Rise and Demise of the New Lakes of Sahara

Multispectral remote sensing data and digital elevation models were used to examine the spatial and temporal evolution of the New Lakes of Sahara in southern Egypt. These lakes appeared in September 1998, when water spilled northwestward toward the Tushka depression due to an unusual water rise in Lake Nasser induced by high precipitation in the Ethiopian Highlands. Five lakes were formed in local depressions underlain by an impermeable Paleocene shale and chalk formation. The lakes developed through three stages. (1) A rise stage occurred from September 1998 to August 2001; the area covered by the lakes reached ~1586 km2. In this stage the rate of water supply far exceeded the rate of water loss through evaporation. This stage was characterized by an early phase (August 1998-August 1999) when the area covered by the lakes increased by ~75 km2/month. This was followed by a late phase (August 1999-August 2001), in which area increase averaged ~28 km2/month. (2) A steady-state stage occurred from August 2001 to August 2003, during which the area covered by the lakes remained relatively unchanged and water lost through evaporation was continuously replaced by water supply from Lake Nasser. (3) A demise stage occurred from August 2003 to April 2007, during which water supply from Lake Nasser stopped completely and water was continuously evaporating. The area covered by the lakes decreased to ~800 km2 with an average loss of ~17 km2/month. If this trend continues, the New Lakes of Sahara will disappear completely by March 2011. The spatial distribution of the New Lakes of Sahara is strongly controlled by morphologically defined east-, north-, northeast-, and northwest-trending faults. The water recharge of the Nubian aquifer by the New Lakes of Sahara is insignificant; much of the lakes\u27 area is above an impermeable formation

If current inhibitor ivabradine in patients with idiopathic dilated cardiomyopathy: Impact on the exercise tolerance and quality of life

Background: Evidence supported a beneficial effect of ivabradine on clinical outcome of patients with systolic heart failure, and a sinus heart rate (HR) ≥ 70 bpm. We explored the effect of ivabradine, vs. placebo, added to evidence-based treatment on exercise tolerance and quality of life in patients with idiopathic dilated cardiomyopathy. Methods: We enrolled 43 consecutive patients with dilated cardiomyopathy of no apparent cause, a left ventricular ejection fraction (LVEF) < 40%, New York Heart Association class ≥ II, sinus HR ≥ 70 bpm, and background evidence-based anti-failure medications. Ischemic heart disease was ruled out. Patients were randomized (1:1) to receive ivabradine or placebo. Ivabradine was titrated up gradually till 7.5 mg twice daily, or a HR < 60 bpm, and continued for 3 months. Symptom-limited exercise tolerance test was performed, and quality of life was assessed by the Minnesota Living With Heart Failure Questionnaire at 0, and 3 months. Results: Forty-three patients were randomized to ivabradine (n = 20), or placebo (n = 23). Mean age was 50.8 ± 14.5 years (53.5% males). Mean HR was 85 ± 12 bpm, and mean LVEF was 32 ± 6%. Mean dose of carvedilol was 31.2% of the target dose. Baseline HR, blood pressure, exercise tolerance, Minnesota questionnaire score, and left ventricular systolic function were comparable between the two groups (p > 0.05 for all). At 3 months, mean dose of ivabradine was 6.8 mg bid. Ivabradine-treated patients had a lower HR, and improved left ventricular dimensions and systolic function, versus placebo-treated ones (p < 0.05 for all). HR dropped by a mean of 14 bpm in the ivabradine group, corrected for placebo. Both exercise tolerance, and Minnesota questionnaire score were better in the ivabradine group (p < 0.05 both). Ivabradine was well-tolerated. Conclusions: In symptomatic patients with idiopathic dilated cardiomyopathy, the addition of ivabradine, vs. placebo, to evidence-based treatment, reduced HR, and improved functional capacity, at short-term follow-up

Blue Nile Incision on the Ethiopian Plateau: Pulsed Plateau Growth, Pliocene Uplift, and Hominin Evolution

The 1.6-km-deep Gorge of the Nile, a rival of the Grand Canyon, resulted from the deep incision of the Blue Nile drainage into the uplifted Ethiopian Plateau. Understanding the incision history of the plateau is crucial to unraveling the Cenozoic tectonoclimatic evolution of the region, particularly because the region has long been used as a natural laboratory to understand the geodynamics of continental rifting and the evolution of hominins. We undertake a quantitative geomorphologic approach integrating field, geographic information system (GIS), and digital elevation model (DEM) data to analyze incision (volume, long-term rates, and spatiotemporal variability) and river longitudinal profiles of the Blue Nile drainage. Previously published isotopic ages of the Cenozoic volcanic rocks are used to constrain long-term incision rates through geologic time. Our data argue that (1) the Blue Nile drainage has removed at least 93,200 km3 of rocks from the northwestern Ethiopian Plateau since ca. 29 Ma (early Oligocene) through a three-phase (ca. 29-10 Ma, ca. 10-6 Ma, and ca. 6 Ma to present) incision, where long-term incision rates increased rapidly and episodically in the late Miocene (ca. 10 Ma and ca. 6 Ma); (2) being out-of-phase with the past climatic events and in-phase with the main volcanic episodes of the region, this episodic increase of incision rate is suggestive of episodic growth of the plateau; (3) of the ~2-km rock uplift of the plateau since ca. 30 Ma, 0.3 km was due to isostatic uplift related to erosional unloading, and the rest was due to other tectonic activities; (4) the extremely rapid long-term incision rate increase, thus a rapid uplift of the plateau, ca. 6 Ma might be related to lithospheric foundering, caused by ponded plume material beneath the Ethiopian Plateau and aided by huge tectonic stresses related to the Messinian salinity crisis of the Mediterranean Sea. These events could have caused the plateau to rise \u3e1 km within a few m.y. in the early Pliocene. This uplift history of the Ethiopian Plateau can shed critical light on the geodynamics of the Afar mantle plume and the evolution of the East African hominins via climate change

Hierarchical Segmentation of the Malawi Rift: The Influence of Inherited Lithospheric Heterogeneity and Kinematics in the Evolution of Continental Rifts

We used detailed analysis of Shuttle Radar Topography Mission-digital elevation model and observations from aeromagnetic data to examine the influence of inherited lithospheric heterogeneity and kinematics in the segmentation of largely amagmatic continental rifts. We focused on the Cenozoic Malawi Rift, which represents the southern extension of the Western Branch of the East African Rift System. This north trending rift traverses Precambrian and Paleozoic-Mesozoic structures of different orientations. We found that the rift can be hierarchically divided into first-order and second-order segments. In the first-order segmentation, we divided the rift into Northern, Central, and Southern sections. In its Northern Section, the rift follows Paleoproterozoic and Neoproterozoic terrains with structural grain that favored the localization of extension within well-developed border faults. The Central Section occurs within Mesoproterozoic-Neoproterozoic terrain with regional structures oblique to the rift extent. We propose that the lack of inherited lithospheric heterogeneity favoring extension localization resulted in the development of the rift in this section as a shallow graben with undeveloped border faults. In the Southern Section, Mesoproterozoic-Neoproterozoic rocks were reactivated and developed the border faults. In the second-order segmentation, only observed in the Northern Section, we divided the section into five segments that approximate four half-grabens/asymmetrical grabens with alternating polarities. The change of polarity coincides with flip-over full-grabens occurring within overlap zones associated with ∼150 km long alternating border faults segments. The inherited lithospheric heterogeneity played the major role in facilitating the segmentation of the Malawi Rift during its opening resulting from extension

Fault Growth and Propagation During Incipient Continental Rifting: Insights from a Combined Aeromagnetic and Shuttle Radar Topography Mission Digital Elevation Model Investigation of the Okavango Rift Zone, Northwest Botswana

Digital Elevation Models (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data and high-resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the Okavango Rift Zone (ORZ), northwest Botswana. Significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben. Faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive, or have large throws and small scarp heights indicating waning activity. Faults on the outer margins of the rift exhibit either (1) large throws or significant scarp heights and are considered older and active or (2) throws and scarp heights that are in closer agreement and are considered young and active. Fault linkages between major fault systems through a process of fault piracy have combined to establish an immature border fault for the ORZ. Thus, in addition to growing in length (by along-axis linkage of segments), the rift is also growing in width (by transferring motion to younger faults along the outer margins while abandoning older faults in the middle). Finally, utilization of preexisting zones of weakness allowed the development of very long faults (\u3e100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift. This study clearly demonstrates that the integration of the SRTM DEM and aeromagnetic data provides a 3-D view of the faults and fault systems, providing new insight into fault growth and propagation during the nascent stages of continental rifting

The Neoproterozoic Keraf Suture in NE Sudan: Sinistral Transpression along the Eastern Margin of West Gondwana

The Keraf Suture, formed during the Neoproterozoic consolidation of Gondwana, is a ~500 km long, ~50 km wide, N-trending suture between the Neoproterozoic Arabian-Nubian Shield in the east and the older Nile Craton to the west. The Keraf Suture is superimposed on E- and NE-trending structures on both sides. The northern part of the suture is dominated by N-trending, upright folds, whereas the southern part is characterized by N- and NNW-trending, sinistral, strike-slip faults. A major antiform defines a structural divide between the northern and southern parts of the suture. 40Ar/39Ar ages on biotite and hornblendes separated from a deformed granitic body indicate that the sinistral movement along the N- and NNW-trending faults took place at ~580 Ma. The difference in structural styles along strike is due to formation of the Keraf Suture by sinistral transpression, which accompanied early NW-SE oblique collision between East and West Gondwana at ~650-600 Ma and terminal collision at ~580 Ma

Thermal Perturbations beneath the Incipient Okavango Rift Zone, Northwest Botswana

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m-2) beneath a ∼60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust ( \u3c 30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m-2), corresponding to thicker crust (∼40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement

Thermal Perturbations beneath the Incipient Okavango Rift Zone, Northwest Botswana

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m-2) beneath a ∼60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust ( \u3c 30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m-2), corresponding to thicker crust (∼40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement

Creating Virtual 3-D Outcrop

Because of the high precision of present-day GPS and reflectorless laser technology, geologic information and remotely sensed data (i.e., seismic and GPR grids, wells) can be positioned accurately in 3-D and reconstructed as a virtual image. Hence, we have developed the “virtual outcrop” for applications that require knowledge about the 3-D spatial arrangements of rock types

Receiver Function Constraints on Crustal Seismic Velocities and Partial Melting beneath the Red Sea Rift and Adjacent Regions, Afar Depression

The Afar Depression is an ideal locale for the investigation of crustal processes involved in the transition from continental rifting to oceanic spreading. To provide relatively high resolution images of the crust beneath the Red Sea rift (RSR) represented by the Tendaho graben in the Afar Depression, we deployed an array of 18 broadband seismic stations in 2010 and 2011. Stacking of about 2300 receiver functions from the 18 and several nearby stations along the ~200 km long array reveals an average crustal thickness of 22 ± 4 km, ranging from ~17 km near the RSR axis to 30 km within the overlap zone between the Red Sea and Gulf of Aden rifts. The resulting anomalously high Vp/Vs ratios decrease from 2.40 in the southwest to 1.85 within the overlap zone. We utilize theoretical Vp and melt fraction relationships to obtain an overall highly reduced average crustal Vp of ~5.1 km/s. The melt percentage is about 10% beneath the RSR while the overlap zone contains minor quantities of partial melt. The observed high Vp/Vs values beneath most of the study area indicate widespread partial melting beneath the southwest half of the profile, probably as a result of gradual eastward migration of the RSR axis. Our results also suggest that the current extensional strain in the lower crust beneath the region is diffuse, while the strain field in the upper crust is localized along narrow volcanic segments. These disparate styles of deformation imply a high degree of decoupling between the upper and lower crust