KINEMATICS OF OROCLINE RELATED SHORTENING IN THE PONGA UNIT: STRUCTURAL ANALYSIS OF THE TARNA SYNFORM AND THE RIO MONASTERIO ANTIFORM, NORTHERN SPAIN

The Cantabrian orocline of NW Iberia is interpreted as a secondary orocline formed by vertical-axis rotation of an originally linear, N-S trending segment of the Western European Variscan belt (WEVB). We present a local-scale structural study of the Ponga Unit, a Cambrian to Carboniferous tectonostratigraphic package within the WEVB foreland fold and thrust belt that lies within the core region of the Cantabrian orocline. Our primary goal is to determine if there is deformation of the fold and thrust belt that is attributable to formation of the Cantabrian orocline.

Mapping and structural analysis within the Ponga unit are restricted to the Laviana and Ri­oseco thrust sheets, and associated bounding thrusts. The Laviana and Rioseco thrust sheets have been deformed into two km-scale anticline-syncline pairs, producing a sin­uous fold interference pattern that is characteristic of the Ponga Unit. The major litho­logical units of the Ponga are the rheologically competent Lower Ordovician Barrios quartzite, and the less competent Barcaliente limestone and Beleño shale and sandstone units, both of which are Carboniferous. Over 800 structural orientation measurements were collected across the 20x20 km map area. These data, coupled with data from regional geological maps, allow for analysis of the crustal structure at an unprecedented level of detail. We find that: 1) the map pattern defines a mushroom-type fold-interference pattern, indicating two distinct deformational events characterized by principle compressive stresses oriented at a high angle to one another; 2) major folds of the Ponga unit plunge 45^o to the West whereas minor folds are steep (~70^o) to vertical indicating a near-vertical axis of rotation during deformation of the WEVB; 3) strike-parallel shortening can be attributed to orocline formation and is greater in the youngest, structurally deepest thrust sheet (Rioseco), which is situated closest to the core of the Cantabrian orocline. The difference in relative shortening from thrust sheet to thrust sheet is in­terpreted as the result of disharmonic folding that is controlled by the Barrios quartzite. Our results indicate that early N-S trending folds, which resulted in WEVB formation, were refolded during an orogen parallel compression event at­tributable to formation of the Cantabrian orocline.