on erosion or sediment transport rates in disturbed vs. adjacent undisturbed areas, the ability of natural vegetation to re-establish itself, or the feasibility of site rehabilitation techniques or methods.     

Geometry and growth of an inner rift fault pattern: the Kino Sogo Fault Belt, Turkana Rift (North Kenya)

A quantitative analysis is presented of the scaling properties of faults within the exceptionally well-exposed Kino Sogo Fault Belt (KSFB) from the eastern part of the 200-km-wide Turkana rift, Northern Kenya. The KSFB comprises a series of horsts and grabens within an arcuate 40-km-wide zone that dissects Miocene–Pliocene lavas overlying an earlier asymmetric fault block. The fault belt is 150 km long and is bounded to the north and south by transverse (N50°E and N140°E) fault zones. An unusual feature of the fault system is that it accommodates very low strains (<1%) and since it is no older than 3 Ma, it could be characterised by extension rates and strain rates that are as low as 0.1 mm/yr and 10⁻¹⁶ s⁻¹, respectively. Despite its immaturity, the fault system comprises segmented fault arrays with lengths of up to 40 km, with individual fault segments ranging up to 9 km in length. Fault length distributions subscribe to a negative exponential scaling law, as opposed to the power law scaling typical of other fault systems. The relatively long faults and segments are, however, characterised by maximum throws of no more than 100 m, providing displacement/length ratios that are significantly below those of other fault systems. The under-displaced nature of the fault system is attributed to early stage rapid fault propagation possibly arising from reactivation of earlier underlying basement fabrics/faults or magmatic-related fractures. Combined with the structural control exercised by pre-existing transverse structures, the KSFB demonstrates the strong influence of older structures on rift fault system growth and the relatively rapid development of under-displaced fault geometries at low strains.     

Fractal analysis of fracture patterns using the standard box-counting technique: valid and invalid methodologies

On the basis of results of simple box-counting analysis, it has been suggested in several recent publications that natural fracture patterns are fractal. Fractal patterns are characterized by self-similarity of structure on a range of scales and provide straight-line distributions on box-counting plots. New analysis of a fracture pattern that provided the most convincing straight-line box-counting curve previously published, shows that the curve is non-linear and, therefore, that the fracture pattern is not fractal. Non-linear box-counting curves are also characteristic of other natural fracture patterns analysed but spurious linear curves can be obtained if the area analysed extends beyond the mapped area.     

Distributions of cumulative displacement and seismic slip on a single normal fault surface

Displacement profiles (normalized displacement vs normalized distance from the point of maximum displacement) have been plotted for 34 horizontal radii from 25 normal faults with maximum displacements ranging from 1.0 to 37.5 m. The composite displacement profile for these faults, when corrected for systematic inaccuracies of the data, is significantly different from the theoretical slip profile for a single seismic slip event. The integration of slip displacement profiles of many slip events on a growing fault shows that a steady-state displacement profile will be established. This theoretical displacement profile is similar to the composite profile derived from the fault data. Analysis of displacement data from 488 fault traces, which do not necessarily pass through the point of maximum displacement of their respective faults, shows that although displacement measurements are strongly influenced by ductile drag the theoretical distribution can still be identified in the data. Although the slip distribution on a fault during a single slip event, or during a period of stable sliding, is not simply related to the distribution of cumulative displacement on the fault, a knowledge of both characteristics places firm constraints on fault growth models.     

Stratigraphy,composition and form of the Deccan Basalts,Western Ghats,India

In the Western Ghats between latitudes 18° 20 N and 19° 15 N, 7000 km² of Deccan Basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts. Using over 70 measured vertical sections, major and trace element analyses of nearly 1200 samples, and rare-earth and⁸⁷Sr/⁸⁶Sr determinations for over 60 samples, we divide the basalt into three subgroups and ten formations. In this paper we describe the seven principal formations in the area and the most prominent individual flows.The Kalsubai Subgroup is formed by the lower five formations, the Jawhar, Igatpuri, Neral, Thakurvadi, and Bhimashankar formations, from botton to top. In these formations amygdaloidal compound flows predominate and have a typically high MgO content, including picrite basalt (> 10% MgO) and picrite (> 18% MgO) with phenocrysts of olivine and clinopyroxene. These flows are separated by others which contain giant plagioclase phenocrysts and have more evolved chamical compositions.The Lonavala Subgroup overlies the Kalsubai and is composed of two formations, the Khandala and the Bushe. Both are readily recognized in the field and by their chemical compositions.The Wai Subgroup includes the upper three formations, the Poladpur, the Ambenali, and the Mahabaleshwar. The whole subgroup is composed of simple flows with well-developed flow tops, small phenocrysts of plagioclase, pyroxene and olivine, and relatively evolved bulk compositions.Distribution and variation in thickness of the straitigraphic units within the Western Ghats provide a first comprehensive view of the development of the Deccan volcanic edifice. The persistent southerly dip and gentle southerly plunging anticlinal form of the flows, the lensoid shape of many of the formations, and nearly randomly oriented feeder-dike system are together interpreted as evidence of a central volcanic edifice formed as the Indian plate drifted northward over a mantle plume or hot spot.