Enrico Tavarnelli

The interaction of extensional and contractional deformations in the outer zones of the Central Apennines, Italy

Abstract The relationships among normal faults and thrusts in the Apennines of Italy are often unclear, and the local absence of syn-tectonic stratigraphic controls have led to contrasting interpretations on the relative chronology for both classes of structures. The activity of normal faults has been variously regarded as due to pre-, syn- or post-orogenic extension, and the contrasting evidence from different sites has produced an ongoing debate on the normal fault–thrust interaction. The results of a kinematic analysis on selected composite structures of the outer zones of the Central Apennines make it possible to unequivocally establish a relative chronology of extensional and contractional deformations. Detailed mapping, outcrop-scale observations and structural overprinting relationships support a positive inversion tectonic history, where normal faults and fault-controlled escarpments formed first, and were later deformed by thrusts and related folds. All normal faults control the distribution of foredeep deposits, thus indicating that the recognised episode of positive inversion is related to the incipient stages of construction of the Apennine thrust belt. The systematic collection of structural data may help to unravel the evolution of adjacent sectors of the Apennine chain, as well as of other belt-foredeep–foreland systems whose extension–contraction relationships are poorly constrained.

Domainal deformation patterns and strain partitioning during transpression: an example from the Southern Uplands terrane, Scotland

The partitioning of deformation into wrench- and contraction-dominated deformation domains is a widely reported but poorly described phenomenon in ancient transpression zones. This paper documents spectacularly exposed examples of such partitioning from the Southern Uplands terrane in SE Scotland (Berwickshire), which was deformed during late Llandovery to Wenlock time. A well-exposed coastal section from Eyemouth to Burnmouth preserves a broadly homoclinal sequence in which a highly heterogeneous array of contemporaneous structures formed during regional triclinic transpression. The deformation involved components of NW–SE contraction with subvertical extension, top-to-the-SE thrusting and top-to-the-SW sinistral shear. In the northern third of the section studied these components are partitioned into a series of fault-bounded, metre- to kilometre-scale structural domains that contain geometrically and kinematically distinct assemblages of variably curvilinear folds, strike-slip detachments and locally transecting cleavages. The structures are all broadly contemporaneous and, in individual domains, record either non-coaxial contractional- or sinistral wrench-dominated strains. Similar highly heterogeneous, domainal structural patterns are likely to be found in other regions of oblique convergence in both ancient and modern settings.

Structural evolution of a foreland fold-and-thrust belt: the Umbria-Marche Apennines, Italy

Abstract Outcrop-scale structures that record a progressive sequence of deformation can be used to clarify the kinematic evolution of an entire fold-and-thrust belt. The temporal progression of the compressional deformation as reconstructed by means of a mesoscopic structural analysis provides a key for unraveling the kinematic history of the Umbria-Marche Apennines (central Italy). Overprinting relationships at outcrop-scale allow for the recognition of three distinct structural stages which are, in sequence: A, layer-parallel shortening; B, folding; C, thrusting. Individual stages are explained in the framework of a progressive deformation model, where shortening of the sedimentary cover was continuous and occurred during a single contractional episode as a consequence of tip-line folding processes. A comparison of this history with those reported from other fold-and-thrust belts shows that layer-parallel shortening, folding and thrusting are sequentially dominant processes in areas which have experienced compressional deformation.

Normal faults in thrust sheets: pre-orogenic extension, post- orogenic extension, or both?

Abstract In fold-and-thrust belts that experienced both pre-orogenic and post-orogenic extension, it may be difficult to establish whether observed normal faults pre-dated, post-dated, or were synchronous with thrusting. Geometrical structural patterns may be insufficient to constrain the relative chronology of extensional and contractional deformations. The systematic use of kinematic criteria makes it possible to unequivocally define the timing relationships of reverse and normal fault development, and hence to correctly unravel complex structural evolutions. Kinematic analysis in the southernmost Umbria–Marche Apennines of Italy, where both normal and thrust faults are present, revealed a history of repeated tectonic inversion, characterised by two distinct stages of extension separated by an episode of folding and thrusting. Structural overprinting relationships observed at thrust–normal fault intersections were useful for: (i) removing sequentially younger deformations; and hence (ii) separating and quantifying the effects of orogenic contraction from those of both pre-orogenic and post-orogenic extension.

Structural inheritance of pre- and syn-orogenic normal faults on the arcuate geometry of Pliocene-Quaternary thrusts: Examples from the Central and Southern Apennine Chain

n the frontal sector of the Central-Southern Apennines, surface geological data integrated with seismic line interpretation provide new constraints into the reconstruction of the structural inheritance of Mesozoic pre-orogenic and Messinian-Pliocene syn-orogenic normal faults on the salient geometry of the Pliocene-Quaternary thrust system.In the Umbria-Marche-Abruzzi area, pre-orogenic normal faults commonly juxtapose the complete Jurassic succession (about 900 metres in thickness) onto coeval condensed successions (about 50 metres in thickness) deposited over structural highs. In the Sibillini Mts and Gran Sasso area, pre-orogenic normal faults are truncated and rotated into Pliocene thrust-sheets according to simple short-cut trajectories. In particular the foreland-dipping Jurassic normal faults in the Sibillini Mts area have been rotated and reactivated during the thrust propagation forming high-angle blind-thrusts in the east verging overturned folds.The Maiella anticline, which involves the Mesozoic-Miocene Apulian carbonate succession and the related slope deposits, joins the Central Apennine fold-and-thrust system to the Apulian Chain buried below the allochthonous Units of the Southern Apennines. Seismic line interpretation allowed us to reconstruct the three-dimensional pattern of the Apulian thrusts, oriented N-S, NNW-SSE and E-W, that are parallel to normal faults related to the Pliocene-Quaternary flexural extension in the foreland. Detailed reconstruction of the Setteporte and Monte Taburno structures shows main N-S/NNE-SSW trending thrusts, branching into NW-SE/E-W trending minor thrusts and back-thrusts, characterized by push-up geometry, typically referable to a transpressive deformation and/or to the positive reactivation of normal faults. Moreover, the sharp westward deepening of the base of the Apulian sedimentary succession (from 4.5 to 6.0 sec in TWT), based on the interpretation of the CROP 11 seismic reflection profile, and the concomitant increase in thickness of the Triassic sequence along the Maiella-Casoli transect, suggest the existence of west-dipping (?)Permian-Triassic normal faults that strongly controlled the distribution and thickness variation of syn-rifting sediments. An inversion of the deepest low angle portions of the pre- and syn-orogenic normal faults is in agreement with surface data (i.e., the structural elevation of the carbonate succession in the Casoli-Bomba anticline) and seismic line interpretation (i.e., deep seated location of the base of Apulian sedimentary succession below the same anticline).In the reconstructed inversion tectonics model, the N-S trending pre-thrusting normal faults are fully inverted as N-S transpressive segments of the salient structures of the chain, whereas, the NW-SE trending thrusts inverted the low angle portion of pre-thrusting normal faults in the middle-lower crust and displaced with a short-cut the normal faults in the upper portion of the crust. As a result, the pattern of the pre-existing normal faults is inherited on the salient structures of the Central and Southern Apennine Chain.

How long do structures take to form in transpression zones? A cautionary tale from California

It is generally assumed that individual sets of coplanar and colinear deformation structures form together during events that are of relatively short duration (1–5 m.y.). The record of deformation in a sequence of Late Cretaceous to Holocene sedimentary rocks from the northern Salinian block of California spans at least 30 m.y. and illustrates that this assumption is sometimes incorrect. At different localities, geometrically and kinematically identical contractional structures either predate or postdate local unconformities of varying age within the succession, so that it is possible to define at least four chronologically distinct, but otherwise indistinguishable, deformation episodes. In the absence of the unconformities, the punctuated nature of the deformation would not be apparent, therefore suggesting that subparallel structures may form during successive, distinct deformations spread out over long time periods. In the northern Salinian block, the inferred contractional strain field is approximately normal to the adjacent San Andreas fault and appears to have been consistently oriented in this direction during deformation events recorded over the past 30–45 m.y. The strain pattern is most easily explained by efficient partitioning of transpressional strains into fault-normal shortening and right-lateral faulting during episodic regional deformations. We propose that reactivation of preexisting structural anisotropies controls the observed partitioning of deformation in many transpression zones.

The Role of Calcining and Basal Fluidization in the Long Runout of Carbonate Slides: An Example from the Heart Mountain Slide Block, Wyoming and Montana, U.S.A.

In order to understand the movement of large rock masses or allochthons on low-angle surfaces, we have studied the 3400-km2 Heart Mountain slide block of northwestern Wyoming and southwestern Montana. The Heart Mountain slide block was initiated on a 2° gradient, with its toe thrust a minimum of 45 km across an early Eocene landscape. The slide block moved on a basal layer that ranges in thickness from a few tens of centimeters to several meters. This basal layer commonly has a concrete-like appearance of rounded, mixed-lithology grains in a fine-grained carbonate matrix, and in some locations it has features similar to sedimentary deposits, including both normal and inverse grading, flow banding, turbidite-like structures, and clastic dikes containing pieces of carbonized wood. Nowhere did we observe crosscutting relationships in the basal layer or overlying clastic dikes, as would be expected from incremental or noncatastrophic emplacement. Results from cathodoluminescence and &dgr;18O, &dgr;13C, and 87Sr/86Sr isotopic compositions from the basal layer support a single movement event followed by hydrothermal and meteoric fluids percolating through a permeable basal layer. These observations suggest that a catastrophic movement on the detachment resulted in frictional heating at the base of the slide. When the generated heat was at least 800°C, calcining of carbonates occurred, yielding calcium and magnesium oxide powders and carbon dioxide gas. The calcium oxide powder became mechanically fluidized by the pressurized carbon dioxide gas, leading to a reduced coefficient of friction at the base of the slide, which in turn permitted the long runout on such a low-angle surface. This mechanism might be applied to explain a wide range of catastrophic sliding events where carbonate rocks are involved.

Repeated reactivation in the Apennine-Maghrebide system, Italy: a possible example of fault-zone weakening?

Abstract Italy owes its complex geological structure to a double switch in tectonic regime, which involved the opening of the Tethys Ocean during Early Mesozoic time, its closure leading to development of the Apennine-Maghrebide fold-and-thrust belt during the Eocene-Recent interval, and the post-orogenic opening of the Tyrrhenian Sea since Miocene time. This history of tectonic inversion is partly preserved within two major fault zones, the Valnerina Line, in the central Apennines, and the Gratteri-Mount Mufara Line, in central-northern Sicily, which were repeatedly reactivated with different kinematic characters. The relatively long life of these structures indicates that strain was localized along anisotropies inherited from early deformation episodes. However, the progressive widening of both fault zones through time may result from strain-hardening fault-rock behaviour during subsequent deformations, thus suggesting that fault reactivation does not imply fault-zone weakening as is often assumed.

Geometry and kinematics of Triassic-to-Recent structures in the Northern-Central Apennines: a review and an original working hypothesis

The geological evolution of the Northern-Central Apennines has been strongly controlled by structural features inherited from pre-thrusting stages. The Apennines consist of a fold-and-thrust belt developed during Neogene time, involving sedimentary successions that were deposited within different paleogeographic domains. The paleogeographic evolution was controlled by the effects of syn-sedimentary tectonics that were active in the Triassic-Neogene time interval, and that are, from Trias to Neogene, related to different geodynamic settings. The Jurassic extensional phase favoured the dissecting of the Triassic carbonate platform and led to the development of different paleogeographic domains. Main oblique and trasversal faults, with transtensive kinematics, characterized the boundaries between different domains. After the Jurassic phase, from Cretaceous to Neogene, the Northern-Central Apennines were characterized by the development of ridges and depressions. These structures were affected by the development of normal fault systems, bending processes within the ridges, with uplift of the crestal sectors and tilting in the peripheripheral ones. The geometries and the structural setting of the foreland domains, of the foredeep and piggy back basins and of the Neogene Apenninic thrust belt are controlled by evolution of the former tectonic elements evolution. The development of extensional faults, the uplift and bending observed in the Apenninic sedimentary sequences during the convergence phase and during a part of the continental collision phase could represent distal effect of the dominant compressional regime. In this geodynamic domain the first tectonic inversion processes in a positive sense occurred along the pre-existing Jurassic listric faults systems. The reactivation first affected the flat sectors of the fault planes and then progressively more superficial ones. In the upper sedimentary cover these processes favoured buckling, flexuring and faulting. Furthermore, during the subsequent involvement of the foreland domains in the foredeep and chain systems, the propagating thrust surface still reused former discontinuities both as a frontal and lateral ramp, completely inverting their movement. During the compressional stage the west-dipping listric normal faults were reactivated in positive sense. The former east-dipping normal faults have been eastward rotated or offset by thrust faults.

Tectonic evolution of the Northern Salinian Block, California, USA: Paleogene to Recent shortening in a transform fault-bounded continental fragment

Abstract The complex structural setting of the western margin of North America is interpreted to result from oblique convergence of the North American and Pacific plates, accommodated by both right-lateral slip along the San Andreas fault and shortening east and west of it. Strike-slip movements along the San Andreas fault led to the detachment of a continental fragment (the Salinian Block) from the North American margin during early Miocene, and its translation northwestwards for over 300 km. Structural analysis in the Northern Salinian Block, west of the San Andreas fault, reveals a NE-SW-directed shortening accommodated by NW-SE-trending folds and thrusts with a dip-slip kinematic character. The stratigraphic record of progressively younger unconformities affected by both folds and thrusts, as well as the overprinting relationships among these contractional structures, enables us to clarify the tectonic evolution of the region since Paleocene time. The Paleocene to Recent history of the Salinian Block was dominated by strike-slip along the San Andreas fault, and by shortening perpendicular to it. The partitioning between strike-slip and dip-slip movements appears to be controlled by a pre-existing tectonic feature. The results from structural analysis along the Salinian Block are integrated into a deformation model for the western margin of North America, providing additional constraints on the timing of deformation and helping to clarify the role of strain-partitioning processes in the obliquely convergent California margin.