U–Pb Ages for Zircon Grains from Nsanaragati Alluvial Gem Placers: Its Correlation to the Source Rocks          下载免费PDF全文

Nguo Sylvestre Kanouo  Rose Fouateu Yongue  Emile Ekomane  Emmanuel Njonfang  Changqian Ma  David R. Lentz  Zhenbing She  Khin Zaw  Akella Satya Venkatesh 《Resource Geology》2015,65(2):103-121

The Mamfe Basin is located in the SW of Cameroon and is an extension of the much bigger Benue Trough in the SE of Nigeria. Along the Asenem River and its tributaries in the western part of the Mamfe Basin (close to the border with Nigeria) gem placers yielding big zircon grains were found in recent river sediments close to Nsanaragati. In order to determine the source area and to establish a possible correlation between the zircons found in the Nsanaragati placers and rocks surrounding the Mamfe Basin 56 detrital zircon grains were analysed regarding their U–Pb ages and selected trace element contents by LA‐ICP–MS techniques. Possible source areas are rocks from the Benue Trough in the West and from the Cameroon Volcanic Line (CVL) in the SE of the study area. Based on microscopic analyses it was possible to distinguish two groups of zircons: reddish and non‐reddish ones, where the latter group comprises color variations from brown to orange, yellow to even colorless. In general, the detrital zircons show high hafnium values (4576 to 12565 ppm) and very variable thorium (7.8 to 1565 ppm) and uranium values (13.4 to 687 ppm). The Th/U ratios vary from 0.4 to 2.3, allowing correlations for some zircon grains to kimberlitic, granitic or syenitic affinities. It was also possible to distinguish zircon grains crystallised in mafic mantle‐derived melts from those crystallised in felsic melts, e.g. from continental rift‐related magmatic systems. In general, the U–Pb zircon ages obtained range from 11.7 to 1949 Ma. All zircons of the reddish group yielded almost similar ages resulting in a Concordia age of 12.4 Ma (Serravallian), an age unknown from the Mamfe Basin so far. The group of non‐reddish zircons showed various ages ranging from Serravallian to Orosirian. It was possible to correlate the youngest ages with rocks known from intrusions along the CVL, dated with K/Ar or Ar/Ar methods. The most probable sources were Mount Bamenda and Mount Bambouto in the east of the Mamfe Basin. Cretaceous ages are interpreted as re‐recycled clastic sediments whose original source had been rocks in the south and the north of the Benue Trough and who had been eroded and deposited within the catchment area of the Asenem river system in the Mamfe Basin in post‐Cretaceous times. The oldest ages are assumed to represent the pan‐African and pre‐pan‐African basement of the Mamfe basin.  相似文献   

Far-field tsunami hazard from mega-thrust earthquakes in the Indian Ocean   总被引：2，自引：0，他引：2  

Emile A. Okal  Costas E. Synolakis 《Geophysical Journal International》2008,172(3):995-1015

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Evidence of prehistoric liquefaction in Kuwait and implications for the seismic vulnerability of the Arabian Gulf Countries     

Bou-Rabee  Firyal  Young  Yin Lu  Okal  Emile A. 《Natural Hazards》2020,103(1):799-813

This paper presents and analyzes paleo-liquefaction features found in the State of Kuwait. The features are cemented sand and gravel-filled dikes of Pleisto–Holocene age with appearance and composition similar to typical “sandstone pipes.” The significant age difference between the cemented dikes and the surrounding loose sand, the size and spatial distribution of the dikes, and the local geologic and hydrologic setting all suggest that the feature probably results from a single large event of seismic origin. Likely hypotheses include shaking during large earthquakes or seiching of tsunami-like waves. Additional research is needed to identify the exact cause of these dike formations, which is important for the purpose of improving seismic risk and vulnerability assessment of the Arabian Gulf countries. The search may also help explain the disappearance of an ancient civilization that lived in the same region approximately seven thousand years ago.

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Introduction     

Emile A. Okal 《Pure and Applied Geophysics》1991,135(2):167-168

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The Structure of the Nazca Ridge and Sala Y Gomez Seamount Chain From the Dispersion of Rayleigh Waves     

Mark T. Woods  Emile A. Okal 《Geophysical Journal International》1994,117(1):205-222

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Contribution à l'étude de la faune du Léman     

Emile André 《Aquatic Sciences - Research Across Boundaries》1922,2(1-2):193-196

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Seismic detection of underwater volcanism: The example of French Polynesia     

Jacques Talandier  Emile A. Okal 《Pure and Applied Geophysics》1987,125(6):919-950

We present a review of the principal methods used for the seismic detection and identification of active underwater volcanism, based on our experience in French Polynesia. In particular, we descrobe the 5-year activity in the Tahiti-Mehetia area, during which more than 32000 earthquakes were detected by the Polynesian network. We discuss the use of the following three types of seismic waves: conventional (mostly body waves), seismic tremor, andT waves propagated in the low-velocity acoustic channel of the ocean. For each of these waves, we discuss the principal characteristics of the signals, their spectral content, the type of information they provide on the activity of the volcano, and the various limitations faced by their use in detection or monitoring of underwater volcanic edifices. We present a review of the principal swarms monitored by the Polynesian network, and discuss their characterization as either volcanic or tectonic.  相似文献   

The depth of the deepest historical earthquakes     

Beth A. Rees  Emile A. Okal 《Pure and Applied Geophysics》1987,125(5):699-715

We useP andS times listed in the International Seismological Summary to relocate 23 historical earthquakes (1927–1963) reported as occurring at or below 670 km. In all cases, our relocated hypocenters are shallower than the starting depths; furthermore, all events converge to 691 km or less, with a precision estimated at ±10 km. This study upholds the results of Stark and Frohlich, who had usedpP–P times of post-WWSSN earthquakes to constrain reliable hypocentral depths to no greater than 684 km. In particular, we reject Rothé's claim that a 1963 event in the vicinity of New Guinea occurred at a depth of more than 780 km.  相似文献   

Slow earthquakes along oceanic fracture zones: evidence for asthenospheric flow away from hotspots?     

Emile A. Okal  Lisa M. Stewart 《Earth and Planetary Science Letters》1982,57(1):75-87

The regime of strain release along transform faults of the Mid-Oceanic Ridge system is studied. It is shown that earthquakes along certain fracture zones exhibit systematic discrepancies between values of their magnitudes measured at short and long periods, implying a regime of slower strain release, also observed in a pattern of complex body waveshapes. These “slow” fracture zones do not correlate with simple geographic or kinematic properties, but usually occur in the neighborhood of hotspot volcanism, frequently also characterized by gravity and bathymetry anomalies. We propose that regimes of slow strain release may be due to a partial lubrication of the fault along these fracture zones, which may itself be due to asthenospheric flow from the nearby hotspots, along the pattern of pipelines described by Vogt and Johnson and by Morgan.  相似文献   

A negative search for an ultra-slow component to the source of the Yunnan earthquakes of May 29, 1976     

Emile A. Okal  Lisa M. Stewart 《Physics of the Earth and Planetary Interiors》1981,26(3):208-216

Following Nagamune's suggestion of a giant, but very slow, component to the source of the Yunnan earthquakes of May 29, 1976, a systematic study of the ultra-low frequency content of a number of tiltmeter, strainmeter, and IDA records of the events is conducted. First, Nagamune's hypothesis is quantified, and it is found that it would require a moment in the range 10²⁹–10³⁰ dyn-cm. Such a large moment would have important consequences for our understanding of stress release in the plates, but is incompatible with the extent of the aftershock zone, and with observations on other instruments: specifically, only one IDA record shows a time-domain oscillation which may be related to the proposed source. In the Fourier domain, however, a spectral analysis fails to identify any of the Earth's modes in any of the available records, including the one originally used by Nagamune. It must therefore be concluded that the apparent signal present in a few records is due to noise—probably of instrumental origin—rather than to an ultra-low-frequency component of the seismic source.  相似文献