Journal of the Geological Society, London, Vol. 157, 2000, pp. 327–334. Printed in Great Britain.
Rotational differences between the northern and southern Tyrrhenian domains: palaeomagnetic constraints from the Amantea basin (Calabria, Italy) 1
FABIO SPERANZA 1,2 , MASSIMO MATTEI 1 , LEONARDO SAGNOTTI 2 & FABIO GRASSO 1 Dipartimento di Scienze Geologiche, Universita` di Roma Tre, Largo S. Leonardo Murialdo 1, 00146 Roma, Italy (e-mail:
[email protected]) 2 Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143 Roma, Italy Abstract: We report on a palaeomagnetic study of upper Miocene sediments from the Amantea basin, located on the Tyrrhenian coast of Calabria. The magnetic mineralogy is dominated by greigite and subordinate magnetite in the Tortonian–Messinian clays (ten sites), and by hemoilmenite and magnetite in the underlying sands and volcanic ashes (three sites), which have not been dated. Data from the Tortonian–Messinian clays pass both a reversal and a fold test, and define a 1911 clockwise rotation (with respect to the geocentric axial dipole field direction) for the whole basin. The variable amounts of westward declinations observed in the underlying sands and volcanic ashes can be due to (1) a large counterclockwise rotation episode occurring before the clockwise rotation, (2) the effects of a transitional geomagnetic field in these rapidly deposited sediments, or (3) the observed complex magnetic mineralogy. These new results, when compared with previous palaeomagnetic studies from other Calabrian basins, show that the Neogene drifting of the Calabro-Peloritan block from the eastern margin of Sardinia to the present-day position was accompanied by a (probably Pleistocene) 15–20 rigid clockwise rotation recorded in both the Tyrrhenian and Ionian margins. This tectonic regime is shown to be very different from the one observed by previous studies in the northern Tyrrhenian domain, where large rotations associated with thrust sheet activity in the external Apennines were coeval with the onset of an irrotational extensional regime in the Tuscan and Latium Tyrrhenian margins. Palaeomagnetism thus confirms the significant geodynamical differences between the southern and northern Tyrrhenian Sea spreadings. Keywords: Calabria, Neogene, palaeomagnetism, tectonics.
The central Mediterranean is characterized by several Neogene extensional basins that developed contemporaneously with the main compressional events recorded at the front of the Apenninic and Maghrebian mountain belts. The investigation of the relationship between the formation of these widespread extensional basins and the onset of the coeval compressional episodes is critical to understanding the complex geodynamic process occurring in the Mediterranean region. Palaeomagnetism is one of the main geophysical techniques which enables us to constrain the kinematics of these tectonic events, since it may show the vertical axis rotations accompanying the displacements of crustal blocks detached at variable depth. Since the mid-1980s, scientific drilling studies in the Mediterranean have shown that the southern Tyrrhenian Sea is mainly floored by stretched fragments of continental crust overlain by upper Tortonian–Pleistocene sediments (Kastens et al. 1986; Mascle et al. 1988). A few areas in the deeper basins are floored by Plio-Pleistocene oceanic crust encircling basaltic seamounts (Sartori 1989). The geological evidence for the opening of the Tyrrhenian Sea demonstrates that the CalabroPeloritan block represents a former part of the eastern Sardinia Alpine belt (Alvarez et al. 1974) and indicates that it rapidly drifted into its present-day position after the late Tortonian, probably in response to the passive sinking of a seismically active Ionian lithospheric slab (Malinverno & Ryan 1986). In this paper we provide new paleomagnetic data from the Tortonian–Messinian sediments of the Amantea basin, located on the Tyrrhenian coast of Calabria. This basin is filled by syn-rift sediments, that were deposited during the early stages of the southern Tyrrhenian rifting, and it is not affected by any
subsequent compressional tectonic event (Argentieri et al. 1997). Therefore these new data allow us to constrain more effectively the kinematics of the spreading of the southern Tyrrhenian Sea and the coeval drifting of the Calabrian block. Moreover, when the paleomagnetic results obtained so far in the southern and northern Tyrrhenian margins are compared, a different rotational behaviour between the southern and northern Tyrrhenian Sea is clearly documented.
Neogene palaeomagnetic data from extensional basins and compressional fronts in central western Mediterranean Palaeomagnetism was successfully employed in several cases to help constrain the geodynamic time-space evolution of the central Mediterranean region. An overall synthesis of the data is presented here, with particular emphasis on the relationships between extensional processes and vertical axis rotations (see Fig. 1). For the Liguro-Provenc¸al basin, in the central-western Mediterranean, it has been demonstrated that the basin opening resulted from the early Miocene 30 counterclockwise rotation of the Corsica–Sardinia block (Montigny et al. 1981; Vigliotti & Langenheim 1995 and references therein). Conversely, the relationships between the complex pattern of Neogene palaeomagnetic rotations measured in the Apenninic chain, Calabrian arc and Sicily, and the contemporaneous Tyrrhenian sea opening are still unclear. This is mainly due to the presence of contemporaneous 327
328
F. SPERANZA ET AL.
Fig. 1. Simplified tectonic map of the central Mediterranean region. Arrows represent the Neogene palaeomagnetic rotations in the several tectonic provinces of the circum-Tyrrhenian region. (1) The extensional northern Tyrrhenian margin was not rotated since late Miocene (Sagnotti et al. 1994; Mattei et al. 1996). (2) The Corsica-Sardinia block rotated c. 30 counterclockwise during early–mid-Miocene (Montigny et al. 1981; Vigliotti & Langenheim 1995). (3) Post-late Miocene clockwise and counterclockwise rotations of the northern Apenninic chain (Dela Pierre et al. 1992; Mattei et al. 1995; Speranza et al. 1997; Muttoni et al. 1998). (4) Post-early Pleistocene 25 counterclockwise rotations of the southern Apenninic chain (Sagnotti 1992; Scheepers et al. 1993; Scheepers 1994). (5) The Apulia–Gargano foreland did not rotate during the Pleistocene (Scheepers 1992). (6) The Calabrian arc rotated 15–20 clockwise during the Plio-Pleistocene (Scheepers 1994; Scheepers et al. 1994). (7) The Sicilian fold-thrust belt underwent variable clockwise rotations during the Neogene (Channell et al. 1990; Oldow et al. 1990; Scheepers & Langereis 1993; Butler et al. 1999; Speranza et al. in press).
compressional and extensional tectonics in contiguous regions, hindering a full understanding of the tectonic meaning of the palaeomagnetic data. In the northern Italian peninsula, recent extensive palaeomagnetic investigations have been performed in the extensional basins of the Tyrrhenian margin (Sagnotti et al. 1994; Mattei et al. 1996) and in the compressional fronts of the central and northern Apennines (Mattei et al. 1995; Speranza et al. 1997). This data set indicated that the northern Tyrrhenian rifting has occurred without any rotation at least since Messinian time. The post-Messinian 10 to 60 rotations observed in the external central northern Apennines were exclusively related to oroclinal bending processes during the main thin-skinned compressional tectonic events. Large counterclockwise rotations were also observed in the Epiligurian units, which represent Eocene–Miocene thrust-top basins of the northern Apennine nappes (Bormioli & Lanza 1995; Muttoni et al. 1998). A close relationship between thrust sheet evolution and Neogene paleomagnetic rotations has also been suggested for the Apenninic units of Sicily (Channell et al. 1990; Oldow et al. 1990). Here differential amounts (0–140) of clockwise rotation have been associated with Neogene tectonic transport and related to the stepwise northward increase of shortening in the external nappe pile.
In the southern Apennines, post-early Pleistocene c. 25 counterclockwise rotations have been documented in the external belt (Sagnotti 1992; Scheepers et al. 1993), whereas no tectonic rotations have been observed in the Apulian foreland since Pleistocene time (Scheepers 1992). In the Calabro-Peloritan arc, Scheepers (1994) and Scheepers et al. (1994) extensively studied the palaeomagnetism of Miocene to Pleistocene sedimentary sequences. In the Plio-Pleistocene sediments a variable amount of northeastward declinations within the sedimentary basins distributed all along the arc was reported. These data were interpreted as being due to a 15 clockwise rotation of the entire Calabrian block, considered as a rigid microplate. In particular Scheepers (1994) suggested that the rotation of the Calabrian block occurred in a very short time interval, between 1 and 0.7 Ma, caused by a mid-Pleistocene large-scale compressional event. The Miocene palaeomagnetic results were obtained in the Ionian coast (Spartivento and Crotone basins) and in northern Calabria (Crati basin), whereas only preliminary results were obtained along the Tyrrhenian coast (Amantea basin). When the overall Miocene dataset from Calabria is considered, clockwise and counterclockwise rotations both at regional and at basin scale are documented (Scheepers 1994; Scheepers et al. 1994; Duermeijer et al. 1998).
UPPER MIOCENE PALAEOMAGNETISM, CALABRIA
329
Fig. 2. Schematic map of the Amantea basin and location of sampling sites.
Geological setting of the Amantea basin The Amantea basin is mainly formed by continental and marine syn-rift sediments which unconformably overlie the metamorphic and sedimentary units of the Calabrian arc (Di Nocera et al. 1974) (Fig. 2). The stratigraphic succession is organized in three depositional sequences, separated by two major angular unconformities (Colella 1995) (Fig. 3). Given the absence of compressive tectonic features, the unconformities are clearly due to the synsedimentary activity of major extensional faults tilting the entire basin, as widely observed elsewhere (e.g. Allen & Allen 1995; Ingersoll & Busby 1995). The basal first sedimentary sequence crops out mainly in the north and northeastern part of the basin and consists of conglomeratic alluvial-fan systems at the base, with a maximum thickness of about 60 m. These deposits evolved toward the centre of the basin and upwards to rather massive sandstones deposited in fan delta systems, generally opening westward, with a thickness of 100–150 m. In the upper part of the sequence an ash layer bed can be used as a key level throughout the whole basin. An angular unconformity, which locally becomes a disconformity, marks the base of the second depositional sequence. This transgressive sequence consists of conglomerates and beach sandstones at the base, followed upwards by ramp-type shelf calcarenites, embedded between two alluvial fan/tidal fan delta conglomerates and sands. These latter deposits pass progressively upward to open marine clays deposited at a depth of some hundreds meters. A few meters of diatomitic marls crop out in the upper part of the second depositional cycle, representing an event of restricted water circulation. The lower portion of this sequence is widespread in the whole Amantea basin, whereas the clays mainly crop out in the southern part of the basin.
Fig. 3. Schematic stratigraphic log of the Miocene sedimentary sequence of the Amantea basin (modified from Colella 1995).
The third depositional sequence can only be observed in the western part of the basin, and is composed of a few meters of gypsum which shows an angular unconformity at the base. The stratigraphic age of the Amantea basin sediments has been precisely defined only for the 80 m thick clay interval within the second depositional sequence, which has been extensively sampled for biostratigraphic analysis by Ortolani et al. (1979). The recognition of the Globorotalia a. acostaensis zone (G. obliquus extremus subzone), followed by the G. suterae subzone, indicated a late Tortonian age for the lower part of the clay interval in the second sequence. The occurrence of G. conomiozea, which marks the Tortonian–Messinian boundary, has been recognized at about the middle of the clay succession, whereas the diatomite levels are characterized by the presence of G. multiloba. The gypsum levels which form the third depositional sequence are related to the Messinian Mediterranean salinity crisis. The tectonic evolution of the basin is mainly controlled by syn-sedimentary normal faults affecting the sediments of both the first and second depositional sequences (Mattei et al. 1999). The faults are mainly oriented N–S to NNE–SSW, and isolate tilted blocks, down-thrown towards the east, that constitute the major structural feature of the basin. Many small-scale unconformities can be observed in the field, due to the
330
F. SPERANZA ET AL.
Fig. 4. Representative results of the thermal demagnetization of a three-component IRM according to the method of Lowrie (1990).
synsedimentary extensional tectonics producing tilted blocks. Moreover, also the two major unconformities separating the three depositional sequences are related to the activity of large-scale extensional faults controlling the geometry of the Amantea basin as a whole. In summary, all the sedimentary and tectonic data concur to indicate that the Amantea basin formed during the late Miocene southern Tyrrhenian rifting phases, and was passively transported upon the Calabrian block during the following drifting episode, as already suggested by several authors (Malinverno & Ryan 1986; Sartori 1989; Patacca et al. 1990).
Sampling and methods After a laboratory study of a preliminary hand-sample collection, we sampled 127 cylindrical core samples from 13 sites (Fig. 2). Cores were drilled using an ASC 280E petrol-powered portable drill and oriented in situ with a magnetic compass. Ten of the 13 sites (AM01 to AM09 and AM11) were sampled in the fine-grained blue-grey strata of the Tortonian–Messinian clays. The remaining three sites were sampled at the top of the first depositional sequence, which is not defined in age. In particular, site AM13 has been sampled in a 10 cm thick clay bed within the sandstone succession just below the unconformity separating the first and second depositional sequences. The last two sites (AM10, AM12) were sampled in a volcanic ash layer about 10 m below site AM13 (Figs 2 and 3). The magnetic mineralogy was investigated using a number of standard rock magnetic techniques (see below). For at least one specimen per site we studied the stepwise acquisition of an isothermal remanent magnetization (IRM) produced in a pulse magnetizer up to 0.9 T. The coercivity of remanence (Bcr) was then estimated by stepwise application of a back-field to remove
the remanence produced in a forward field of 0.9 T. We also thermally demagnetized a composite IRM (Lowrie 1990) produced by the sequential application of 0.9 T, 0.5 T and 0.12 T fields along the three orthogonal specimen axes. The palaeomagnetism of one standard cylindrical specimen per sample was studied at each site. Measurements of the natural remanent magnetization (NRM) were carried out on a JR-5A spinner magnetometer, in the magnetically shielded room of the Istituto Nazionale di Geofisica. Stepwise thermal demagnetization was employed to resolve the NRM components in each specimen. The low-field magnetic susceptibility was measured after each heating step, as a check for thermally induced changes in the magnetic mineralogy. The demagnetization data were analyzed on orthogonal vector diagrams and directions of the remanence components were estimated using principal component analysis (Kirschvink 1980).
Results The magnetic mineralogy of the studied sediments appears rather complex. Only low-coercivity magnetic carriers were identified in the ten sites sampled in the Tortonian–Messinian clays. The unblocking temperature spectra observed in these sediments indicate that the magnetic mineralogy is constituted by iron sulphides, sometimes associated with magnetite (Fig. 4a and b). The dominant presence of greigite (Fe3S4) was positively identified at three sites (AM07, AM09 and AM11) with specific rock magnetism studies performed on the same samples by Sagnotti & Winkler (1999). Greigite is often confused with pyrrhotite, but the following evidence showed the exclusive presence of greigite in the samples (see Sagnotti & Winkler 1999 for details): (1) thermal decomposition above 230C, (2) high saturation isothermal remanent magnetization
UPPER MIOCENE PALAEOMAGNETISM, CALABRIA
(SIRM) to low-field magnetic susceptibility ratios, (3) tendency to acquire a significant rotational remanent magnetization and (4) sensitivity to field-impressed anisotropy. On the other hand, high-coercivity magnetic carriers were recognized at all the three sites sampled in the first depositional sequence. Among these three sites, low-coercivity minerals are present with the high-coercivity minerals in the two volcanic ash layer sites (AM10 and AM12). These two sites approach saturation at the maximum applied field of 0.9 T, whereas the clay sampled at site AM13 appears far from saturation at the maximum applied field of 0.9 T. The ratio S 0.3T = IRM 300mT/(SIRM) is often used as an estimate of the relative quantities of low-coercivity and high-coercivity magnetic phases in a rock specimen (e.g. Bloemendal et al. 1988); the S 0.3T value should theoretically be equal to 1 if only low-coercivity minerals are present. Using the IRM intensity at 0.9 T as an estimate of the SIRM in the S 0.3T computation, we obtained S 0.3T values of 0.88 and 0.61 for the two sites in the volcanic ash layer (AM10 and AM12, respectively) and of +0.2 for the clay interval sampled in the sands of the first depositional sequence (AM13). The latter S-0.3T value clearly indicates that at this site the low-coercivity minerals are only minor magnetic components, since a field of 300 mT is sufficient to reverse the forward IRM0.9T of even elongated single-domain low-coercivity ferrimagnetic minerals (e.g. Dunlop & O } zdemir 1997). The blocking temperature spectrum observed in the first depositional sequence of rocks indicates a mixture of magnetite and hematite in the volcanic ash layer, the latter suggesting some degree of oxidation and alteration at these sites (Fig. 4c and d). At site AM13, the combination of a high-coercivity with a maximum unblocking temperature of c. 550C (Fig. 4d) can only be explained by supposing that the main magnetic carrier is a hemoilmenite (Fe2yTiyO3) with a Ti content y of c. 0.15 (e.g. Nagata 1961). This mineral has a volcanic origin and can only be detrital in the clay sediments of site AM13. The magnetic susceptibility was stable during thermal demagnetization of the NRM in clay samples containing magnetite and in the ash layer samples. In the clay samples containing iron sulphides, a strong decay in susceptibility was observed between 200 and 330C, thus testifying to the thermal decomposition of greigite (Torii et al. 1996; Sagnotti & Winkler 1999). In the samples from site AM13, the susceptibility increases continuously from 300 to 500C. NRM from three clay sites (AM01, AM04, and AM06) was found to be too weak (that is NRM
