Abstract The North Yemen earthquake (Mb = 6.0) of 13 December 1982 is the first earthquake in the southern Arabian Peninsula known to be accompanied by surface displacements. The extensive destruction and loss of lives resulted entirely from widespread collapse of unreinforced masonry and mud brick structures; maximum Modified Mercalli intensity was probably VII to VIII. The only surface manifestation of tectonic activity was the occurrence of earthquake-related extensional ground cracks in the epicentral region. The cracks occur mainly in four relatively continuous north- to northwest-trending linear zones that range from a few hundred meters to 15 km in length and in irregular areas of polygonal extension cracks. The area within which the cracks occur is 22.5 km long by about 10 km wide. Continued dilation across some cracks was measured almost 1 month after the main shock. Seismicity and active volcanism in this region are inferred to reflect slow extension of the southwestern margin of the Arabia plate perpendicular to the Red Sea spreading axis.
The region covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrane, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrane. The geology of the area is well understood, and this geochronologic and isotopic study confirms that the Afif terrane was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield. U-Pb zircon age data from a pelitic garnet-sillimanite gneiss of the Kabid formation shows that this part of the continental basement in the Afif terrane may be as old as 1830 Ma. Isotope data indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1830 Ma, and require that Archean rocks exist at depth beneath the Afif terrane. Lead isotope data reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic, marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a fades change within a pre-collision marginal basin. Miogeosynclinal continental shelf fades of the Siham group lie unconcormably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures. U-Pb zircon age determinations show that this "Andean" continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 690-640 Ma, the continental Afif microplate collided with the Asir terrane as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip orogen commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcano-sedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike slip motion at about 620 Ma ago.
The presence of ancient continental crust in the Afif terrane of the Arabian Shield has been indicated by radiometric data. Detailed mapping in the central Shield reveals a volcano-sedimentary succession older than one of the final cratonizing events of the Shield and unconformable upon high grade paragneisses, providing direct evidence for a western continental margin to the Afif terrane in late Proterozoic times. The basement gneisses were metamorphosed to almandine-sillimanite amphibolite facies and preserve evidence of a deformation not seen in any other rocks of the area. The Siham group, unconformable upon this basement, was metamorphosed to upper greenschist facies. It has been divided into six formations which provide an E–W section through the group. The lower members are volcanic and are absent in the easternmost formation. Their clastic content increases eastward. A carbonaceous shale with exhalative chert and marble, tuffs and associated volcanogenic massive sulphides is associated with the volcanics. It is absent in the east where shallow marine marbles and quartzites lie unconformably upon basement. Rhyolite and conglomerate complete the succession. In the west, the base of the group is not seen but the shale is associated with numerous sheet-like bodies of mafic and ultramafic rock emplaced in an extensional tectonic setting. Other plutonic rocks are calc-alkaline and are emplaced mainly in the west. Deep water oceanic conditions prevailed in the west and a shallow marine continental shelf environment existed in the east, with deposition of the group across this continental margin. The stratigraphy of the Siham group is particularly akin to the Mesozoic of Peru. Magmatic arc and marginal basin were coincident in space and time, and the marginal basin was probably extensional. There is some limited evldence that it was ensialic. The Siham group is bounded to the west by a N-S suture zone formed in one of the final collisional events in the cratonization of the Shield. This event caused the deformation of the Siham group and marked the termination of the magmatic are.
Cu-Fe-Mo mineralization in the Rio Pisco section of the Peruvian Coastal batholith is spatially associated with the Linga superunit, a suite of monzonitic rocks intruded into Albian volcanics. Petrochemical studies of this superunit indicate emplacement of a differentiation series at a subvolcanic level in the crust. The Cu-Fe-Mo mineralization is located principally in the Albian volcanic envelope and is essentially a low-grade porphyry copper type, although the grade is enhanced where structural controls on the movement of the ore-bearing fluids produced more sulfide-rich vein- and manto-type deposits. Alteration patterns associated with both the mineralization and the Linga superunit suggest a close, predominantly magmatic control on the nature of the hydrothermal fluids. Fluid inclusion studies of quartz from the Linga superunit support this and indicate that emplacement of magmas and mineralization took place at a depth of approximately 3 km. The characteristics of the Linga porphyry copper are compared to those of other such deposits and used to suggest a possible telescoping of geometry of the Andean model of Lowell and Guilbert (1970). Thus, in magmatic hydrothermal systems like the Linga, the deeper parts of the model are effectively brought nearer the surface.
The use of remote sensing in mineral exploration has evolved from basic photo-geology to interpretation of more sophisticated satellite and airborne multi-spectral data sets. Although the mineral mapping capabilities of Geoscan airborne multi-spectral scanners have been demonstrated for well-exposed and arid terrains, the question remains as to their effectiveness in deeply weathered regimes such as Western Australia.
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