The metamorphic core of the Northern Apennines is exposed in the Alpi Apuane which form a topographic high located to the west of the axial zone of the range. The metamorphic rocks of the Alpi Apuane are bordered by a tectonic contact which separates a footwall affected by maximum P and T conditions ranging between 0.6-0.8 Gpa and 420-500°C (Massa unit) and 0.4-0.6 Gpa and 350-420°C (Apuane unit), from a hangingwall of unmetamorphosed Macigno sandstone. The metamorphic rocks of the Alpi Apuane have long been interpreted as an example of tectonic exhumation formed during active convergence in the Apennines. Here we report 53 new zircon and apatite (U-Th)/He (ZHe and AHe, respectively) ages on rocks from the Alpi Apuane and the Macigno sandstones to provide a much more detailed understanding of when the Apuane fault was active and how much section was cut out. The ZHe and AHe systems provide cooling ages corresponding to closure temperatures of 180°C and 70°C, respectively. ZHe ages in the crystalline core of the Alpi Apuane are all reset and vary around an average of 5.7 Ma (std deviation 1.2 Ma). In the hangingwall, the Macigno sandstones have ZHe ages which vary between 47.1 and 9.3 Ma, indicating partial resetting. This result indicates that the hangingwall saw a temperature no greater than 160-190 °C, followed by cooling around 9 Ma. AHe ages in the Alpi Apuane and in the Macigno sandstone show no differences and vary around a mean of 4.9 Ma (std deviation 1.2 Ma). Our data clearly constrain different exhumation paths for the Alpi Apuane and the Macigno sandtones between 180°C and 70°C, whereas their paths are similar from 70°C to the surface. The different thermal histories from 180°C to 70°C suggest that between 6 and 4 Ma, 4±1 km have been removed along the contact between the Macigno sandstones and the Alpi Apuane. The amount of crustal section removed has been determined assuming a geothermal gradient of 25±°C/km, and a cooling age of 8±1 Ma at 110°C for the Macigno sandstones. The geometry of the major Apuane boundary and the different exhumation times suggest that extension under brittle conditions played a major role in the exhumation of the metamorphic core of the Apennines.
Exhumation of the Northern Apennines core: new thermochronological data from the Alpi Apuane / Fellin M.G.; Reiners P.W.; Brandon M.T.; Molli G.; Balestrieri M.L.; Zattin M.. - ELETTRONICO. - (2004). (Intervento presentato al convegno 32nd International Geological Congress tenutosi a Firenze nel 20-28 Agosto 2004).
Exhumation of the Northern Apennines core: new thermochronological data from the Alpi Apuane
FELLIN, MARIA GIUDITTA;ZATTIN, MASSIMILIANO
2004
Abstract
The metamorphic core of the Northern Apennines is exposed in the Alpi Apuane which form a topographic high located to the west of the axial zone of the range. The metamorphic rocks of the Alpi Apuane are bordered by a tectonic contact which separates a footwall affected by maximum P and T conditions ranging between 0.6-0.8 Gpa and 420-500°C (Massa unit) and 0.4-0.6 Gpa and 350-420°C (Apuane unit), from a hangingwall of unmetamorphosed Macigno sandstone. The metamorphic rocks of the Alpi Apuane have long been interpreted as an example of tectonic exhumation formed during active convergence in the Apennines. Here we report 53 new zircon and apatite (U-Th)/He (ZHe and AHe, respectively) ages on rocks from the Alpi Apuane and the Macigno sandstones to provide a much more detailed understanding of when the Apuane fault was active and how much section was cut out. The ZHe and AHe systems provide cooling ages corresponding to closure temperatures of 180°C and 70°C, respectively. ZHe ages in the crystalline core of the Alpi Apuane are all reset and vary around an average of 5.7 Ma (std deviation 1.2 Ma). In the hangingwall, the Macigno sandstones have ZHe ages which vary between 47.1 and 9.3 Ma, indicating partial resetting. This result indicates that the hangingwall saw a temperature no greater than 160-190 °C, followed by cooling around 9 Ma. AHe ages in the Alpi Apuane and in the Macigno sandstone show no differences and vary around a mean of 4.9 Ma (std deviation 1.2 Ma). Our data clearly constrain different exhumation paths for the Alpi Apuane and the Macigno sandtones between 180°C and 70°C, whereas their paths are similar from 70°C to the surface. The different thermal histories from 180°C to 70°C suggest that between 6 and 4 Ma, 4±1 km have been removed along the contact between the Macigno sandstones and the Alpi Apuane. The amount of crustal section removed has been determined assuming a geothermal gradient of 25±°C/km, and a cooling age of 8±1 Ma at 110°C for the Macigno sandstones. The geometry of the major Apuane boundary and the different exhumation times suggest that extension under brittle conditions played a major role in the exhumation of the metamorphic core of the Apennines.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
