Anatomy and Structural Evolution of a Mesozoic Structural High: An Example from the Western Po Plain (Italy)

Structural highs formed during the extensional tectonics of the Mesozoic rifting phase, represent important petroleum systems in Italy and hold potential for geothermal energy exploitation and geofluid storage. These structural highs, commonly exposed in the Southern Alps, Northern Apennines, or buried beneath the Po Plain foredeep, were generally overprinted by later tectonic phases. Specifically, the compressional tectonics associated with the Alpine orogeny since the Eocene often mask the original structure of mesozoic structural highs. The Gaggiano structure, located in the southwestern Po Plain, offers a rare opportunity to study a Mesozoic high only partially affected by subsequent tectonics.

This study investigates the geological and structural evolution of the Gaggiano area using a 3D seismic volume covering an area of approximately 180 km². Seven seismic horizons, spanning from the Lower/Middle Triassic to the Miocene, were interpreted with detailed mapping, using a spacing of 300 m and 250 m between adjacent inline (N-S) and crossline (E-W) sections, respectively. The interpreted fault network, combined with thickness and top maps of key horizons, allowed the reconstruction of the structural evolution of the area and its associated fault system.

Well data from the topographically most elevated portion of the Gaggiano high highlight a stratigraphic succession that includes continental to evaporitic rocks (Lower Triassic), overlain by organic-rich mudstone limestones and dolomitized intervals (Middle Triassic), followed by Jurassic and Cretaceous pelagic limestones. The succession is highly condensed on the structural high, with significant stratigraphic gaps, including the absence of Upper Triassic rocks, most of the Jurassic (~30 m preserved), and Lower Cretaceous deposits. From the Middle Eocene onward, siliciclastic sediments of the Po Plain foredeep were deposited.

During the Middle Triassic, the structural evolution of the Gaggiano high was controlled by a N-S trending, east-dipping domino-style extensional fault system, with associated E-W striking normal faults. During the Late Triassic to Early Jurassic, the N-S striking east-dipping faults dominated. The Gaggiano high formed at the footwall of an E-dipping fault which accommodates a maximum throw of ~700 m. By the Early Jurassic, pelagic carbonates were deposited unconformably over the structural high, progressively leveling the paleotopography during the deposition of the Scaglia Formation (Upper Cretaceous-Middle Eocene). During or shortly after the deposition of Scaglia Fm., the Gaggiano area was affected by extensional tectonics testified by NW-SE striking normal faults affecting the top of Scaglia Fm. and of older formations. The NW-SE striking faults locally reactivate the pre-existing fault system. Partial involvement in Miocene compressional tectonics is evident from gentle folds affecting the sedimentary succession near Mesozoic extensional faults, suggesting positive fault reactivation.

The findings provide key insights into the interplay of extensional and compressional tectonics in shaping the evolution of the Gaggiano area. This study contributes to a better understanding of Mesozoic reservoirs and their potential reuse in sustainable energy applications.