THE SPACE PROBLEM OF CALDERA RESURGENCE - AN EXAMPLE FROM ISCHIA ISLAND, ITALY

A space problem can arise in a resurgent caldera when the resurgent bl ock is non-cylindrical, such as, for example, when it is bounded by in ward-or outward-dipping faults. Ischia caldera (Italy) is an excellent case study because it is well exposed and resurgence is ongoing. On t he western and eastern flanks of the Ischia resurgent horst, uplift oc curred along NNW-striking normal faults with inclination from sub-vert ical to vertical (>85 degrees). The geometry of these faults suggests negligible extension within the horst. Along the northern flank, uplif t was accomplished by ENE-striking normal faults that dip 60-85 degree s outward; a few bear striae which indicate almost pure dip-slip. The southern flank of the horst is a monocline trending ENE associated wit h vertical faults. In a NNW-SSE section, the resulting resurgent horst has a wedge shape with an upward apex. The uplift of this wedge can b e accommodated by contemporaneous regional extension along NE-to east- west striking normal faults whose motions create space for resurgence without deformation of the caldera floor. Similar interaction with reg ional tectonics could exist in other calderas, such as Yellowstone (US A) in an extensional setting, Los Azufres (Mexico) in a transtensional regime and Chalupas (Ecuador) in a transpressional one. At other cald eras, resurgence was accommodated by caldera-floor arching as at Valle s (USA) or by shortening deformations between the caldera rim and the uplifting block as at Latera (Italy).