What Causes the Non-closure of the Cocos-Nazca-Pacific Plate Motion Circuit?

By far the largest plate-circuit misfit on the planet for geologically current plate motion is that of the Cocos, Nazca, and Pacific plates. This plate motion circuit fails closure by a linear velocity of 12 mm a^–1 ±4 mm a^–1 (DeMets et al., 2010, Zhang et al., 2017).  Here we investigate this nonclosure.  In an initial test, we omit the spreading rates along the Cocos-Pacific plate boundary north of the Orozco transform fault where it appears that the Pacific and Rivera plates are separated by a diffuse boundary.  With this omission, the non-closure linear velocity shrinks to 9 mm a^–1 ±4 mm a^–1 (95% confidence limits) with a non-closure angular velocity of 0.22° Ma^–1 (± 0.12° Ma^-1; 95% confidence limits) about a pole at 22°N, 92°W.  The size of the misfit remains too large to be explained by any known processes of intraplate deformation and suggests that there is an unrecognized plate boundary somewhere in the circuit.

We argue that undiscovered plate boundaries (or intraplate deformation large enough to explain the observed non-closure) within the Pacific plate and most of the Nazca plate are implausible, which leaves either a boundary within the traditionally defined Cocos plate or possibly a boundary within the northeast corner of the currently defined Nazca plate.  If the spreading rates and transform faults along the traditionally defined Cocos-Nazca plate boundary east of ≈87°W are eliminated from the Cocos-Nazca data set, the non-closure velocity is reduced to 3 mm a^–1 ±4 mm a^–1 (95% confidence limits), small enough to be within uncertainty or to be explained by expected horizontal thermal contraction.

This result indicates that the traditionally defined Cocos-Nazca plate boundary east of ≈87⁰W may not record motion between the Cocos and Nazca plate after all, but instead records motion between a small previously unrecognized plate and either the Cocos or Nazca plate.  The distribution of earthquakes suggests that the better candidate is a small plate within the traditionally defined Cocos plate.  We propose to call this hypothesized plate the Kahlo plate.  A possible location for a hypothesized narrow plate boundary and an alternative hypothesized diffuse plate boundary will be presented and discussed.