PALEOMAGNETIC ANALYSIS OF THE INTERACTION BETWEEN THRUST SHEET ROTATION AND BASEMENT STRUCTURES IN THE ESLA NAPPE TECTONIC UNIT, NW IBERIA

The Esla Nappe tectonic unit lies along the southern boundary of the Cantabrian Zone – a highly curved foreland fold-thrust belt that was deformed during the final amalgamation of the Pangea supercontinent. Previous structural and paleomagnetic analysis of the Cantabrian Zone suggests a two stage tectonic history in which an originally linear belt was bent into the orocline we see today. The Esla Nappe is a particularly complex region due to the interaction of classic thin-skinned tectonic structures and larger duplex systems that underlie the Esla Nappe. Protracted deformation during late-stage oroclinal bending caused reactivation of the underlying duplex system, which complicated the overlying thrust sheets and resulted in significant localized vertical-axis rotation. Theoretical models have been created to quantify the contribution of these preexisting basement structures to the overall kinematics of the region, but paleomagnetic analysis of vertical-axis rotation is required to constrain the models and to gain a better understanding of the interaction of mid-crustal structures to upper-crustal thrust sheets.

A total of 74 paleomagnetic sites were collected from the Portilla and Santa Lucia formations, two carbonate reef platform units from the middle Devonian. Similar to other regions of the Cantabrian Zone, Esla Nappe samples have a characteristic magnetization that is secondary in nature and was acquired in the Late Carboniferous during the final stages of Variscan deformation. Results from preliminary data show varying trends in the paleomagnetic declination. From west to east on the western flank of the Esla Nappe, declination change from E directed to SSE, similar to the core of the thrust sheet and to the likely Late Carboniferous reference direction for stable Iberia. Along the northern edge of the nappe, declinations are easterly with more noticeable local variation. These new paleomagnetic data help elucidate the thrust sheet kinematics as they relate to existing basement folds and will help clarify how the existing crustal structure helped contribute to the overall deformation.