Effects of Mechanical Layering on the Development of Normal Faults and Dykes in Iceland

Abstract

Dykes and normal faults are the most common large-scale tectonic elements in Iceland. The dykes are commonly offset, and some arrested, at contacts between layers. At most such contacts there are stiff (high Young's modulus) lava flows in contact with soft (low Young's modulus) scoria, soil, or pyroclastic or sedimentary layers. Soft layers and weak contacts tend to alter the local stresses, suppress the tensile stresses at dyke tips, and dissipate the surface stresses generated by arrested dykes. Mechanical layers of similar properties are normally thicker in the Tertiary lava pile (mainly basaltic lava flows) than in the Pleistocene pile (basaltic lava flows alternating with basaltic breccias and sediments). The heights of dyke segments in individual mechanical layers largely control the segment thicknesses; the segments in the Tertiary pile tend to be higher, and the dykes thus thicker (4-6 m), than in the Pleistocene pile (1-2 m). The normal faults also tend to be segmented on passing from one mechanical layer to another. In particular, the dips of the faults are commonly steeper in the stiff basaltic lava flows than in the softer sedimentary and hyaloclastite layers. As an active fault develops, the fault rocks (the fault core and the damage zone) gradually become softer so that, for a given driving shear stress, the fault slip in individual seismic events tends to increase with time.