Al Haruj Research Articles

Emplacement and inflation of the Al-Halaq al Kabir lava flow field, central part of the Al Haruj Volcanic Province, Central Libya

Numerous of lava rise plateaus and tumuli developed in the medial and distal portions of the Al-Halaq al Kabir lava flow field during the last (Holocene) eruption in the central part of the Al Haruj Volcanic Province (AHVP). These inflation structures provide important data on the lava-emplacement mechanism in this part of the Al Haruj region where the widespread occurrence of tumuli and lava rise plateaus are good indicators of the mode of emplacement. We report the results of detailed measurements of the maximum and minimum diameters in plan-view of 551 lava-rise plateaus and 289 tumuli from the distal part of the Al-Halaq al Kabir lava field using ArcGIS and field observations. Tumuli and lava rise plateaus may be divided into subpopulations according to abrupt changes in the scaling exponents on log-log plots of their frequency versus diameter. Using the estimated stiffness (Young's modulus) of the basaltic rocks in the study area of 10–34 GPa, numerical and analytical results show that theoretical maximum tensile stresses in the inflated upper (solidified) crustal layers of the developing tumuli and lava rises at tens of mega-pascals (MPa). This theoretical stress is orders of magnitude higher than typical in-situ tensile strengths of rocks (0.5–9 MPa) and thus high enough to rupture the crustal layers of the lava rises and tumuli. The high tensile stresses are generated during crustal doming driven by a magmatic overpressure of only about 1 MPa. Our results partly explain the abundance of tension fractures at the surface of the Al-Halaq al Kabir lava flow field. Tumuli observed in the area are remarkably similar in morphology and aspect (height/width) ratios to the flow-lobe tumuli in Holocene lava flow fields in Iceland, suggesting an analogous mechanics of formation. There appears to be a coincidence between the age of an initial volcanism in the AHVP (7.9–5.3 Ma) and a local fauna and flora disappearance in the As-Sahabi area, NE Sirt Basin, during Messinian-Zanclean time (7–5 to 5 Ma). We speculate that the volcanism may have had a local negative environmental impact and contributed to the reported decline in the fauna and flora in the area.

Mechanical interaction between volcanic systems in Libya

The spatial distributions of monogenetic volcanoes, primarily volcanic craters, within the four principal volcanic provinces of Libya are examined and presented on a volcano-density map. Six main volcanic clusters have been identified, referred to as volcanic systems. Remarkably, the Al Haruj (AHVP) and Nuqay (NVP) volcanic provinces have double-peak volcano-density distributions, while the Gharyan (GVP) and As Sawda (SVP) volcanic provinces have single-peak volcano-density distributions. We interpret each volcano-density peak as corresponding to a separate volcanic system, so that there is a total of six systems in these four provinces. There was an overlap in volcanic activity in these provinces with at least three simultaneously active. We propose that each of the 6 volcanic systems was/is supplied with magma from a large sill-like reservoir – similar in lateral dimensions to the systems/clusters themselves. Numerical results show zones of high tensile and shear stresses between the reservoirs that coincide roughly with the main swarms of extension (dykes and volcanic fissures) and shear (faults) fractures in the areas. The most recent volcanic eruptions in Libya fall within the modelled high-stress concentration zones, primarily eruptions in the volcano Waw an Namus and the Holocene Al Mashaqaq lava flow. There are no known eruptions in Libya in historical time, but some or all the volcanic systems may have had one or more arrested historical dyke injections. In particular, part of the recurrent seismic events in the Hun Graben in the northwest Libya may be related to dyke propagation and arrest. If some of the inferred magma reservoirs are still fluid, as is likely, they pose earthquake and volcanic hazards to parts of Libya, particularly to the city of Gharyan and Zallah town, as well as to many oil-field operations.

Distribution and size of lava shields on the Al Haruj al Aswad and the Al Haruj al Abyad Volcanic Systems, Central Libya

The Al Haruj Volcanic Province (AHVP) consists of two distinct volcanic systems. In the north is the system of Al Haruj al Aswad, covering an area of 34,200km2, while in the south the system of Al Haruj al Abyad, covering an area of 7,850km2. The systems have produced some 432 monogenetic volcanoes, primarily scoria (cinder) cones, lava shields, and maars. The density distribution of the volcanoes in each system, plotted as eruption points or sites, has a roughly elliptical surface expression, suggesting similar plan-view geometry of the magma sources, here suggested as deep-seated reservoirs. More specifically, the Al Haruj al Aswad magma reservoir has major and minor axes of 210km and 119km, respectively, and an area of 19,176km2, the corresponding figures for the Haruj al Abyad reservoir being 108km and 74km, for the axes, and 6209km2 for the area. We measured 55 lava shields on the AHVP. They are mostly restricted to the northern and southern parts of AHVP and date from late Miocene to (at least) the end of Pleistocene, while some may have been active into Holocene. In fact, although primarily monogenetic, some of the lava shields show evidence of (possibly Holocene) fissure eruptions in the summit parts. The early lava shields tend to be located at the edges of volcanic systems and with greater volumes than later (more central) shields. The average lava shield basal diameter is 4.5km and height 63m. There is strong linear correlation between lava shield volume and basal area, the coefficient of determination (R2) being about 0.75. When 22 Holocene Icelandic lava shields are added to the dataset, for comparison, the correlation between volume and basal area becomes R2=0.95. Numerical models suggest that the local stress fields favoured rupture and dyke injection at the margins of the source reservoirs during late Miocene – early Pliocene, in agreement with the distribution of the early, large-volume shields.

Volcano-tectonics of the Al Haruj Volcanic Province, Central Libya

The Al Haruj intra-continental Volcanic Province (AHVP), located at the south-western margin of the Sirt Basin, hosts the most extensive and recent volcanic activity in Libya - which is considered typical for plate interiors. From north to south the AHVP is divided into two subprovinces, namely Al Haruj al Aswad and Al Haruj al Abiyad. The total area of the AHVP is around 42,000km2. Despite the great size of the AHVP, its volcano-tectonic evolution and activity have received very little attention and are poorly documented and understood. Here we present new field data, and analytical and numerical results, on the volcano-tectonics of the AHVP. The length/thickness ratio of 47 dykes and volcanic fissures were measured to estimate magmatic overpressure at the time of eruption. The average dyke (length/thickness) ratio of 421 indicates magmatic overpressures during the associate fissure eruptions of 8–19MPa (depending on host-rock elastic properties). Spatial distributions of 432 monogenetic eruptions sites/points (lava shields, pyroclastic cones) in the AHVP reveal two main clusters, one in the south and another in the north. Aligned eruptive vents show the dominating strike of volcanic fissures/feeder-dykes as WNW-ESE to NW-SE, coinciding with the orientation of one of main fracture/fault zones. Numerical modelling and field observations suggest that some feeder-dykes may have used steeply dipping normal-fault zones as part of their paths to the surface.

The petrology and geochemistry of Gharyan volcanic province of NW Libya

Gharyan Volcanic Province (GVP) is one of the four major outpouring volcanic provinces (Jabal as Sawda, Jabal Al Haruj, and Jabal Nuqay) in Libya. It culminates the extreme NNW–SSE linear trend with the other province which defines a systematic decrease in age from NWW to SSE. The voluminous petrologic and chronologic episode in GVP is the Old Lava Series (OLS). This OLS (55–50Ma) is flown over 1000km of Mesozoic rocks that are sliced by Wadi Ghan fault zone. The second cycle is represented by the phonolite–trachyte intrusions (40Ma). Those intrusions occur in the form of laccoliths and plugs where Wadi Ghan fault zone has a conspicuous effect in their emplacement behavior. The Late Volcanic Center (LVC) is the main young volcanic activity in the province (<12Ma). They show different mode of eruptions on the continuous plateau of OLS. Young Lava Series (YLS) are minor eruptions that have a distinctive appearance on the field and landsat image by occupying an ancient buried wadies.The compound phonolite laccoliths in Garyat Gamatat al–Gharyiha area increase in relief northwardly due to the imparity of denudation with the OLS flows at the southern portions of the area. One of those phonolite samples shows a crustal contamination due to its anomalous in Sr content. The radial pattern of OLS flows in urban area has some differentiation degree by their relative evolution from basalts to basaltic andesites. The chemistry of LVC in the same area does not show close concordance with the LVC basanitic suite of Busrewil and Wadsworth (1980). On other hand, the basaltic sill in Garyat Gamatat al–Gharyiha area is belonging to the LVC activity in GVP by their concordant with the geochemistry of LVC basanitic phase. The ultrabasic rocks in the investigated areas are four-phase lherzolite (olivine+orthopyroxene+clinopyroxene+spinel) restrict only in LVC phase as sub-rounded nodules. These xenoliths penetrate the whole crust with relatively large velocity by melts with sub-continental mantle source (OIB) beneath GVP.

Geochemistry and Petrology of Basic Volcanic Rocks of Jabal Al Haruj Al-Aswad, Libya

The Al Haruj Intra-continental Volcanic Province is the largest part of the extensive volcanic activity in Libya which is considered to be a typical within plate basalts. The volcano-tectonics evolution of this province, as well as its origin, are still widely disputed. According to K-Ar dating previously studied, the volcanic activity started in the Late Miocene and lasted until at least the Late Pleistocene. The field may still be volcanically active. The mafic rocks of Jabal Al Haruj have been classified into six major phases or groups. These phases have been differentiated using Landsat images together with aerial photographs of different scales as well as field observations. The topographic forms of the earliest phase are highly eroded while the forms of the latest phase are usually fresh and very well preserved as regards primary features. Mafic lavas of this field consist of alkali basalts to olivine tholeiites (transitional basalt) which contain olivine as essential constituent together with clinopyroxene, plagioclase and glass. The basalt exhibits intergranular, intersertal, ophitic and subophitic relations. Amygdaloidal and glomerporphyritic textures are also observed. The basaltic rocks of different ages and from different localities are petrographically rather similar. Phenocrysts of olivine probably the result from slow cooling in crustal magma chambers prior to eruptions, suggesting that magmas ascended slowly through the crust. 109 samples have been carefully collected from various phases, some of these samples have been chosen for major and trace elements analyses, using XRF in order to determine the characteristics of the mantle source and investigate crustal interaction. The major and trace elements revealed a slightly significant chemical diversity among the phases and within each phase. The normative classification of most of these rocks shows close agreement with their modal classification. A vague correlation between MgO and most major oxides in the studied samples suggests different degrees of partial melting rather than fractional crystallization. A characteristic feature of the studied volcanic rocks is the relatively constant ratios of certain incompatible trace elements (Nb/Zr, Rb/Zr), which provides strong evidence of a common source. In addition, the rocks display similar patterns of the peaks and troughs; this strongly suggests that they have a common parent and common subsequent processes. The compatible transitional metals Ni (81 - 193 ppm) and Cr contents (238 - 361 ppm) and relatively low Mg# (Mg/(Mg + Fet)) (52 - 62) give an indication that the studied basaltic rocks have slightly to moderately fractionated olivine and/or spinel. The magmatism of this volcanic field seems to be related to reactivation of pre-existing structures during the passive rifting of the Sirt Basin that most likely produced in response to convergence between European and African plates since Jurassic until Holocene times.

The Kufrah paleodrainage system in Libya: A past connection to the Mediterranean Sea?

Paillou et al. (2009) mapped a 900 km-long paleodrainage system in eastern Libya, the Kufrah River, that could have linked the southern Kufrah Basin to the Mediterranean coast through the Sirt Basin, possibly as long ago as the Middle Miocene. We study here the potential connection between the terminal part of the Kufrah River and the Mediterranean Sea through the Wadi Sahabi paleochannel, which may have constituted the northern extension of the lower Kufrah River paleodrainage system. New analysis of SRTM-derived topography combined with Synthetic Aperture Radar images from the Japanese PALSAR orbital sensor allowed the mapping of seven main paleochannels located west of the Kufrah River, each of which is likely to have formed a tributary that supplied water and sediment to the main paleodrainage system. The northernmost four paleochannels probably originated from the Al Haruj relief, a Pliocene alkaline basaltic intracontinental volcanic field, and potentially connected to the Wadi Sahabi paleochannel. The remaining three paleochannels are in the more southerly location of the Sarir Calanscio, North-East of the Tibesti mountains, and barely present a topographic signature in SRTM data. They end in the dunes of the Calanscio Sand Sea, forming alluvial fans. The most southern paleochannel, known as Wadi Behar Belama, was previously mapped by Pachur (1996) using LANDSAT-TM images, and was interpreted by Osborne et al. (2008) as representing part of an uninterrupted sediment pathway from the Tibesti mountains to the Mediterranean Sea. Processing of SRTM topographic data revealed local depressions which allow to connect the seven paleochannels and possibly the terminal alluvial fan of the Kufrah River to the Wadi Sahabi paleochannel, through a 400 km-long, south-north oriented, paleocorridor. These new findings support our previous hypothesis that proposed a connection between the lower Kufrah River in the region of the Sarir Dalmah and the Wadi Sahabi paleochannel, which connected to the Mediterranean Sea. Including the newly mapped paleochannels, the Kufrah River paleowatershed, at its maximum extent, would have covered more than 400,000 km2, representing close to a quarter of the surface area of Libya.

Miocene to recent alkaline volcanism between Al Haruj and Waw an Namous (southern Libya)

Unspiked K–Ar ages, petrological, geochemical and isotopic data are reported on samples from southern Libya (Wan an Namous—Al Haruj area). The Wan an Namous intracaldera cone dated at 0.2 Ma consists of unusually undersaturated foidite, representing the most extreme compositions among Libyan and Tibestian lavas. A basanitic and a basaltic lava flow located north-west of Wan an Namous, and probably belonging to the Al Haruj volcanic field, were dated at 5.1 Ma and 8.1 Ma. These data extend the range of ages previously reported for Al Haruj lavas. REE and multi-element patterns are typical of alkaline intraplate magmas. Sr ratios range from 0.70314 to 0.70812, whereas Nd ratios are very homogeneous (0.51290–0.51293). Pb ratios (19.231 < 206Pb/204Pb < 19.547, 15.607 < 207Pb/204Pb < 15.641 and 38.859 < 208Pb/204Pb < 39.242) are typical of HIMU-FOZO compositions. Such isotope characteristics are very similar to those available on two Gharyan (northern Libya) lavas and largely overlap those of Hoggar and Cameroon Line alkaline rocks. These lavas were produced by low and variable degrees of partial melting of a garnet- and amphibole-bearing mantle source, constraining the depth of melting between 80 and 150 km. Crustal contamination was also probably involved for the oldest sample. Mineral compositions of a dunite–harzburgite xenolith clearly indicate that the lithospheric mantle was affected by partial melting and metasomatic processes by magmatic liquids, probably associated with the genesis of Cenozoic lavas. Lithospheric delamination and asthenospheric upwelling, due to the reactivation of lithospheric megastructures induced by the Africa-Europe convergence, could represent a model for the genesis of Libyan lavas, as in Hoggar.

Petrogenesis of the eastern part of the Al Haruj basalts (Libya)

The eastern part of the Al Haruj volcanic field (central Libya) is dominated by lava flows with subordinate dykes as well as scoria and spatter cone deposits which formed around 4–0.5 Ma. These rocks are olivine- clinopyroxene- and rarely plagioclase-phyric alkaline to transitional basalts with intersertal to intergranular groundmass. They are fairly primitive rocks (MgO and Ni contents ∼10 wt.% and >130 ppm, respectively). A positive correlation between MgO contents and Ni and Cr concentrations is suggestive of olivine (±spinel, ±Fe–Ti-oxide) fractionation. The rocks show LREE- and incompatible trace element enriched patterns on chondrite- and primitive mantle-normalized spider diagrams, respectively. Geochemical modeling revealed that the composition of most East Al Haruj basalts can be reproduced by ∼5% melting of an undepleted, garnet-bearing peridotitic source, similar in composition to primitive asthenospheric mantle and typical of within-plate setting. The most incompatible element enriched samples could either represent extremely small degrees of melting of a similar source or, more probably, magmas derived from a more enriched, (lithospheric?) source. East Al Haruj magmatism occurred from predominantly asthenospheric-derived melts which formed and transported along fundamental sub-lithospheric faults of the Tibesti-Sirt basement. These faults were likely related to the late-stage evolution of the Sirt Basin characterized by NW–SE compression and perpendicular tension.

Palaeohydrology of the Fazzan Basin and surrounding regions: The last 7 million years

We have integrated information on topography, geology and geomorphology with the results of targeted fieldwork in order to develop a chronology for the development of Lake Megafazzan, a giant lake that has periodically existed in the Fazzan Basin since the late Miocene. The development of the basin can be best understood by considering the main geological and geomorphological events that occurred thought Libya during this period and thus an overview of the palaeohydrology of all Libya is also presented. The origin of the Fazzan Basin appears to lie in the Late Miocene. At this time Libya was dominated by two large rivers systems that flowed into the Mediterranean Sea, the Sahabi River draining central and eastern Libya and the Wadi Nashu River draining much of western Libya. As the Miocene progressed the region become increasingly affected by volcanic activity on its northern and eastern margin that appears to have blocked the River Nashu in Late Miocene or early Messinian times forming a sizeable closed basin in the Fazzan within which proto-Lake Megafazzan would have developed during humid periods. The fall in base level associated with the Messinian desiccation of the Mediterranean Sea promoted down-cutting and extension of river systems throughout much of Libya. To the south of the proto Fazzan Basin the Sahabi River tributary know as Wadi Barjuj appears to have expanded its headwaters westwards. The channel now terminates at Al Haruj al Aswad. We interpret this as a suggestion that Wadi Barjuj was blocked by the progressive development of Al Haruj al Aswad. K/Ar dating of lava flows suggests that this occurred between 4 and 2 Ma. This event would have increased the size of the closed basin in the Fazzan by about half, producing a catchment close to its current size (~ 350,000 km 2). The Fazzan Basin contains a wealth of Pleistocene to recent palaeolake sediment outcrops and shorelines. Dating of these features demonstrates evidence of lacustrine conditions during numerous interglacials spanning a period greater than 420 ka. The middle to late Pleistocene interglacials were humid enough to produce a giant lake of about 135,000 km 2 that we have called Lake Megafazzan. Later lake phases were smaller, the interglacials less humid, developing lakes of a few thousand square kilometres. In parallel with these palaeohydrological developments in the Fazzan Basin, change was occurring in other parts of Libya. The Lower Pliocene sea level rise caused sediments to infill much of the Messinian channel system. As this was occurring, subsidence in the Al Kufrah Basin caused expansion of the Al Kufrah River system at the expense of the River Sahabi. By the Pleistocene, the Al Kufrah River dominated the palaeohydrology of eastern Libya and had developed a very large inland delta in its northern reaches that exhibited a complex distributary channel network which at times fed substantial lakes in the Sirt Basin. At this time Libya was a veritable lake district during humid periods with about 10% of the country underwater.

Mineral chemistry of Al Haruj low-volcanicity rift basalts, Libya: Implications for petrogenetic and geotectonic evolution

Al Haruj basalts represent the largest volcanic province in Libya (∼16,000 km 2). Field, petrographic and mineral compositional studies of the NW corner of this province have been undertaken in order to better understand their petrogenetic and geotectonic evolution. These basalts have been distinguished into two phases of different field and mineralogical characteristics. Clinopyroxene compositions classified the earlier phase (AHB-I) as subalkaline, tholeiitic basalts and the later, more voluminous, phase (AHB-II) as alkaline ones. Clinopyroxene and chromian spinel compositions suggest anorogenic within plate affinity for basalts of the two phases. Chromian spinel of the Al Haruj basalts shows compositional similarities to those of the ocean island basalts (OIB). Field and mineralogical features of the Al Haruj and Libyan volcanics in general are typical of the low-volcanicity rifts. The Libyan continental rift zone is probably related to the larger Afro-Arabian rift system. The low Mg# of clinopyroxene phenocrysts (59–81 and 50–73 for AHB-I and AHB-II, respectively) and olivine groundmass crystals (60–71 and 53–66 for AHB-I and AHB-II, respectively), implying the evolved nature of these rocks with the AHB-II are relatively more fractionated. Clinopyroxene, olivine and chromian spinel compositions suggest that the AHB-II were crystallized at higher temperature and pressure, probably at greater depth, compared to the AHB-I. It is proposed that the tholeiitic basalts (AHB-I) represent pre-rift stage magma, probably generated by higher degree of partial melting of asthenospheric mantle source, at shallow depth, whereas the dominant alkaline basalts (AHB-II) represent the rift stage magma formed by low degree of partial melting of the same source, at greater depth.

How Strombolian is a “Strombolian” scoria cone? Some irregularities in scoria cone architecture from the Transmexican Volcanic Belt, near Volcán Ceboruco, (Mexico) and Al Haruj (Libya)

Scoria cone modelling focuses on ballistic (no-drag) ejection that are termed Strombolian as a result of weak-intensity, strongly intermittent activity associated with bursting of gas bubbles resulting ballistic emplacement of clasts 10+ cm. Particles with a size < 10 cm are normally unable to follow ballistic trajectories instead depositing from sub-Plinian eruption clouds. The Neogene volcanic field near Volcán Ceboruco in the San Pedro–Ceboruco graben, Mexico includes fissure fed flows, domes, and monogenetic scoria cones. The scoria cones near Ceboruco consist of normal (proximal) to inverse (distal) graded welded and/or non-welded scoria lapilli and coarse ash. The grain size pattern observed from scoria cones at Ceboruco is not consistent with the classic ballistic model of cone growth. Instead it is more consistent with recently proposed model where cones grow by accumulation of clasts falling from a sustained eruption column. Al Haruj a Miocene to Holocene intracontinental basaltic volcanic field in Libya preserves pyroclastic rocks indicating hot emplacement from eruptions of scoria cones and lava fountains. However, craters are commonly wide and surrounded by rims of strongly welded tephra, they closely resemble maar-structures. It is inferred that magmatic fragmentation of the uprising melt often has changed to phreatomagmatic interaction leading to enigmatic explosive events that have removed the top of the volcanic cones and produced maar-like depressions. In the two fields at least four different types of scoria cones have been distinguished that indicates a far greater diversity in eruptive mechanisms of scoria cones than those proposed by earlier researchers.

The morphology and origin of wide craters at Al Haruj al Abyad, Libya: maars and phreatomagmatism in a large intracontinental flood lava field?

The morphology and origin of wide craters at Al Haruj al Abyad, Libya: maars and phreatomagmatism in a large intracontinental flood lava field?