Fracture and surface crust development in a Holocene pahoehoe lava flow onthe Island of Tenerife, Canaries

An almost horizontal pahoehoe surface in a Holocene plagioclase basanite la va on Tenerife displays three scales of fracture within the surface crust. An early-formed jet of large-scale fractures divides up the surface into an orthogonal set of rectangular slabs with dimensions of several metres and depths of 10-12 cm. The shortest slab dimension is parallel to the Row empl acement direction, inferred from a strong surface lineation. The slabs are domed with the centre an average of 9.6 cm (with range 4-19.6 cm) above the edges of the slabs. Profiles of the slabs normal and perpendicular to the margins and through the crest indicate that they can be described by a powe r law in which the deflection of the slab, h, is related to the distance fr om the crest, x, with an exponent between 2 and 3. Analysis of joints withi n the slabs indicates two smaller scale networks. An intermediate scale joi nt network bounds blocks with rectilinear to polygonal shapes in plan-view and has a characteristic mean spacing of 24.2 cm (range 10.5-48 cm). The ma jor Fractures in this set are normal and parallel to the slab margins. A sm aller-scale joint network bounds polygonal equant blocks in plan-view and h as characteristic spacing of 6.4 cm (range 3.7-10.5 cm). A model of cooling from the pahoehoe surface is used to constrain the growth of the crust aci d timing of fracture development. The large-scale slabs are attributed to l ocalised accumulation of gas beneath the growing crust causing buoyant forc es. The tensile stresses caused by uplift are sufficient to form the large- scale fractures after 2 or 3 h of cooling, The intermediate scale fracture network is attributed to the flexure of the slab crust. The smaller scale p olygonal joint network is related to the build up of isotropic tensile stre sses in the cooling slab crust due to thermal contraction with fracture dev elopment being promoted by the flexure of the slabs. An analysis of the sla b deformation indicates that lava crust is weak. The weakness is explained by division of the crust into thr rr zones: an outer zone with small scale joints that cause negligible strength, a middle zone of elastic behaviour i n which stress can build up, and a lower zone of plastic deformation. The c rustal slabs display profiles similar to that expected in a bending elastic plate. The deformation of the 10-cm-thick crust can be explained if the el astic zone was about 2-cm thick. This result agrees with an independent cal culation of elastic zone thickness based on the position of the brittle-duc tile transition being located Lit the 600 degreesC isotherm at a depth of a bout 2 cm when the crustal slabs were rifted apart. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.