The Neoproterozoic Najd Fault System extends for 2000km across the East African Orogen, yet its history of motion and tectonic significance are widely debated. The Halaban-Zarghat Fault is the northeastern-most of the major NW-strildng Najd faults in the Arabian Shield. Several sedimentary basins of the Neoproterozoic Jibalah Group are bounded by strands of the Halaban-Zarghat Fault and other Najd faults, particularly along right steps in the fault trace. Among the largest of the basins is the Jifn. The geometry of the Jifn Basin and the sedimentary facies of Jibalah Group indicate that it is a dextral pull-apart basin between strands of the Halaban-Zarghat Fault. A zone of high-grade mylonitic gneiss is located along a left step in the fault zone and may be a deeply eroded pop-up structure related to dextral transpression. Analysis of structural data from around and within the Jifn Basin, the position of other pull-apart basins and high-grade mylonite zones along the Halaban-Zarghat Fault are all consistent with early dextral movement along the Halaban-Zarghat Fault. Offsets of distinctive older rock units and transection of the Jifn Basin by sinistral faults, however, show that the latest and most significant sense of offset on the Halaban-Zarghat Fault and other Najd faults was sinistral.A U-Pb zircon date of 624.9 _-+4.2Ma from rhyolitic basement of the Jifn Basin gives a lower limit for the formation of the basin and initiation of dextral movement along the Halaban-Zarghat Fault. This age is interpreted as the earliest age for the collision of East and West Gondwana. A 621 _-_7Ma pluton is offset 10km dextrally along the Halaban-Zarghat Fault, showing that dextral motions continued for some time past 621Ma, before switching to sinistral motions, and accreted terranes caught between the two continents were forced toward an oceanic-free face to the north. A 576.6 __. 5.3 Ma U-Pb zircon date from an undeformed felsite dyke that intrudes the Jibalah Group gives an upper time limit for movement along the Halaban-Zarghat Fault. This may mark the time that collision and escape tectonics ended, or it may reflect the time that displacements were transferred to other Najd faults in more interior parts of the East African Orogen.

In arid regions, the analysis of wadi morphometric parameters and their interrelationships are fundamental for describing the topographic, geologic evolution, structures and hydrologic potential efficiency of the watershed. Unfortunately, the spatial characteristics and the analyses of these variables are not taken into consideration in an efficient way, especially in arid regions. In this paper, digital elevation model, geographic information system and multivariate statistical techniques are integrated for the identification and the assessment of main morphometric parameters. For this purpose, Q and R modes of cluster analyses and principal component analysis (PCA) are applied to ten different-size watersheds in the western region of Saudi Arabia utilizing 18 morphometric descriptors (variables). The results show that the R-mode cluster analysis classifies the variables into three groups, whereas the Q-mode cluster analysis classifies watersheds according to the similarities in their major variables such as area, perimeter, total stream length and peak discharge. The first three components of PCA accounted for 86% of the total variance in the data and show more details concerning the parameter loadings and the degree of variable significance.

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Wadi Fatima fold-thrust (FAT) belt is a distinctive foreland FAT belt in the Arabian-Nubian Shield (ANS) involving unmetamorphosed to slightly metamorphosed sedimentary sequence of Fatima Group, deposited over a metamorphic/igneous basement, comprising ortho-amphibolites, orthoand para-schists (with chaotic unmappable blocks of marbles, pyroxenites and metagabbros), older granite (773 ± 16 Ma) and younger granite. The basement exhibits structural fabrics, such as attenuated tight isoclinal folds, sheared-out hinges, NE-SW penetrative foliation and subhorizontal stretched and mineral lineations, related to an oldest prominent dextral shearing phase affected the main Wadi Fatima during the Neoproterozoic. In Wadi Fatima FAT belt, the style of deformation encompasses flexural-slip folding forming mesoscopicand map-scales NE to ENE plunging overturned antiforms and synforms, and a thrust duplex system bounded by floor thrust and sole thrust (basal detachment) dipping gently towards the hinterland (SE to SSE direction) and rises stratigraphically upwards towards the foreland. Such style is affiliated to thin-skinned deformation. Several lines of evidence, such as geometry of interacting outcropand map-scale folds and thrusts, patterns of thrust displacement variations and indications for hinge migration during fold growth, strongly suggest that folding and thrusting in Wadi Fatima FAT belt are geometrically and kinematically linked and that thrusting initiated as a consequence of folding (fold-first kinematics). Thrusts frequently show flat-ramp-flat geometry, and every so often give an impression that they are formed during two main sub-stages; an older sub-stage during which bedding sub-parallel thrusts were formed, and a younger sub-stage which generated younger ramps oblique to bedding. Thrust ramps with SE to SSE dipping regularly show sequential decrease in dip or inclination (due to piggy-back imbrication) into their transport direction which is proposed to be towards NW to NNW. Evidence indicating this transport direction of Wadi Fatima FAT belt embrace NW to NNW oriented stretching lineations recorded along thrust planes, NW to NNW folding vergence, and diminishing of the intensity of deformation and thrust stacking and imbrication from SE to NW; <i>i.e.</i> from hinterland to foreland. The tectonic transport vector is congruent with the mean orientation of slickenline striae formed by layer-parallel slipping along folded bedding planes. The mean orientation of slickenline lineations, after their host beds were rotated to horizontal about their strikes, is found to be N25°W - S25°E. Two tectonic models are proposed to unravel the structural history of the study area and to illustrate the tectonic evolution of Wadi Fatima FAT belt which represents one of interesting foreland FAT belts recorded worldwide. In the first model, the area was evolved from dextral shearing during the early convergence and amalgamation between East and West Gondwana, to emplacement of the older granite during a period of c...

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