Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.
Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.
The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.
Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of dmax/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (dmax) along a fault and increase the fault length (L).
Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven. 相似文献
The rock magnetic properties of the Chinese loess and paleosols constitute a unique and sensitive record of East Asian paleoclimate through the Quaternary Period. Systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal, which can be easily and rapidly measured at many sites across the Loess Plateau. Variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size, but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages, as well as glaciations and interglaciations. Past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations. The susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the Loess Plateau. Our rainfall reconstructions identify enhanced summer monsoonal activity in the Chinese Loess Plateau region in the early Holocene and the last interglaciation. In the presently semiarid western area of the plateau, annual precipitation in interglacial times was up to 80% higher than at present; in the more humid southern and eastern areas, values were up to 20% higher than today's levels. During the last glaciation, precipitation decreased across the entire plateau, typically by 25%. The relationship between pedogenic susceptibility, climate, and weathering age was examined over the Northern Hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate, rather than soil age, is the predominant factor that controls pedogenic susceptibility enhancement in loess soils. 相似文献
Summary. Teleseismic P and S arrival times to North American stations are obtained from the ISC bulletins for the 10-yr period 1964–73, and relative travel-time delays are calculated with respect to standard tables. Station anomalies as well as variations of the delays with azimuth and epicentral distance from station are analysed, and the location of the velocity anomalies responsible for them is discussed. Inversion of the P delays to infer upper mantle velocity structure down to a depth of 700 km is obtained using three-dimensional blocks, as proposed by Aki, Christofferson & Husebye. Three layers can be resolved in this depth range. It is found that the heterogeneities responsible for the travel-time delays are primarily located in the first 250 km of the upper mantle, and that they correlate with surface features. Significant heterogeneities subsist to depths of at least 700 km and their broad scale pattern also correlates with the surface features: in the third layer (500 to 700 km depth) there is an increase of velocity from the West to the East of the United States, while the second layer (250 to 450 km depth) exhibits a reversed pattern. A tentative interpretation of these deeper anomalies is made, as being due mainly to topography of the major upper mantle discontinuities, near 400 and 650 km depth. 相似文献
Rhizophora mangle L. (red mangrove) is the dominant species of mangrove in the Americas. At Twin Cays, Belize (BZ) red mangroves are present in a variety of stand structures (tall >5 m in height, transition ~2–4 m and dwarf ~1–1.5 m). These height differences are coupled with very different stable carbon and nitrogen isotopic values[1] (mean tall δ13C = -28.3‰, δ15N = 0‰; mean tall δ13C = -25.3‰, δ15N = -10‰). To determine the utility of using these distinct isotopic compositions as 'biomarkers' for paleoenvironmental reconstruction of mangrove ecosystems and nutrient availability, we investigated the distribution and isotopic (δ13C and δ15N) composition of different biochemical fractions (water soluble compounds, free lipids, acid hydrolysable compounds, individual amino acids, and the residual un-extractable compounds) in fresh and preserved red mangrove leaves from dwarf and tall trees. The distribution of biochemicals are similar in dwarf and tall red mangrove leaves, suggesting that, regardless of stand structure, red mangroves use nutrients for biosynthesis and metabolism in a similar manner. However, the δ13C and δ15N of the bulk leaf, the biochemical fractions, and seven amino acids can be used to distinguish dwarf and tall trees at Twin Cays, BZ. The data support the theory that the fractionation of carbon and nitrogen occurs prior to or during uptake in dwarf and tall red mangrove trees. Stable carbon and nitrogen isotopes could, therefore, be powerful tools for predicting levels of nutrient limitation at Twin Cays. The δ13C and δ15N of biochemical fractions within preserved leaves, reflect sedimentary cycling and nitrogen immobilization. The δ15N of the immobilized fraction reveals the overlying stand structure at the time of leaf deposition. The isotopic composition of preserved mangrove leaves could yield significant information about changes in ecosystem dynamics, nutrient limitation and past stand structure in mangrove paleoecosystems. 相似文献
This paper describes the fusion of information extracted from multispectral digital aerial images for highly automatic 3D map generation. The proposed approach integrates spectral classification and 3D reconstruction techniques. The multispectral digital aerial images consist of a high resolution panchromatic channel as well as lower resolution RGB and near infrared (NIR) channels and form the basis for information extraction.Our land use classification is a 2-step approach that uses RGB and NIR images for an initial classification and the panchromatic images as well as a digital surface model (DSM) for a refined classification. The DSM is generated from the high resolution panchromatic images of a specific photo mission. Based on the aerial triangulation using area and feature-based points of interest the algorithms are able to generate a dense DSM by a dense image matching procedure. Afterwards a true ortho image for classification, panchromatic or color input images can be computed.In a last step specific layers for buildings and vegetation are generated and the classification is updated. 相似文献
