View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Aberdeen University Research Archive INVITED PAPER Ital. J. Geosci., Vol. 138 (2019), pp. 371-389, 13 Figs. (https://doi.org/10.3301/IJG.2019.11) © Società Geologica Italiana, Roma 2019 Syn-kinematic sedimentary systems as constraints on the structural response of thrust belts: re-examining the structural style of the Maghrebian thrust belt of Eastern Sicily ROBERT W.H. BUTLER (1), ROSANNA MANISCALCO (2) & PATRICIA R. PINTER (3) ABSTRACT drilling (reviewed by BUTLER et alii, 2018). Indeed, seismic Structural evolution of thrust wedges is influenced by syn- reflection data can yield entirely misleading images kinematic deposition ahead and upon them. It dampens uplift of as a consequence of spatially complex heterogeneous synclines and promotes amplification of anticlines. High-resolution seismic velocity and correspondingly tortuous ray-paths. marine seismic images and analogue experiments indicate that Consequently, maps of surface geology remain important emergent thrusts only form upper thrust flats (detachments, required to form far-travelled tectonic allochthons) when depositional rates sources of data from which structure can be projected at the thrust front are very low. These interactions are illustrated by to depth. However, challenges arise in subsurface re-examining tectonostratigraphic evolution of the Neogene eastern interpretation when structures are disharmonic, separated Sicily, a rotational thrust belt forming part of the Maghrebian orogen by multiple detachments. The approach developed here of the central Mediterranean. Existing interpretations of the thrust belt which invoke stacking of far-travelled thrust sheets (e.g. the “Sicilide”) arises from the recognition that emergent thrust systems are incompatible with stratigraphic data that are better explained by are geometrically distinctive from buried systems (Fig. 1). deposition upon a simple emergent imbricate fan. The distribution of In buried thrust systems, imbricate thrusts can recombine pre-kinematic successions reflects basin-structuring before Neogene up-dip to form duplexes (e.g. BOYER & ELLIOTT, 1982). thrusting, so that Mesozoic depositional units are unreliable guides for structural interpretation and associated palinspastic restoration. Yet when thrusts climb to the syn-orogenic surface they Deformation in the thrust wedge is marked by spaced anticlines that can become progressively over-stepped by syn-kinematic amplify together in an array, with individual structures active for sediments. Not only do these sediments provide key at least 6 million years. The shortening across individual structures information on the growth of structures, they can also in the thrust wedge is rather low (a few km). Yet, reconstructing the palaeomagnetic rotation history of the thrust wedge requires influence the trajectories of faults and thus the geometry >200km displacement, which must have chiefly localised on the of the thrust wedge (e.g. STORTI & MCCLAY, 1995). The basal detachment. This behaviour was strongly facilitated by the syn- aim of this paper is to examine structural evolution in kinematic thrust front remaining sediment-starved for much of its history. In contrast, during much of the Mio-Pliocene, sedimentation the Maghrebian thrust system of central-eastern Sicily, was ponded on top of the thrust wedge. In eastern Sicily, only in a setting with unrivalled syn-kinematic successions and the Pleistocene, and briefly during the Tortonian, did significant existing structural interpretations that, as discussed below, sedimentation occur at the thrust front and this temporarily changed are becoming increasingly untenable in the light of new thrust localisation. Displacement partitioning reflects this inferred distribution of syn-kinematic deposition. Characterising these geological data. This paper necessarily discusses regional interactions is important for interpreting the structural style in thrust geology and, while attempting to be concise, unavoidably systems, especially when assessing the role of allochthonous thrust focusses on specific structural-stratigraphic relationships sheets. that are primarily relevant to Sicilian geology. Brief introductory notes are provided below. Some of the more KEY WORDS: tectono-stratigraphy, thrust geometry, generic observations and deductions are developed in a palaeogeographic reconstruction, Neogene. companion paper (BUTLER, in review), which has examples from the Himalayas and Apennines. INTRODUCTION EMERGENT VS BURIED THRUST SYSTEMS Structural interpretation of continental thrust systems is inherently uncertain. Even in settings with numerous Many of the basic geometric elements of thrust belts well-penetrations and seismic reflection data acquired were developed from preserved parts of ancient systems and processed to high industry standards, subsurface such as the Appalachians and Moine Thrust Belt (e.g. interpretations commonly fail when tested by further HATCHER, 1978; ELLIOTT & JOHNSON, 1980) that did not directly interact with their syn-orogenic surface. It is these locations that provide type-examples of duplexes and related structures: thrust sheets formed at depth, well- (1) Fold-thrust Research Group, School of Geosciences, Univer- sity of Aberdeen, Aberdeen AB24 3UE, United Kingdom. below the syn-orogenic surface. They are entirely enclosed (2) Department of Biological, Geological and Environmental by thrust surfaces (e.g. BOYER & ELLIOTT, 1982; BUTLER, 1987; Sciences, University of Catania, Corso Italia, 57, 95129 Catania, Italy. Fig. 1) and are therefore said to be “buried”. In contrast, (3) School of Geosciences, University of Aberdeen, Aberdeen “emergent” thrust systems are represented by imbricate AB24 3UE, United Kingdom (present address: CGG Robertson, Tyn-y- coed, Llandudno LL30 1SA, United Kingdom). fans and related folds that influence accumulation of syn- Corresponding author e-mail: [email protected] kinematic sedimentation and associated local erosion (e.g. 372 R.W.H. BUTLER ET ALII WILLIAMS, 1993). On restored sections, emergent thrust TOTAKE et alii, 2018). Therefore, they offer exceptional systems have segments of syn-kinematic sedimentation images of structures and their relationship to stratigraphy. displayed on their restored templates (Fig. 1b), the bed- Consider an example from the deep-water fold-thrust belt lengths of which decrease up-section in response to the of the outer Niger Delta (Fig. 2c-e). In this structure, the gradual accumulation of shortening. Restored sections for forelimb of a thrust-associated anticline is cut by an array buried thrust systems cannot show syn-kinematic deposits of thrusts. Stratal reflectors continuous with an unfaulted because the thrust slices were stacked entirely in the stratigraphic section ahead of the fold onlap faulted strata subsurface (Fig. 1c). in the forelimb. However, faults shallower in the section A variety of approaches reveal fundamental differences cut deeper stratal reflectors but are themselves overstepped in the structure of buried thrust systems in comparison by younger ones. Thus, the thrust splays repeatedly climb with those that develop into strata that accumulated during section but cease displacement, to be replaced by younger deformation (Fig. 2). Many analogue experiments have fault strands progressively developed in their hangingwalls. explored the large-scale dynamics of thrust wedges and a Syn-kinematic sedimentation inhibits the activation few examine the role of sedimentation on the trajectories of upper thrust detachments. This type of behaviour is of individual thrusts. STORTI & MCCLAY (1995) show thrusts directly analogous to salt-canopies climbing ramps as a that form without syn-kinematic sedimentation climb form of progressive unconformity, in halokinetic systems section and then follow upper detachments (thrust flats). (e.g. HUDEC & JACKSON, 2009). Syn-kinematic sedimentation The resultant thrust wedge structure is marked by closely- at the thrust front forces this thrust to climb a ramp. spaced faults (Fig. 2a). At large displacements these types Insights from analogue experiments and numerous of thrusts can recombine up-dip to entirely isolate thrust high-quality seismic images from deepwater thrust slices, forming duplexes (in the manner and sense of BOYER belts indicate that syn-kinematic sedimentation exerts a & ELLIOTT, 1982). In contrast, when there is syn-kinematic primary control on thrust geometry. Therefore, emergent sedimentation the imbricate thrusts continue to climb thrusts systems are expected to develop in a distinctly ramps into the growth strata. Upper thrust detachments different fashion to buried systems. Simple application are not activated. The resultant thrust wedge comprises an of duplex models with upper thrust detachments would array of widely spaced anticlines separated by synclines appear unwarranted for thrust belts that hosted significant within which thrust-top sediments are ponded. The syn-kinematic sedimentation. These are the deductions generality of the results of STORTI & MCCLAY (1995) have that inspire a re-examination of thrust system evolution in since been repeatedly reproduced (e.g. BONNET et alii, 2008; the Maghrebian thrust belt of Sicily, building on our long- GRAVELEAU et alii, 2012). Insights on natural relationships between strata standing studies of syn-kinematic deposition. and deformation are best gained from marine seismic reflection data. Compared