Compaction‐induced stress variations with depth in an active anticline: Northern Apennines, Italy

Abstract

[1] It is shown that the stress field can vary with depth, even within a homogeneous tectonic setting, as documented for the active Mirandola fault-related fold along the buried front of the northern Apennines. Analyses of borehole breakouts and other well data, integrated with seismological information and field evidence, show that extension perpendicular to the fold axis, above approximately 1200 m, changes to a strike-slip stress field and finally to compression near the main detachment at depth. Similar along-depth strain variations recognized in non-active anticlines are usually explained by invoking tangential longitudinal strain folding (i.e., stretching above and shortening below a neutral surface) or gravitational instabilities. However, in this study, we propose that differential compaction may play a significant and generally overlooked role. With the aid of numerical modeling, it is shown that where fold limbs are onlapped by highly compactable deposits (as in the Mirandola area), differential compaction induces stretching at shallow (<1-2 km) depths. The amount of stretching is a function of the shape of the fold and of the thickness of the syn-tectonic sediments. We conclude that in the Mirandola case study, the stress variations observed with depth are the result of a combination of a regional compression at depth and local tension driven by differential compaction of growth strata on the limbs of the anticline with respect to its crest.

Frontal compression along the Apennines thrust system: The Emilia 2012 example from seismicity to crustal structure

“…Deep features revealed by our tomography complement results from seismic profiles (Pieri and Groppi, 1981;Picotti and Pazzaglia, 2008;Toscani et al, 2009b;Carminati et al, 2010;Fantoni and Franciosi, 2010;Boccaletti et al, 2011;Lavecchia et al, 2012;Bonini and Toscani, 2014) and geologic reconstructions (Burrato et al, 2003). The strong model complexity observed in the aftershocks volume, responsible for impressive hypocentral refinements of this study, is helpful, with the aftershock distribution itself, in defining the fault geometry.…”

“…The tip of the low-angle eastern segment struck by the Mw 6.0 May 20 event emerges close to middle thrust of the Ferrara thrust-fold system as defined by seismic reflection data (section B in Fig. 6; Carminati et al, 2010). In addition, relocated aftershocks of the May 20 main-shock preferentially cluster on the deeper, low-angle portion of the middle Ferrara thrust (section B in Fig.…”

“…Hence, the deep geometry of the thrusts is not defined univocally. In particular, the Ferrara outer thrusts are defined as either high-angle structures deeply rooted in the basement (Picotti and Pazzaglia, 2008) or listric faults rooting on a regional basal detachment at the bottom of the Mesozoic carbonates (8-10 km depth, Carminati et al, 2010;Boccaletti et al, 2011;Bonini and Toscani, 2014).…”

On the geodynamics of the northern Adriatic plate

“…As the subduction hinge retreats toward the foreland, the subsidence rate increases subduction zones (e.g., Doglioni et al 1999) reveals the presence of an active accretionary wedge delimited at its footwall by the external thrust front and at its hangingwall by a thrust responsible for the rapid uplift of the chain which generally shows an ''out-ofsequence'' character. Recent studies have confirmed this interpretative scheme also for the central-northern portions of the Apennines (Basili et al 2008;Montone et al 2004;Scrocca et al 2007;Carminati et al 2010b). The accretion occurs at the expenses of the upper layers of the lower plate.…”

“…4). In fact the Cremona salient propagated into the Lombard Graben-basin to the north; the Parma recess developed at the encroachment with the Trento Horst, whereas the Ferrara salient or arc occurs along the southern propagation of the Belluno Graben (Carminati et al 2010b). This is a tectonic terminology, since we are defining horst and gaben as controlling thickness variations, rather than lateral facies (e.g., Trento Platform and Belluno Basin).…”