This work is now published in the July 2010 issue of Geological Society of America Bulletin
Thomson, S.N., Brandon, M.T., Reiners, P.W., Zattin, M., Isaacson, P.J. & Balestrieri, M.L. (2010). Thermochronologic evidence for orogen-parallel variability in wedge kinematics during extending convergent orogenesis of the northern Apennines, Italy, Geological Society of America, Bulletin, 122, p. 1160-1179, doi: 10.1130/B26573.1
The northern Apennine orogenic wedge is one of several active convergent orogens world-wide that exhibit syn-convergent extension. Numerous geodynamic models have been proposed to explain this apparent paradox, including slab retreat, slab detachment, orogenic collapse, buoyant wedge escape, and wedge underplating. However, no single model currently explains well all observed tectonic, geomorphic and geologic features in such orogens, and none does a good job of coupling mantle dynamics with critical wedge mechanics. The advent of advanced numeric geodynamic and surface process computer models are helping advance our understanding of orogen dynamics. However, to validate such models, they must be able to satisfy real-world observations, including the regional pattern and history of erosional and tectonic denudation within the orogen, and by inference, the long-term record of erosional mass flux as recorded by multi-chronometer low-temperature thermochronology.
Our investigation has yielded 146 new surface apatite (U-Th)/He (AHe) ages from 93 bedrock samples, including three age-elevation transects, as well as 14 new AHe ages and 11 new apatite fission track ages (AFT) from three boreholes. Our results complement an existing regional database of about 160 AFT ages collected by our numerous Italian collaborators over the last 15 years.
Complementary analysis of 146 apatite (U-Th)/He (AHe) and 6 apatite fission track (AFT) thermochronometric ages highlights strong post-late Miocene orogen-parallel differences in wedge kinematics and exhumation history of the northern Apennine extending convergent orogen, with a western segment dominated by vertical material motion and an eastern segment dominated by horizontal motion. The transition is situated at ca. 11°30'E and coincides with the previously recognized orogen transverse Sillaro line. Age patterns and age-elevation relationships (AER) in the eastern segment are diagnostic of ongoing frontal accretion and slab retreat consistent with a northeastward migrating 'orogenic wave'. Enhanced erosion at rates of ca. 1 mm/yr occurs for ca. 3 to 5 Ma on the transient contractional frontal flank and ca. 0.3 mm/yr on the extending internal flank accompanied by relative material velocities for frontal accretion and retro-flank excretion of ca. 13-17 km/Myr, and a ca. 7 to 8 km/Myr rate of horizontal material motion through the core of the orogen. West of the Sillaro line the post-late Miocene age pattern and AER record ongoing exhumation restricted to the core of the range since at least ca. 8 Ma at rates of ca. 0.4 mm/yr increasing to ca. 1 mm/yr in the Pliocene (ca. 3 Ma) accompanied by post-Pliocene tilting and associated out-of-sequence faulting. Over the same time the frontal flank saw less than ca. 2 km of erosion. This pattern can be attributed to either continued convergence, but a switch in the transfer of material into the wedge to a regime dominated by underplating or shortening in the form of out-of-sequence thrusts, or a slow down or cessation of frontal accretion and slab retreat with related enhanced Pliocene uplift and erosion triggered by a deeper seated process such as lithospheric delamination, complete slab detachment, or slab tear. These findings emphasize that no single model of wedge kinematics is likely appropriate to explain long-term northern Apennine orogenesis and syn-convergent extension, but rather that different lithospheric geodynamic processes have acted at different times in different lateral segments of the orogen.
Figure 1. Location map of new apatite (U-Th)/He ages obtained from the northern Apennines overlain on SRTM 90m resolution topography
Figure 2. Apatite fission-track age contour map.
Figure 3. Apatite (U-Th)/He age contour map.
Figure 4. Apatite fission-track time-averaged erosion rate (edot) contour map.
Figure 5. Apatite (U-Th)/He time-averaged erosion rate (edot) contour map.
Figure 6. Apatite (U-Th)/He and fission-track age transects across the northern Apennines (see Figure 1 for location of profiles). The top figure for each transect shows the max, mean, and min elevation profiles for a 20 km wide transect with the location and age for each sample projected onto the plane of the transect. The lower figure shows the AFT (blue) and AHe (green) ages. Note the relative offset of the reset front and age minima of the higher temperature AFT system in the direction of material flux in profile B-B'.
Figure 7. AFT and AHe age-elevation relationships (AER) for three high-relief transects (Mt. Cimone, Mt. Falterona, and Val d'Arno). Upper plots (a, b, c) show unmodified AER. Best-fit exhumation rates and errors calculated from difference in slope from x on y, and y on x least squares linear regression for each thermochronometer. Lower plots (d, e, f) show relationship between age and the height of each sample above the closure isotherm calculated using a 3D topographic and heat advection corrected thermal model. The slopes calculated from these plots give exhumation rates adjusted for both the 3D topography of the closure isotherm surface and the advection of heat during enhanced exhumation.
Figure 8. Paired Apatite (U-Th)/He and fission-track ages converted to closure depth using the Brandon et al. (1998) "Age2Edot" thermal model. This plot shows the apparent NE migration (at a rate of 18-25 km/Myr) of a locus of maximum denudation rates of ca. 1mm/yr.
Figure 9. (a) Simple wedge model to predict AFT and AHe age data and total erosion estimate across northern Apennines shown in (b). Profile predicted ages are calculated from the time to erode to mean closure depth of 3720m for AFT and 1730m for AHe (as calculated using a topographic and heat advection corrected 3D thermal model ). Dotted lines in (b) are predictions for different velocity and erosion rate input values - see text for details. (c and d) Comparison of model predicted values with actual AFT and AHe age data obtained from NW-SE sample swath profiles B-B' and A-A' (Figures 2 and 4), respectively.
Last Modified: August 6th, 2010