Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy

摘要

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U-Th)/He and fission-track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally overlying, very low grade Macigno Formation. Stratigraphic, metamorphic, and thermochronologic data indicate that the Apuane rocks were structurally buried to 15-30 km and ～400℃ at about 20 Ma. Exhumation to 240℃ and 9 km depth (below sea level) occurred at 10-13 Ma. By 5 Ma the Apuane rocks were exhumed to 70℃ and ～2 km. The Macigno and associated Tuscan nappe were also structurally buried and the Macigno reached its maximum depth of 7 km at ～15 to 20 Ma. Stratigraphic evidence indicates that the Apennine wedge was submarine at this time. Thus we infer that initial exhumation of the Apuane was coeval with tectonic thickening higher in the wedge, as indicated by synchronous structural burial of the Tuscan nappe. From 6 to 4 Ma, thinning at shallow depth is indicated by continued differential exhumation between the Apuane and the Tuscan nappe at high rates. After 4 Ma, differential exhumation ceased and the Apuane and the Tuscan nappe were exhumed at similar rates (～0.8 km/Ma), which we attribute to erosion of the Apennines, following their emergence above sea level.