Orocline timing through joint analysis: Insights from the Ibero-Armorican Arc

Abstract

The timing and kinematics of oroclinal bending in the core of the Ibero-Armorican Arc (IAA) has recently been constrained using paleomagnetic data from the Cantabrian Zone in northern Iberia. This study analyzes the joint-patterns present in rock units deposited pre-, syn- and post-oroclinal bending. Systematic changes in the orientations of tensional joint-sets in superimposed stratigraphic units are interpreted to record the progressive stages of oroclinal bending in the core of the IAA. Time constrains for joint set development are constrained by the known ages of the bounding unconformities that limit the studied stratigraphic units. Joint azimuth variability in the pre-orocline rocks (Neoproterozoic to pre-Upper Carboniferous) is comparable to the present arc curvature of the orocline (about 180°); the joints in the syn-orocline rocks (Upper Pennsylvanian or Stephanian, 304 to 299 Ma) show a lower azimuthal variability that is comparable to about 50–70% of the total curvature seen in pre-orocline rocks. Finally, post-orocline rocks (Permian) contain joints that have uniform azimuths for each set across the entirety of the present-day arc. Together these spatially and temporally distinct joint sets suggest that rotations in the Cantabrian Zone took place in the Upper Pennsylvanian during a ca. 10 Ma time period, which agrees well with previous paleomagnetic arguments. The data also provides supporting evidence for oroclinal bending by rotation around vertical axes of an initially linear, or nearly linear, orogenic belt. And finally, these data highlight the potential power in using tectonic joint sets for constraining thrust belt kinematics in curved orogenic systems when unconformity bounded stratigraphic sequences are present that are coeval with orocline development.