Understanding paleomagnetic rotations in Sicily: thrust vs. strike-slipe tectonics

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The paleomagnetic investigation of the western Sicily Maghrebian belt has revealed since the 1970s thatlarge clockwise (CW) rotations up to 140° with respect to the Hyblean-African foreland occurred synchronouswith Tertiary shortening of the chain. The observation that rotations decrease stepwise from internal to externaltectono-stratigraphic units led in the 1990s to a widely accepted model postulating that rotational thrust-sheetemplaced during forward orogenic propagation. More recently, other authors suggested that CW rotations fromSicily are conversely the result of late orogenic dextral strike-slip tectonics. Here we report on a paleomagneticinvestigation of 30 Jurassic-Eocene sedimentary sites sampled mainly across the WNW-ESE Mt. Kumeta andRocca Busambra ridges (Trapanese Unit), both bounded to the north by high-angle reverse faults with dextralstrike-slip components. We find rotations of 110°-120° at faults of northern ridge margins, that decrease to80°-90° at ~200 m to the south and rise again moving further south. Thus an excess rotation of 20°-40° dueto dextral-strike slip shear is annulled to the regional rotational background of the Trapanese Unit at only200 m from fault traces, translating to paleomagnetically-calculated strike-slip offsets not exceeding 600 m.Further north, seven sites sampled in the Imerese Unit, tectonically stacked above the Trapanese Unit, yielda ~130° rotation. Thus our data confirm that CW rotations in Sicily are predominantly related to thrust-sheetemplacement. Strike-slip tectonics has very limited relevance, and gives local rotations that fade out at only200 m from fault planes.No differential rotation occurred between the Panormide and Imerese units, both characterized by 130°rotation values and likely representing contiguous paleogeographic domains separated by secondary thrustfaults. Considering data from Mt. Kumeta, we constrain at 80° the rotation of the Trapanese Unit. The upperCretaceous-Eocene Scaglia cover of both Mt. Kumeta and Rocca Busambra records an additional 20°-30°rotation with respect to the Jurassic ridge backbones, implying that it was décolled from the substratum.Assuming rigid nappe rotations and a rotation pole along the west Sicily coast, we derive (at a 13.5°Elongitude) a total 230 km rotational shortening of the chain, and individual nappe displacements in the 20-120km range, although further non-rotational shortening might have occurred. Thus paleomagnetism definitelyrepresents a proof for the high allochthony of the Maghrebian chain of Sicily, consistently with recent seismicreflection data interpretations. By further assuming that rotations occurred during late Miocene thrusting eventsin the 12-5 Ma age window (except the Saccense Unit, later stacked onto the foreland), we derive an average17°/Myr rotation rate, and a paleomagnetically-calculated average shortening rate of 3 cm/yr, that is againconsistent with recent geological estimates.
Lingua originaleEnglish
Numero di pagine1
Stato di pubblicazionePublished - 2018


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