The worldwide active middle/late eocene geodynamic episode with peaks at ±45 and ±37 m.y. B.P., and implications and problems of orogeny and sea-floor spreading

Two remarkable geodynamic events in earth history at ± 45 and ± 37 m.y. ago, corresponding to the early and late Pyrenean orogenic phases of Middle and Late Eocene age are described in this paper. Based on numerous data, each of these events is manifested by a set of many various worldwide geologic activities, such as orogenic shortening, granodioritic plutonism, regional metamorphism, change in rate or direction of sea-floor spreading, and global marine regression. These activities shed light on the kinematic relation between plate motions and orogeny because they are widespread and coeval.According to the plate tectonic theory, mountain building is attributed mainly to three types of convergent plate motions: collision, subduction, and obduction. However, an extensive orogenic process does not occur randomly and locally, and does not proceed diachronously by steady plate motion or subduction. The data presented here indicate short duration and synchronism of

1. (a) worldwide orogenic deformations at specific times and

2. (b) abrupt changes of plate motion during the orogeny.

In this paper it is shown that there were two geodynamic peaks (±45 and ±37 m.y. ago) which involved more than 80 separate processes of displacement. Such a major reorganization in the plate tectonic pattern reflects a turning point in the geotectonic history.The generally steady, in several oceans variable sea-floor spreading on the one side and the episodic orogenic compressions correspond neither temporally nor kinematically. The two types of movement represent dissimilar kinematic actions. Ocean-floor spreading and orogeny are two different mechanisms that alternate in earth history. It is not the long-term sea-floor movement but a short-term readjustment in the plate tectonic pattern that is related to orogeny. The normal, continuous sea-floor spreading becomes disarranged; its motion is mainly slowed or stopped at times of orogeny which is discontinuous, pulsatory and of short duration. The driving forces of the two processes are different, as is apparent from the contrasting rates of motion.The essential prerequisite for crustal shortening during alpinotype viscoplastic deformation seems to be a worldwide penetrative remobilization of continental marginal zones. Due to the loss of rigidity, the bordering plates then converged. The associated migmatization and granitization point to a thermal origin of this crustal mobilization. Episodic remobilization and folding of continental margins are the expression of such an endodynamic pulsation, released by processes in the earth's interior.