Modes and times of caldera resurgence: The < 10 ka evolution of Ischia Caldera, Italy, from high-precision archaeomagnetic dating

摘要

Ischia is a well exposed and densely populated late Quaternary caldera in the Campanian magmatic province of Italy. Ischia Caldera experienced an average uplift rate of 3.3 cm/year in the last ca. 30 ka and is still actively resurging. During the last 10 ka, coeval with the resurgence, a volcanic field of alkali-trachytic to trachyandesitic lava domes, lava flows, tuff and scoria rings, and pumice cones developed, mainly on the eastern sector of the caldera, along both resurgence-related faults and regional NNW- and NE-striking faults. In order to improve both our understanding of the recent volcanic history and the evaluation of future risks on Ischia Island, a high-precision archaeomagnetic dating method was applied to the products of 12 volcanic centres with probable age <10 ka. Accurate paleofield directions with a median 95% confidence angle of 1.47 were measured on 277 lava and spatter samples (25 sites). Coupled with the reference curves for secular variation in the western Mediterranean sea (during the last 3000 years) and eastern Europe (from 3000 to 8000 years ago), ages of between 4100 BC and 355 AD were obtained. These archaeomagnetic data were supported by a volcanologic and stratigraphic survey and are consistent with written sources, archaeological findings, and previous isotopic (K/Ar and ~(14)C) ages.rnArchaeomagnetic and other geochronological data, as well as stratigraphic constraints, show that, during the studied time interval, Ischia volcanism occurred in five periods separated by phases of quiescence and coeval with earthquake and landslide events. This fact suggests a pulsating mode of uplifting and deformations of the Ischia resurgence.rnDuring the two oldest periods of activity (7200-6800 BC and 4100-2300 BC), resurgence probably produced a dome-shaped structure. Location and geometry of vents suggest the occurrence of magma uprise along the fractures produced by bending of the overburden crustal block. Most of magma was emplaced as intrusions at the interior of the resurgent block, whereas volcanism was represented by very viscous, differentiated, and crystallized lavas that emplaced as domes and high aspect-ratio flows. The resurgent dome caused recurrent lateral collapses that removed about 2.5 km~3 of rocks. During the three youngest periods of activity (1800-1000 BC; 650 BC -355 AD; and 1302 AD), resurgence affected a fault-bounded, asymmetric block. This resulted from both (a) hydrostatic rebound of the crustal block after removal of material involved into huge slope instability triggering an increase in uplift rate, and (b) new influxes of less evolved magma batches into the shallow reservoir that, in turn, favoured the intense volcanism of the last 4000 years.