Geology; March 2008; v. 36; no. 3;
p. 243-246; DOI: 10.1130/G24344A.1
© 2008 Geological Society of America
Tectonic burial and "young" (<10 Ma) exhumation in the southern Apennines fold-and-thrust belt (Italy)
S. Mazzoli1,
M. D'Errico1,
L. Aldega2,
S. Corrado2,
C. Invernizzi3,
P. Shiner4 and
M. Zattin5
1 Dipartimento di Scienze della Terra, Università di Napoli Federico II, 80138 Naples, Italy
2 Dipartimento di Scienze Geologiche, Università Roma Tre, 00146 Rome, Italy
3 Dipartimento di Scienze della Terra, Università di Camerino, Piazza Cavour 19/f, 62032 Camerino, Italy
4 Shell Italia E & P, Via Dei due Macelli, 66, 00187 Rome, Italy
5 Dipartimento di Scienze Geologiche e Ambientali, Università di Bologna, 40126 Bologna, Italy
 |
ABSTRACT
|
|---|
In the southern Apennines fold-and-thrust belt, thermal indicators record exhumation of sedimentary units from depths locally in excess of 5 km. The thrust belt is made of allochthonous sedimentary units that overlie a 6–8-km-thick, carbonate footwall succession. The latter, continuous with the foreland Apulian Platform, is deformed by reverse faults involving the underlying basement. Therefore, a switch from thin-skinned to thick-skinned thrusting occurred as the Apulian Platform carbonates—and the underlying thick continental lithosphere—were deformed during the latest shortening stages. Apatite fission track data, showing cooling ages ranging between 9.2 ± 1.0 and 1.5 ± 0.8 Ma, indicate that exhumation marks these late tectonic stages, probably initiating with the buttressing of the allochthonous wedge against the western margin of the Apulian Platform. Pliocene-Pleistocene foreland advancing of the allochthonous units exceeds the total amount of slip that, based on cross-section balancing and restoration, could be transferred to the base of the allochthon from the underlying thick-skinned structures. This suggests that emplacement of the allochthon above the western portion of the Apulian Platform carbonates was followed by gravitational readjustments within the allochthonous wedge, coeval—and partly associated with—thick-skinned shortening at depth. The related denudation processes are interpreted to have played a primary role in tectonic exhumation.
Key Words: low-temperature thermochronology • thin-skinned thrusting • thick-skinned thrusting • gravitational instability • extensional faulting
|
INTRODUCTION
|
|---|
The Tyrrhenian Sea–Apennines system is a well-known paired tectonic belt with shortening on the foreland side of the orogen and extension in the hinterland, the two processes coexisting since at least late Miocene times (e.g., Patacca et al., 1990). Although backarc extension in the Tyrrhenian domain and high-angle normal faulting in the axial zone of the mountain chain have been extensively studied, the occurrence of low-angle normal faults within the chain has been convincingly described only in recent years (e.g., Casciello et al., 2006). The activity of these (mainly east-dipping) extensional detachments appears to be coeval—and probably kinematically linked—with thin-skinned thrusting in the frontal part of the orogen (Mazzoli et al., 2006). It is suggested here that the linked detachments and thrusts played a fundamental role in controlling tectonic exhumation in the southern Apennines. Based on clay mineralogy, vitrinite reflectance (Ro%), apatite fission tracks (AFTs), and fluid inclusion data, exhumation of sedimentary units from depths locally in excess of 5 km has been documented (Corrado et al., 2005). In this study we integrate the large amount of thermal and thermochronological data published in the latter work, and provide a new interpretation for the exhumation processes occurring at upper crustal levels in the southern Apennines. Our model, emphasizing the role of gravity-driven processes interacting with thrust tectonics, sheds new light on this key area of the Alpine-Mediterranean region, and provides new insights into the evolution of fold-and-thrust belts.
|
TECTONIC SETTING
|
|---|
The southern Apennine chain is a NE-directed fold-and-thrust belt, with the Apulian promontory representing the orogenic foreland (Fig. 1). Except for so-called "internal" (Sicilide and ophiolite-bearing Liguride) tectonic units that occur on top of the thrust pile, outcropping thrust sheets consist of Mesozoic and Cenozoic rocks derived from the sedimentary cover of the foreland plate (Fig. 2A). These include carbonate platform and pelagic basin successions, locally covered by Neogene foredeep and/or thrust-top basin sediments. The structure at shallow levels is dominated by low-angle tectonic contacts separating the platform/slope carbonates of the Apennine Platform, in the hanging wall, from the pelagic successions of the Lagonegro Basin. The latter experienced superposed folding, with upright second-phase folds being characterized by a steep crenulation cleavage (Mazzoli et al., 2001). Furthermore, out-of-sequence faults locally produced significant tectonic burial of limited portions of the platform/slope carbonates and stratigraphically overlying Miocene siliciclastics (Monte Croce Unit; Fig. 1).
View larger version (69K):
[in this window]
[in a new window]
|
Figure 1. Geological sketch map of the study area, showing location of regional cross section X–X' (after Butler et al., 2004) and analyzed samples. Thermal and thermochronological data (Ro, KI, %I in I/S, Th, AFTs; see text) are from Corrado et al. (2005), integrated with new data. The data are shown in the different colored blocks for the main tectonic units, with period or epoch referring to age of the sampled rocks. AFT cooling ages are shown in Ma ± 1 , in color also relating to the sampled tectonic unit (ages referred to the Apulian Platform also include data from the mélange zone exposed on top of Monte Alpi). Asterisks mark samples showing no AFT annealing (upper thermotectonic plate); for all other samples (lower thermotectonic plate), showing total annealing of AFTs, the data represent exhumation ages (cooling through the isotherm  110 °C).
|
|
View larger version (39K):
[in this window]
[in a new window]
|
Figure 2. Three simplified sketches showing interpreted tectonic evolution of the southern Apennines. Carbonate slope facies associated with the eastern margin of the Apennine Platform and western margin of the Apulian Platform are shown with white and black dots, respectively. The location of the eastern margin (M) of the Apennine Platform is held fixed through the three panels for reference. A: Mesozoic setting of the continental margin later telescoped in the southern Apennines. The Liguride oceanic domain was located to the west of the Apennine Platform. The Lagonegro Basin is shown in reduced width (note that the original width of the basin remains poorly constrained). B: "Closure" of the Lagonegro Basin, initiating buttressing of the allochthonous wedge against the western inherited rifted margin of the Apulian Platform (BU). C: Following emplacement of the allochthonous wedge above the westernmost portion of the Apulian Platform, gravitational readjustments dominate within the allochthonous wedge, triggering denudation and tectonic exhumation (assisted by erosion). Active shortening migrated to the underlying Apulian crust, producing basement-involved inversion at depth.
|
|
|
AMOUNT OF DEFORMATION: STRUCTURAL AND STRATIGRAPHIC CONSTRAINTS
|
|---|
Large amounts of surface geological information coupled with subsurface data, particularly oil wells, demonstrate large-scale thin-skinned thrusting in the shallow part of the southern Apennines (e.g., Shiner et al., 2004). As a whole, the outcropping thrust belt forms a displaced allochthon that has been carried onto a footwall of foreland strata continuous with those exposed in the Apulian promontory to the northeast (Fig. 1). The detachment between the allochthon and the buried Apulian Platform carbonates is marked by a mélange zone up to several hundred meters thick. It consists mainly of intensely deformed and over-pressured deepwater mudstones and siltstones of Miocene to lower Pliocene age, including blocks of material derived from the overlying allochthon (Mazzoli et al., 2001). Beneath the mélange zone, under a variable thickness of Pliocene shales, are Mesozoic-Tertiary shallow-water carbonates of the Apulian Platform. This tectonically buried portion of the Apulian Platform was involved in the final shortening phases, giving rise to reverse-fault-related, open, long-wavelength folds that form the hydrocarbon traps for the significant oil discoveries in the thrust belt (Shiner et al., 2004). The structural style—thin-skinned or thick-skinned—of shortening within the buried Apulian Platform carbonates has been debated. However, in recent years subsurface data sets have provided evidence that deep-seated reverse faulting within the Apulian Platform carbonates is characterized by (1) steep reverse faults, (2) a substantial lack of thrust flats, and (3) involvement of the underlying basement. In particular, cross-section balancing and restoration of buried Apulian Platform carbonates, carried out by Shiner et al. (2004) based on high-quality seismic data and well logs, favor an inversion tectonics model involving reactivation of pre-existing (Permo-Triassic) basement normal faults. Therefore, in late Pliocene times, a switch from thin-skinned to thick-skinned shortening occurred in the southern Apennines as the Apulian carbonates—and the underlying thick continental lithosphere—were shortened. The related deformation is characterized by limited horizontal displacements: balancing and restoration of the Apulian Platform carbonates in the cross section of Figure 1 yields 14 km of horizontal shortening (Butler et al., 2004). Similar amounts of shortening can be derived from the regional geological sections of Menardi Noguera and Rea (2000), and Turrini and Rennison (2004), all of them based on sub-surface data sets provided by the oil industry. On the other hand, forward displacement of the allochthon since early Pliocene times exceeds 50 km along the cross section of Figure 1 (Butler et al., 2004). Based on high-resolution stratigraphy applied to synorogenic sequences, Patacca and Scandone (2001) unraveled the discontinuous nature of this motion. For the SE part of the area shown in Figure 1, the latter authors demonstrated that the most recent motion of the allochthon, producing a cumulative displacement of 30 km, occurred during two distinct time intervals (i.e., 3.70–3.30 and 1.83–1.50 Ma), each characterized by average slip rates of 4 cm/yr.
|
TECTONIC BURIAL AND EXHUMATION: THERMAL AND THERMOCHRONOLOGICAL CONSTRAINTS
|
|---|
Thermal and thermochronological constraints (see the GSA Data Repository1 for analytical details) indicate that the study area is divisible into two distinct thermotectonic plates—an upper plate that records limited burial and only minor heating, and a lower plate that has been extensively tectonically buried and heated. The upper plate consists of the rocks of the Apennine Platform domain and the tectonically overlying "internal" (Liguride and Sicilide) units. The former are characterized by (1) illite contents in illite-smectite mixed layers (%I in I/S in Fig. 1) and Ro% values in the range of the early diagenetic zone, (2) immature to early mature stage of hydrocarbon generation, (3) homogenization temperature (Th in Fig. 1) of both primary and secondary fluid inclusions in syntectonic veins, never exceeding 70 °C, and (4) no annealing of AFTs. Fresh volcanic apatites from upper Tortonian siliciclastics (Monte Sierio site in Fig. 1) provided an age of 7.5 ± 1.8 Ma, compatible with the stratigraphic age. Thermal modeling (Corrado et al., 2005) suggests the minor effect of burial (never exceeding 2 km) for the rocks of the Apennine Platform domain. A maximum temperature lower than the total annealing temperature of AFTs is also shown by a cooling age of 86.9 ± 11.8 Ma (Fig. 1) from a sample from an "internal" (Sicilide) unit.
The lower plate includes rocks of the Lagonegro Basin, the Monte Croce, and the Apulian Platform domains. For the Lagonegro Basin successions, thermal constraints mostly record late diagenetic conditions and overmature hydrocarbon generation. The homogenization temperature for primary fluid inclusions ranges between 120 and 160 °C. Illite crystallinity, expressed as Kübler Index (KI in Fig. 1), ranges from 0.60 to 1.10 
°2
. AFTs are totally annealed, both in Mesozoic strata and in unconformably overlying Miocene siliciclastics, although the latter display lower thermal maturity. Total annealing of AFTs also characterizes the Monte Croce Unit exposed in a tectonic window in the foot-wall to Mesozoic Lagonegro Basin strata.
The data from Monte Alpi (Fig. 1), representing the unique outcrop of Apulian Platform rocks within the thrust belt (e.g., Mazzoli et al., 2006), consistently fall into the late diagenetic zone and late mature stage of hydrocarbon generation. The homogenization temperature of primary fluid inclusions is mostly in the range of 100–130 °C (although higher temperatures were sporadically recorded; Mazzoli et al., 2006), whereas AFTs show total annealing for all samples.
The postdepositional history of the sedimentary units is particularly well constrained by AFT data (Fig. 1). As opposed to the rocks belonging to the upper plate, all samples from the lower plate were affected, during the Neogene, by maximum temperatures higher than total annealing temperature. Therefore, ages reflect time of recent cooling through the isotherm 110 °C, effectively dating the exhumation of the previously buried successions.
|
DISCUSSION
|
|---|
Thermal indicators suggest that parts of the allochthonous units cropping out in the southern Apennines have been buried to depths in excess of 5 km (Corrado et al., 2005). In particular, the Lagonegro Basin rocks, though recording non-metamorphic conditions, appear to form a series of "cores" of exhumed, previously deeply buried rocks surrounded by rocks (those of the Apennine Platform) showing lower thermal maturity. The latter parameter does not show a gradual increase from upper to lower tectonic units. Rather, a break in the paleothermal characteristics occurs across the tectonic contacts between the Apennine Platform carbonates and the underlying Lagonegro Basin strata. This feature is independent of the fact that most of the Apennine Platform data come from Miocene deposits that overlie the carbonates. In fact, the thickness of the hanging-wall carbonates tectonically overlying the Lago negro Basin rocks is limited to a few hundred meters (never exceeding 1 km). This thickness cannot account for the burial and thermal conditions recorded by the Lagonegro units sampled in the immediate footwall (Fig. 1). Therefore, tectonic omission can be interpreted to occur across the tectonic contacts, forming the boundary between two distinct thermotectonic plates. This interpretation is consistent with the common truncation of structures observed in the footwall (Lagonegro) units (Mazzoli et al., 2001), as well as the significant structural reworking of the pre-existing fold-and-thrust belt. For instance, Casciello et al. (2006) unraveled the occurrence of important top-down-to-the-ESE extensional detachments in the Salerno area (Fig. 1). These low-angle normal faults superpose younger rocks on older rocks and also cut across the original thrust contact between the Apennine Platform carbonates and the footwall Lagonegro Basin strata. Mazzoli et al. (2006) documented tectonic exhumation in the footwall of an east-dipping extensional detachment in the Monte Alpi area (Fig. 1) and suggested that the related extensional process was thin-skinned, being kinematically linked with shallow thrusting farther east. Dominantly eastward hanging-wall transport characterizes the low-angle extensional detachment faults and the Pliocene-Pleistocene frontal thrusts to the east. According to this interpretation, both low-angle normal faults and thin-skinned thrust faults share a common detachment located at the base of the allochthon. Pliocene-Pleistocene foreland advancing of the allochthonous units (in excess of 50 km) exceeds the total amount of slip that, based on cross-section balancing and restoration, could be transferred from the buried Apulian Platform carbonates to the overlying allochthon (e.g., 14 km along the geological section of Fig. 1). All together, these observations suggest that gravitational instability within the allochthonous wedge is likely to have substantially contributed to the forward motion of the allochthon by a process of extension linked with thin-skinned thrusting. This process may have been enhanced by the development of structurally positive features (large antiforms and pop-ups) within the Apulian Platform carbonates beneath the allochthon, as active shortening migrated to deeper structural levels. Hence within a dynamic thrust belt, crustal thickening by reverse faulting at depth may produce normal faults within the overlying wedge (e.g., Gamond, 1994). These gravitational readjustments are induced by an increase in the wedge slope, as well as by tilting of weak layers—the main one being, in this instance, the thick mélange zone at the base of the allochthon—overlying the Apulian Platform. Gravitational readjustments may lead to the reactivation of pre-existing thrusts as normal faults and may also generate rootless forethrusts by denudation of the upper part of the tilted cover (Gamond, 1994), thereby triggering tectonic exhumation. Within this framework, the previously mentioned, large—and fast—discrete pulses of forward displacement of the allochthon documented by Patacca and Scandone (2001) at 3.70–3.30 and 1.83–1.50 Ma likely represent major gravity collapse events within the allochthon, linked to thin-skinned thrusting at the front.
|
TECTONIC MODEL
|
|---|
AFT data indicate a late Miocene (<10 Ma) onset of exhumation of previously deeply buried rocks in the southern Apennines (Fig. 1). In a first stage, this process probably involved buttressing of the allochthonous wedge against the outer (eastern) crustal ramp of the inherited Lagonegro Basin (i.e., inner or western margin of the Apulian Platform). Numerical modeling suggests that buttressing at this scale is able to produce efficient exhumation (Jamieson and Beaumont, 1989). The development, in the Lagonegro Basin rocks, of second-phase folds with subvertical axial surfaces and a steep crenulation cleavage was probably associated with the buttressing process, which involved shortening of previously folded and imbricated rocks.
In the proposed model (Fig. 2), the tectonically advancing Apennine wedge encountered the Apulian Platform, resulting in complex faulting and thickening of the previously accreted Lagonegro units. Subsequent uplift and emplacement of the allochthon on top of the western portion of the Apulian Platform induced gravitational instabilities within the thickened and elevated tectonic wedge. Active shortening continued, at lower rates, by thick-skinned reactivation of high-angle faults within the Apulian Platform. Within this framework, forward displacement of the allochthonous units resulted from a combination of (1) slip transferred to the base of the allochthon from the underlying Apulian Platform carbonates, which were undergoing active shortening, and (2) gravitational readjustments triggering extension linked with thin-skinned thrusting at shallow levels. The latter process was coeval—and associated with—thick-skinned shortening at depth, producing tilting of weak interfaces in the allochthonous overburden, denudation, and related tectonic exhumation.
The late Miocene onset of exhumation was coeval with the onset of rifting in the Tyrrhenian Sea (Patacca et al., 1990). Extension related to Tyrrhenian opening may, therefore, have played a role in the initiation of tectonic exhumation in the thrust belt. However, the Tyrrhenian extensional processes affected the SW side of the orogen and involved the development of high-angle normal faults (e.g., Patacca et al., 1990). As such, they cannot account for the widespread exhumation recorded by the thermochronological data throughout the study area (Fig. 1).
|
CONCLUSIONS
|
|---|
Combined organic and inorganic indicators indicate that a significant part of the sedimentary rocks exposed in the southern Apennines experienced substantial tectonic burial (locally in excess of 5 km). Exhumation of such deeply buried sedimentary rocks followed late Miocene "closure" of the Lagonegro Basin, involving buttressing of the allochthonous wedge against the eastern crustal ramp of the rifted margin of the Lagonegro Basin. This suggests a strong control on the onset of exhumation due to the architecture of the continental margin.
Recent (<10 Ma) tectonic evolution and exhumation in the southern Apennines were controlled by the interplay between deep-seated shortening (in the buried Apulian Platform, where thick-skinned reverse faulting and basement-involved inversion are dominant) and extension linked with thin-skinned thrusting in the allochthonous overburden, induced by gravitational readjustments within the wedge.
|
ACKNOWLEDGMENTS
|
|---|
Peppe Cello, a friend we lost, provided invaluable inspiration and support. Reviews by S.T. Johnston, M.S. Steckler, an anonymous reviewer, and T.M. Niemi substantially improved the paper. Financial support from the Italian Ministry of University and Research, projects PRIN2005043180_002 (S. Mazzoli) and PRIN2004044497_002 (C. Invernizzi), is gratefully acknowledged.
|
FOOTNOTES
|
|---|
GSA Data Repository item 2008060, analytical details, is available online at www.geosociety.org/pubs/ft2008.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. 
|
REFERENCES CITED
|
|---|
Butler, R.W.H., Mazzoli, S., Corrado, S., De Donatis, M., Di Bucci, D., Gambini, R., Naso, G., Nicolai, C., Scrocca, D., Shiner, P., and Zucconi, V., 2004, Applying thick-skinned tectonic models to the Apennine thrust belt of Italy: Limitations and implications: in McClay, K.R., ed., Thrust tectonics and petroleum systems: American Association of Petroleum Geologists Memoir 82, pp. 647-667.
Casciello, E., Cesarano, M., and Pappone, G., 2006, Extensional detachment faulting on the Tyrrhenian margin of the southern Apennines contractional belt (Italy): Geological Society [London] Journal, v. 163 pp. 617-629 doi: 10.1144/0016-764905-054.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]
Corrado, S., Aldega, L., Di Leo, P., Giampaolo, C., Invernizzi, C., Mazzoli, S., and Zattin, M., 2005, Thermal maturity of the axial zone of the southern Apennines fold-and-thrust belt (Italy) from multiple organic and inorganic indicators: Terra Nova, v. 17 pp. 56-65 doi: 10.1111/j.1365-3121.2004.00584.x.[CrossRef][ISI][GeoRef]
Gamond, J.-F., 1994, Normal faulting and tectonic inversion driven by gravity in a thrusting regime: Journal of Structural Geology, v. 16 pp. 1-9 doi: 10.1016/0191-8141(94)90013-2.[CrossRef][ISI][GeoRef]
Jamieson, R.A., and Beaumont, C., 1989, Deformation and metamorphism in convergent orogens: A model for uplift and exhumation of metamorphic terrains: in Daly, J.S., et al., eds., Evolution of metamorphic belts: Geological Society [London] Special Publication 43, pp. 117-129.
Mazzoli, S., Barkham, S., Cello, G., Gambini, R., Mattioni, L., Shiner, P., and Tondi, E., 2001, Reconstruction of continental margin architecture deformed by the contraction of the Lagonegro Basin, southern Apennines, Italy: Geological Society [London] Journal, v. 158 pp. 309-319.[Abstract/Free Full Text][ISI][GeoRef]
Mazzoli, S., Aldega, L., Corrado, S., Invernizzi, C., and Zattin, M., 2006, Pliocene-Quaternary thrusting, synorogenic extension and tectonic exhumation in the southern Apennines (Italy): Insights from the Monte Alpi area: in Mazzoli, S., and Butler, R.W.H., eds., Styles of continental contraction: Geological Society of America Special Paper 414, pp. 55-77 doi: 10.1130/2006.2414(04).
Menardi Noguera, A., and Rea, G., 2000, Deep structure of the Campanian-Lucanian Arc (southern Apennines): Tectonophysics, v. 324 pp. 239-265 doi: 10.1016/S0040-1951(00)00137-2.[CrossRef][ISI][GeoRef]
Patacca, E., and Scandone, P., 2001, Late thrust propagation and sedimentary response in the thrust-belt–foredeep system of the southern Apennines (Pliocene-Pleistocene): in Vai, G.B., and Martini, I.P., eds., Anatomy of an orogen: The Apennines and adjacent Mediterranean basins: Dordrecht, Netherlands, Kluwer Academic Publishers, pp. 401-440.
Patacca, E., Sartori, R., and Scandone, P., 1990, Tyrrhenian Basin and Apenninic arcs: Kinematic relationships since late Tortonian times: Memorie della Società Geologica Italiana, v. 45 pp. 425-451.[GeoRef]
Shiner, P., Beccacini, A., and Mazzoli, S., 2004, Thin-skinned versus thick-skinned structural models for Apulian carbonate reservoirs: Constraints from the Val D'Agri Fields: Marine and Petroleum Geology, v. 21 pp. 805-827.[CrossRef][ISI][GeoRef]
Turrini, C., and Rennison, P., 2004, Structural style from the southern Apennines' hydrocarbon province—An integrated view: in McClay, K.R., ed., Thrust tectonics and petroleum systems: American Association of Petroleum Geologists Memoir 82, pp. 558-578.
Received for publication 16 August 2007
Revised manuscript received 12 November 2007
Manuscript accepted 25 November 2007
Copyright © 2008 by Geological Society of America
TOP
ABSTRACT
INTRODUCTION
