Abstract
The Cingoli arcuate anticline is part of the Apennines fold-thrust belt in Italy. The anticline involves sedimentary carbonate strata generally affected by syn-thrusting contractional structures such as bed-normal pressure solution seams, folds, and reverse faults. An exception is constituted by an outcrop in the anticline hinge, where sub-horizontal carbonate and chert beds are affected by joints and intraformational short normal faults. These faults are poorly-systematic and conceivably polygonal in map view. They cut through the carbonate beds while usually stop against the chert layers that are bent and extended along the faults themselves. At the fault tips, the displacement is generally transferred, via a lateral step, to an adjacent similar fault segment. The fault surfaces are often characterized by slickolites, greenish clayey residue, and micro-breccias including chert and carbonate clasts. Fault displacement is partly or largely accommodated by pressure solution. The faults, in effect, are usually accompanied by bed-parallel pressure solution seams in the two contractional quadrants located at the present or past fault tips. The pressure solution features fade away departing from the faults. This evidence and others are analytically explained with fault tip stress distributions. The faults are interpreted as polygonal normal faults syn-tectonically (syn-thrusting) nucleated in response to multi-directional stretching processes occurred at the Cingoli triple-folded anticline extrados. The faults then grew through a four-stage process: (1. stop) the faults stopped at the competent chert beds; (2. shrink) faulting produced shrinkage (pressure solution) of carbonate beds at the fault compressive tips; (3. shrink and step) the faults stepped laterally at the competent chert beds; (4. shatter) the chert beds were shattered along the fault surfaces. The case presented constitutes the first reported one of syn-thrusting non-diagenetic polygonal normal faults.
Highlights
Polygonal faults (Figure 1) form a relatively-recent new class of normal faults, which were first observed along the North Sea basin margin in two-dimensional seismic reflection images (Henriet et al, 1991)
We present a few X-ray diffractograms to better understand some observed pressure solution structures; (3) we focus on the fault terminations by presenting a photographic documentation and an analytical explanation for some field observations involving fault tip displacement and associated bed-parallel pressure solution; and (4) we present similarities and contrasts between the normal faults studied in the Cingoli anticline hinge and some polygonal normal faults buried in the North Sea basin (Cartwright and Lonergan, 1996; Cartwright, 2011)
The Scaglia Bianca and Rossa Fms. in the Cingoli anticline and in most thrust sheets of the northern Apennines are usually characterized by multiple sets of contractional structures including bed-normal and bed-oblique pressure solution seams and reverse faults, which are variably oriented
Summary
Polygonal faults (Figure 1) form a relatively-recent new class of normal faults, which were first observed along the North Sea basin margin in two-dimensional seismic reflection images (Henriet et al, 1991). We present our previous and new results that are organized as follows: (1) we synthesize our results from previous structural studies (background) on the Cingoli anticline (Petracchini et al, 2012, 2015; Antonellini et al, 2014) These results are useful to understand the timing (relative to the Cingoli anticline formation) of the nucleation and growth of the normal faults analyzed ; (2) we present the field observations and data with particular focus on exposure images and on the attitude of faults, slickenlines, joints, and bedding. To make results as comprehensible as possible, this latter method is better explained below together with the description of data
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