Plate kinematics during the last 20 m.y. and the problem of “present” motions

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Abstract
While increasingly precise plate reconstructions are being made at intervals of the order 10 m.y. for times corresponding to well-defined magnetic lineations (Klitgord and Schouten, this volume; Srivastava and Tapscott, this volume), much less is known about possible irregularities in plate motions over shorter time scales. Obviously the most recent geologic past offers the best hope for detecting any “kinematic fine structure.” This paper discusses the last 10–20 m.y., with special emphasis on the “present,” which, of course, is not an instant but involves a finite averaging interval, whether several years for geodetic or earthquake measurements, or several million years for a measurement based on magnetic anomalies. The chapter begins with Wegener’s attempts to measure continental drift, reviews the geodetic measurements on Iceland where the MAR (Mid-Atlantic Ridge) plate boundary is exposed, and discusses the latest attempts to measure plate motions over the “historical” present (several years time scale). The latter field is just being launched (see Anderle, this volume), and whatever is written today will soon be outdated. The main part of this paper is devoted to the question of relative and absolute plate motions over the “geological” present (m.y. time scale) resolved by magnetic lineations, transform faults, and hotspot traces. There is very little information about variability of plate motion over time scales intermediate between the “historical” and “geological” present. A case is made for significant geologically “short-period” (a few m.y.) global fluctuations in rates of plate motions over the last 10–20 m.y. The latest episode began

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