Recent Reactivation of a Miocene Fold-and-Thrust Belt (Northern Apennines, Italy). Implications for the Petroleum System

Picotti, Vincenzo ¹; Capozzi, Rossella ¹; Minola, Matteo ²; Terazzi, Francesco ²; Oppo, Davide ¹; Ponza, Alessio ¹
(1)Earth and Environmental Science, Università di Bologna, Bologna, Italy. (2) Exploration and Production, Edison Spa, Milan, Italy.

The Northern Apennines fold and thrust belt formed throughout the Neogene, following a dramatic flip in subduction around 35 Ma, changing the destination of the Europe-verging accretionary wedge, eventually defining the main tectonic units of the belt: the uppermost, so-called Ligurian unit, a nappe thrusting the lower unit, a pro-foreland basin, developed over the Adriatic lower plate, that was telescoped until the present-day Po Plain.

In the Apennines near Parma, the Ligurian front advanced rapidly during the Burdigalian, then slowed down due to the growth of the Salsomaggiore thrust sheet that affected the foreland during the Serravallian. The Ligurian was locally hindered by the Salsomaggiore anticline, but went on overthrusting until the end of the Miocene. The main external fronts of the so-called Emilia arc, formed during Tortonian and Messinian, are documented by the clear divergent pattern of seismic reflectors. As a whole, the Ligurian unit acts as a seal for the culminations of the deformed foreland. During the Pliocene and Pleistocene, the thrust belt has been deeply affected by an out-of sequence reactivation of the thrust sheets. This was due to the diminishing space on the foreland that encroached the Southern Alps and the fact that the new depocenter was basically located near the Middle-Late Miocene one.

The reactivation produced a tightening of the anticlines, with smaller pop-up decreasing the width and length of the culminations. Several transverse structures, lateral ramps of previous NE- verging thrust, were reactivated with NW-vergence producing dome-and swell culminations, locally highlighted by the distribution of the Pliocene deposits. Finally, new NW-verging structures cut across the whole edifice, locally deforming Late Pleistocene continental deposits. All these structures are kinematically connected with basement ramps, flooring at the middle crust.

The main effect on the petroleum system is fluid migration along the strike of the belt, affecting traps and resulting in HC remobilization due to the changing geometry. As a consequence of the renewed subsidence, the source is presently below of the oil window as documented by the thermogenic methane trapped in the new culminations.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.