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.