Deposition of rheomorphic ignimbrite D (Mogan Formation), Gran Canaria, Canary Islands, Spain

Rheomorphic ignimbrite D (13.4 Ma, Upper Mogan Formation on Gran Canaria), a multiple flow-single cooling unit, is divided into four major structural zones that differ in fabric and finite strain of deformed pyroclasts. Their structural characteristics indicate contrasting deformation mechanisms dur ing rheomorphic flow. The zones are: (a) a basal zone (vitrophyre) with pur l uniaxial flattening perpendicular to the foliation; (b) an overlying shea r zone characterized by asymmetric fabrics and a significantly higher finit e strain, with an ellipsoid geometry similar to stretched oblate bodies; (c ) a central zone with a finite strain geometry similar to that of the under lying shear zone but without evidence of a rotational strain component; and (d) a slightly deformed to non-deformed top zone where the almost random o rientation of subspherical pyroclasts suggests preservation of original, sy n-depositional clast shapes. Rheomorphic flow in D is the result of syn- to post-depositional remobilization of a hot pyroclastic flow as shown by kin ematic modeling based on: (a) the overall vertical structural zonation sugg ested by finite strain and fabric analysis; (b) the relation of shear sense to topography; (c) the interrelationship of the calculated vertical coolin g progression at the base of the flow (formation of vitrophyre) and the rel ated vertical changes in strain geometry; (d) the complex lithification his tory; and (e) the consequent mechanisms of deformational now. Rheomorphic f low was caused by load pressure due to an increase in the vertical accumula tion of pyroclastic material on a slope of generally 6-8 degrees. We sugges t that every level of newly deposited pyroclastic flow material of D first passed through a welding process that was dominated by compaction (pure fla ttening) before rheomorphic deformation started.