Depths of magma chambers at three volcanic provinces in the Karliova region of Eastern Turkey

Abstract

The size of a volcanic eruption, and thus the associated potential hazards, depends partly on the depth, geometry,and size of the source magma chamber. To estimate magma chamber depths and sizes, we apply a newly developedanalytical method, based on the aspect ratio (length/thickness) of dikes, to three volcanoes in the Karlıova regionof Eastern Turkey, namely Turnada˘g, Varto, and Özenç. The results indicate that the depths of the source chambersare between 2 and 4 km at Turnada˘g, 2 to 5 km at Varto, both of which are located in transtensional tectonicregimes, but from 22 to 27 km at Özenç, which is located in a convergent tectonic regime. A similar reservoir depthat Özenç is indicated by seismic tomography, and this data also suggests that the reservoir is laterally continuousfor more than 40 km. The large volume of ignimbrites (> 40 km3) associated with Varto, a collapse caldera, indicatesthat caldera subsidence may have maintained the excess magmatic pressure (through tectonic forcing) inthe chamber over a longer time than during normal pyroclastic eruptions. The dike aspect ratios further indicatemagmatic overpressures of 13–21 MPa for Varto, 13–17 MPa for Turnada˘g, and 26–31 MPa for Özenç. The combinedresults from seismic tomography, analytical models and magma compositions indicate that both Turnada˘gand Varto volcanoes, which are typical stratovolcanoes composed of mostly intermediate, and more rarely, acidicmagmas, were fed by two very shallow and comparatively small magma chambers (2–5 km depth). Whereas lessevolved magmas were erupted from Özenç, which hosts predominantly basaltic and intermediate lavas and dikesthat were fed by a deep reservoir at 22–27 km depth. Our tomographic models show that none of the volcanoes arelocated directly over the center of a deep magma reservoir. Our data also indicates that the magma in the reservoirhas migrated between 34 and 40 km in a right lateral motion (to the east) below Varto and Turnada˘g, respectively,and 23 km in a left lateral motion (to the west) at Özenç over the past 3 Ma. This lateral propagation of magmacan be explained by tectonic escape of the Anatolian block to the west through the Northern Anatolian Fault andthe Varto Fault Zone over the last 6 Ma.