Reconstruction of rotational history in the Apennines arcs
The huge quantity of paleomagnetic data collected in the last few decades in Northern Apennines and the Calabrian Arc enables reconstruction of the rotational history of these two arcs. In particular, the totality of data allows to describe a realistic distribution of tectonic rotations in space and time along the arcs.
NORTHERN APENNINES
In order to describe the rotational history of the Northern Apennines, we collected published paleomagnetic data available in this region. A total of 375 data have been computed, collected either for tectonic or magnetostratigraphic purposes from Jurassic to Pleistocene strata. In this analysis, paleomagnetic rotations were calculated, according to Demarest (1983), by comparing Northern Apennines paleomagnetic results with coeval expected African paleopoles computed by Besse and Courtillot (2002).
The overall data set is represented in Fig. 3, where paleomagnetic rotations have been differentiated according to the age of the sampled rocks. In this figure the different rotational pattern between the external and the internal sector of the chain is clearly highlighted. While the internal sector of the Tyrrhenian margin is characterised by an irrotational pattern with any tectonic rotations in the last 5 million years, the external sector is characterised by a complex pattern of tectonic rotations, which involves Jurassic to Pleistocene strata. In the following,paleomagnetic data from the Tyrrhenian margin will be excluded whereas paleomagnetic data collected in the orogenic nappes, where the rotations are confined, will be considered in our analysis.
In general, in the external arc, paleomagnetic data show a correlation between the changes in paleomagnetic declination and changes in the orogenic trend, with the exception of the northwestern sector, represented by the Tertiary Piedmont Basin (Fig. 3). In order to analyze the spatial distribution of paleomagnetic rotations in the Northern Apennines, we plotted them as a function of their location along the length of the arc. Starting from the northwestermost sector of Northern Apennines where paleomagnetic data are available, paleomagnetic rotations were analyzed in relation to their relative distance, in percentage, along the arc. A hypothetical axis of the chain was traced across the arc and tectonic rotations were represented as the projection of the sampled sites on this axis (Fig. 5a). A progressive variation of the tectonic rotation is observed moving from SE toward NW, except for the Tertiary Piedmont Basin, where the counterclockwise rotation pattern do not fit with the general trend expected in an orogenic arc. This sector is located in a controversial tectonic setting, at the boundary between the Northern Apennines and the Western Alpine arc, which constitute two orogenic segments characterized by opposite tectonic transport and by a different tectonic history. This distribution of paleomagnetic data indicates that the rotations of the Tertiary Piedmont Basin are not related to the northern Apennine curvature processes. If we exclude this sector, we observe a gradual change (even if with a large scatter) in the magnitude of along-strike paleomagnetic rotations (Fig. 5b), as expected in an orocline.
Figure 5. Spatial distribution of paleomagnetic rotations in Northern Apennines.
Distribution of paleomagnetic rotations in Northern Apennines, as a function of the distance along a hypothetical axis of the chain, from NW to SE. (TPB = Tertiary Piedmont Basin)
The totality of data sets allows a valuable time extension, and it is possible to describe the rotational history through time. In Fig. 6, paleomagnetic rotations are plotted as a function of the age, indicating with different colours paleomagnetic rotations detected in Jurassic-Eocene, Oligocene-Miocene and Plio-Pleistocene rocks. In this type of diagrams, the structural influence on paleomagnetic rotations is not detectable and, as a consequence, paleomagnetic sites of the same age may show very different values of paleomagnetic rotations if come from opposite sectors of the curved arc. This is the case of the upper Jurassic-lower Cretaceous Maiolica formation (Hirt and Lowrie, 1988), whose large range of paleomagnetic rotations at 140 My is due to the fact that sites are distributed in a very wide area along the arc. A similar dispersion is noted in the Scaglia formation (around 50 and 90 Ma), sampled by several Authors along different portions of the arc. Late Miocene deposits show a scatter distribution around 6 Ma; these data come from the external flysch deposits and their geographic distribution cover the entire arc length, characterized by different structural trend. In general, data from younger sedimentary sequences show a more regular trend with paleomagnetic rotations decreasing with time.
Figure 6. Temporal distribution of paleomagnetic rotations in Northern Apennines.
Paleomagnetic rotations (in degrees) versus age in Northern Apennines. Paleomagnetic data shown are those reported in Fig. 3, except the Tertiary Piedmont Basin and the Tyrrhenian margin.
CALABRIAN ARC
Paleomagnetic rotations available for Sicily, Southern Apennines and Calabria-Peloritane Domain can be used to unravel the history of paleomagnetic rotations through space and time of the Calabrian Arc. In Fig. 7, the distribution of paleomagnetic rotations is plotted as a function of distance (from north to south) along the arc. In Southern Apennines and Sicily, sedimentary deposits underwent counterclockwise and clockwise rotations, respectively. The magnitude of paleomagnetic rotations appears to be a function of their position within the thrust belt, being the more internal tectonic units systematically more rotated with respect to the external ones (Oldow et al. 1990; Speranza et al. 2003). In correspondence of the transition between Southern Apennines and the Calabria-Peloritane Domain, the distribution of paleomagnetic rotations along the arc shows an abrupt change in paleomagnetic rotations, in an area where a complex pattern of paleomagnetic rotations has been detected (Cifelli et al. 2007b). This abrupt change is not expected in a typical orocline, where a gradual change of rotations through the arc should occur. Moreover, in the Calabria-Peloritane Domain, measured paleomagnetic rotations are almost uniform along the entire region, and in particular there is no significant difference between paleomagnetic rotations measured in the Ionian fore-arc basins and in the Tyrrhenian postorogenic extensional basins, suggesting that the Calabria-Peloritane Domain behaved as an almost rigid block.
Figure 7. Spatial distribution of paleomagnetic rotations in the Calabrian Arc.
Distribution of paleomagnetic rotations in the Calabrian Arc as a function of distance (from N to S) along a hypothetical axis of the chain (modified from Cifelli et al., 2008).
The distribution of paleomagnetic rotations as a function of the age (Fig. 8) shows as the Southern Apennines and Sicily thrust belts rotational pattern is remarkable different from that observed in the Calabria-Peloritane Domain, reflecting the peculiar tectonic evolution of the Calabrian Arc. In Sicily and Southern Apennines, from Jurassic to middle Miocene time rotations are very high but remain constant during the entire time span (highlighted b y dashes ellipses in Fig. 8). From middle Miocene a progressive decrease in the magnitude occurs (Figs. 7 and 8). The observed trend marks the progressive incorporation of different paleogeographic domains in the Apennines and Maghrebian orogenic wedge and suggest that the observed Miocene paleomagnetic rotations initiated during middle to late Miocene, when internal Apennines and Maghrebian strata started to be included in the orogenic wedge (Cifelli et al., 2008). The significant magnitude of opposite sense rotations measured in lower Pleistocene sediments in Sicily and Southern Apennines indicate that vertical axis rotations played a very important role during the recent history of the Calabrian Arc (e.g., Scheepers and Langereis 1993; Scheepers et al. 1993; Speranza et al. 1999). The ‘rotational’ behavior of the Calabria-Peloritane Domain is completely different. In this sector, there is no evidence of a decrease of paleomagnetic rotations with time, because an almost constant value of about 20° of clockwise rotations has been obtained from Serravallian to lower Pleistocene strata (Fig. 8). These data suggest that during the early Pleistocene the Calabria-Peloritane Domain rotated as an almost homogeneous block, which underwent significant 15–20° clockwise rotations, and that no significant rotations occurred during late Miocene–Pleistocene time (Cifelli et al., 2007a).
Figure 8. Temporal distribution of paleomagnetic rotations in the Calabrian Arc.
Paleomagnetic rotations (in degrees) versus age for Sicily (green squares) and Southern Apennines (red circles). (Modified from Cifelli et al., 2007a).