Rapid formation of large coastal sand bodies after emplacement of Magdalena river jetties,northern Colombia

The Magdalena River is noted for its high discharge of river sediment and its importance as the sediment source for a large delta complex and downdrift coastal sand bodies. The emplacement of jetties, completed in 1935 to stabilize the river mouth, contributed to major changes in the downstream coastal sand bodies. The western delta front retreated an average 65 m/yr. Puerto Colombia spit detached and migrated toward Puerto Colombia at rates of 230–430 m/yr, ultimately running into the town's quay and port facility. Galerazamba spit alternately elongated and shortened over the short term, leading to the destruction or damage of coastal town sites. Isla Cascajo acted as a significant sand trap with nearly 12 km² of accretion over a 47-year period. Sand is now bypassing the tombolo, and the accretion zone continues migrating southwest. The small Punta Canaos spit also has shown significant accretion since 1974. The changes imply high rates of sediment transport; furthermore their growth is probably dependent on jetty-caused alterations of wave patterns, causing remobilization of shelf sands as well as delta-derived sand.Understanding sand body evolution and behavior is important to future development of the northern Colombia coast. Placement of port facilities, recreational beaches, tourist villages, and related support facilities on these sand bodies, as well as utilizing the sand bodies for aggregate, beach nourishment sands for other areas, or heavy mineral resources will require significant planning.     

Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.     

Some geophysical constraints on the chemical composition of the earth's lower mantle

The physical properties(?, K, K′) of the adiabatically decompressed lower mantle are interpreted in terms of an (Mg,Fe)SiO₃ perovskite + magnesiowüstite mineralogy. The approach employed in this paper involves the removal of the relatively better characterised magnesiowüstite component from the two-phase mixture in order to highlight the physical properties required of the perovskite phase for consistency between the seismological data and any proposed compositional model. It is concluded that a wide tradeoff (emphasized by Davies [1]) between composition, temperature and the physical properties (especially thermal expansion) of the perovskite phase accommodates most recently proposed compositional models including Ringwood's [2] pyrolite and the more silicic models of Burdick and Anderson [3], Anderson [4], Sawamoto [5], Butler and Anderson [6], Liu [7,8] and Watt and Ahrens [9].     

Oceanic magnetic anomaly amplitudes: variation with sea-floor spreading rate and possible implications

A qualitative examination of oceanic magnetic anomalies suggests that their amplitudes increase with the spreading rate of the ocean floor. This suggestion has been investigated quantitatively by inversion of the anomalies. A variance matching technique has been used to calculate the average magnetisation of the crust with crustal thickness held constant or to calculate the thickness with the magnetisation fixed. In spite of variations in the computed intensities of magnetisation and thickness when compared to spreading rates the results show an over-all increase in the intensities with the rate of spreading. The mechanisms responsible for this could include greater irregularity in distribution of magnetic polarities within the crust at slow spreading ridges, greater intensity of magnetisation in crust produced at fast spreading due to initial chemical properties or enhanced hydrothermal alteration, and a dependence of crustal thickness on spreading rate.     

Effect of a kelp forest on coastal currents

Ocean currents supply a kelp ecosystem with nutrients, planktonic food, and larvae. We have found that these currents in a kelp forest (Macrocystis pyrifera) are slower than currents outside. At the Pt. Loma, San Diego, California, site that we studied, current velocities were about a third of those outside. A comparison of frequency spectra shows that semi-diurnal frequencies are preferentially passed by the kelp. This effect of a kelp forest on the currents that nurture it is similar to that of a terrestrial forest on local winds.     

The seismicity of Kohistan, Pakistan: Source studies of the Hamran (1972.9.3), Darel (1981.9.12) and Patan (1974.12.28) earthquakes

Long period body waves are examined to show that the Hamran (1972.9.3), Darel (1981.9.12) and Patan (1974.12.28) earthquakes in Kohistan had focal depths of about 8–10 km. All involved high angle reverse faulting (thrusting) and had seismic moments of about 2.2 to 2.7·10²⁵ dyne cm. These shallow depths contrast with the deeper hypocentres found in the Hindu Kush and northeast Karakoram to the north and in Hazara to the south. The Hamran and Patan shocks were assigned depths of 45 km by the ISC, indicating that even well-recorded events in this region may have focal depths in error by 30 km     

Initiation and growth of salt-based thrust belts on passive margins: results from physical models

Scaled sandbox models simulated primary controls on the kinematics of the early structural evolution of salt‐detached, gravity‐driven thrust belts on passive margins. Models had a neutral‐density, brittle overburden overlying a viscous décollement layer. Deformation created linked extension–translation–shortening systems. The location of initial brittle failure of the overburden was sensitive to perturbations at the base of the salt. Salt pinch‐out determined the seaward limit of the thrust belt. The thrust belts were dominated by pop‐up structures or detachment folds cut by break thrusts. Pop‐ups were separated by flat‐bottomed synclines that were partially overthrust. Above a uniformly dipping basement, thrusts initiated at the salt pinch‐out then consistently broke landward. In contrast, thrust belts above a seaward‐flattening hinged basement nucleated above the hinge and then spread both seaward and landward. The seaward‐dipping taper of these thrust belts was much lower than typical, frictional, Coulomb‐wedge models. Towards the salt pinch‐out, frictional resistance increased, thrusts verged strongly seawards and the dip of the taper reversed as the leading thrust overrode this pinch‐out. We attribute the geometry of these thrust belts to several causes. (1) Low friction of the basal décollement favours near‐symmetric pop‐ups. (2) Mobile salt migrates away from local loads created by overthrusting, which reduces the seaward taper of the thrust belt. (3) In this gravity‐driven system, shortening quickly spreads to form wide thrust belts, in which most of the strain overlapped in time.