Volume 44, Issue 16 p. 8213-8218
Research Letter
Free Access

The Malpelo Plate Hypothesis and implications for nonclosure of the Cocos-Nazca-Pacific plate motion circuit

Tuo Zhang

Tuo Zhang

Department of Earth Science, Rice University, Houston, Texas, USA

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Richard G. Gordon

Corresponding Author

Richard G. Gordon

Department of Earth Science, Rice University, Houston, Texas, USA

Correspondence to: R. G. Gordon,

[email protected]

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Jay K. Mishra

Jay K. Mishra

Department of Earth Science, Rice University, Houston, Texas, USA

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Chengzu Wang

Chengzu Wang

Department of Earth Science, Rice University, Houston, Texas, USA

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First published: 28 July 2017
Citations: 12

Abstract

Using global multiresolution topography, we estimate new transform-fault azimuths along the Cocos-Nazca plate boundary and show that the direction of relative plate motion is 3.3° ± 1.8° (95% confidence limits) clockwise of prior estimates. The new direction of Cocos-Nazca plate motion is, moreover, 4.9° ± 2.7° (95% confidence limits) clockwise of the azimuth of the Panama transform fault. We infer that the plate east of the Panama transform fault is not the Nazca plate but instead is a microplate that we term the Malpelo plate. With the improved transform-fault data, the nonclosure of the Nazca-Cocos-Pacific plate motion circuit is reduced from 15.0 mm a−1 ± 3.8 mm a−1 to 11.6 mm a−1 ± 3.8 mm a−1 (95% confidence limits). The nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered.

Key Points

  • Transform fault azimuths from multibeam sonar show that the direction of Cocos-Nazca plate motion is ~3 degrees clockwise of prior estimates
  • The plate east of the main part of the Panama transform fault is not the Nazca plate but the Malpelo plate
  • Pacific-Nazca-Cocos plate circuit nonclosure is less than that found before but large enough that more undiscovered plate boundaries may exist

Plain Language Summary

The central tenet of plate tectonics is that the tectonic plates are rigid. In sharp conflict with this assumption is the prior result that the relative motions between the Cocos, Nazca, and Pacific tectonic plates, which lie in the Pacific Ocean basin, do not sum to zero as expected if the plates are indeed rigid. From an analysis of plate-motion data, we show that part of the traditionally defined Nazca plate, which lies off the west coast of South America, is really a separate tectonic plate, which we refer to as the Malpelo plate. Recognition of this new tectonic plate reduces the inconsistency in the plate-motion circuit, but a large and significant inconsistency remains. This remaining inconsistency suggests that there may be one or more plate boundaries still remaining to be discovered within these three plates.

1 Introduction

The central tenet of plate tectonics is that the plates are rigid. In sharp conflict with this assumption is the result that the Cocos-Nazca-Pacific plate-motion circuit fails to close by 14 ± 5 mm a−1 (95% confidence limits) [DeMets et al., 2010]. Absent serious errors in the plate-motion data (spreading rates and the azimuths of transform faults), the magnitude of this misfit is difficult to explain from known processes of intraplate deformation, such as horizontal thermal contraction [Collette, 1974; Kumar and Gordon, 2009; Kreemer and Gordon, 2014; Mishra and Gordon, 2016] or movement of plates over a nonspherical Earth [McKenzie, 1972; Turcotte and Oxburgh, 1973]. Alternatively, there may be one or more unrecognized plate boundaries in the circuit, but no such boundary has been found or hypothesized to date.

To make progress on this problem, herein we report three new Cocos-Nazca transform fault azimuths from multibeam data now available through GeoMapApp's global multiresolution topography data sets [Ryan et al., 2009], but unavailable to DeMets et al. [2010].

2 Cocos-Nazca Transform Fault Azimuths

Figure 1 shows the location of the transform faults with useful azimuths along the conventionally defined Cocos-Nazca plate boundary. DeMets et al. [2010] used an azimuth from the Inca transform fault (at 85.3°W) and the easternmost transform fault, the Panama transform fault, to estimate the direction of Cocos-Nazca plate motion (Figures 1 and 2). From a combination of precision depth recorder data and limited multibeam data, DeMets et al. [2010] estimated the azimuth of the Inca transform fault to be 002.0° ± 0.8° (±1σ). Using the more extensive multibeam data now available through GeoMapApp [Ryan et al., 2009], we estimate the strike to be 005.0° ± 0.7° (±1σ) (Figure S1 in the supporting information), which is 3° clockwise of the estimate of DeMets et al. [2010]. We also obtain a new well-constrained estimate of the strike of the 84.7°W transform fault of 003.0° ± 1.2° (±1σ) and a new less well-constrained strike for a short portion of the 84.3°W transform fault of 003.0° ± 3.3° (±1σ) (Figures 2 and S1). Azimuths for these faults were not available to DeMets et al. [2010]. We retain the azimuth (358.0° ± 1.2°) of the Panama transform fault adopted by DeMets et al. [2010] based on satellite altimetry and crossings of precision-depth-recorder profiles (Figure S2).

Details are in the caption following the image
Map of the region surrounding the Cocos-Nazca plate boundary from Global Multiresolution Topography [Ryan et al., 2009]. The white arrows indicate the locations and azimuths of four transform faults discussed herein. Abbreviations: GSC, Galapagos Spreading Center; ESC, Ecuador Spreading Center; CRSC, Costa Rica Spreading Center; TF, transform fault. The image is illuminated from the north.
Details are in the caption following the image
Transform-fault azimuth versus longitude along the Cocos-Nazca and Cocos-Malpelo plate boundaries. Data and results from DeMets et al. [2010] are in blue and from this study in red. Solid curves: Cocos-Nazca plate-motion direction calculated along the Cocos-Nazca plate boundary from the best-fitting angular velocity of DeMets et al. [2010] (blue) or this study (red). Circles: Plate-motion directions calculated at each transform fault location from the best-fitting angular velocity of DeMets et al. [2010] (blue) or this study (red). Stars: Transform fault strikes used to estimate Cocos-Nazca plate motion by DeMets et al. [2010] (blue) or this study (red). Dashed error bars: 1σ uncertainties of the observed transform fault azimuths of DeMets et al. [2010] (blue) or this study (red). Red solid error bars: 1σ uncertainties of the plate-motion directions at transform fault locations calculated from our new best-fitting angular velocity (i.e., this study).

We determined a new Cocos-Nazca best-fitting angular velocity from the three new transform-fault azimuths (while excluding the transform-fault azimuths of DeMets et al. [2010]) combined with the spreading rates of DeMets et al. [2010]. (We take the term best-fitting angular velocity to be the angular velocity determined only from data along the mutual boundary of a plate pair.) The three new azimuths are mutually consistent and fit well (Figure 2). Azimuths calculated from the new best-fitting angular velocity are 3.3° ± 1.8° (95% confidence limits) clockwise of those calculated from the best-fitting angular velocity of DeMets et al. [2010] and agree better with the earlier results of DeMets et al. [1990] and of Wilson and Hey [1995] than they do with the results of DeMets et al. [2010]. Moreover, the azimuth predicted for the Panama transform fault, which we did not use as input to the new best-fitting angular velocity, is 4.9° ± 2.7° (95% confidence limits) clockwise of the observed value (Figure 2), demonstrating that the Panama transform fault does not parallel Nazca-Cocos plate motion.

3 Malpelo Plate Hypothesis

While we still assume that the Cocos plate lies west of the Panama transform fault, we hypothesize that the lithosphere east of it moves independently of the Nazca plate and constitutes a microplate, which we term the Malpelo plate (Figure 3). We further hypothesize that a diffuse plate boundary separates the Malpelo plate from the much larger Nazca plate (Figure 3). In most diffuse oceanic plate boundaries, the pole of rotation lies in the diffuse boundary [Gordon, 1998; Zatman et al., 2001, 2005; Cande and Stock, 2004; Jellinek et al., 2006], and we speculate that is also the case for the Malpelo-Nazca boundary (Figure 3).

Details are in the caption following the image
(a) Map of region surrounding the Malpelo plate from Global Multiresolution Topography [Ryan et al., 2009]. Centroid-moment-tensor solutions are shown for earthquakes from 1976 to 2016 with depths shallower than 50 km and Mw > 5.0 [www.globalcmt.org]. Yellow-filled circle: the hypothesized rotation pole between the Malpelo plate and the Nazca plate. The shaded magenta lines indicate the plate boundaries outlining the Malpelo and Coiba plates. The Yaquina Trough is a former transform fault between the Cocos and Nazca plates [Lonsdale and Klitgord, 1978]. Abbreviations: GSC, Galapagos Spreading Center; ESC, Ecuador Spreading Center; CRSC, Costa Rica Spreading Center. (b) Vertical gravity gradient map of region surrounding the Malpelo plate [Sandwell et al., 2014]. Earthquake epicenters are shown by small color-filled circles for earthquakes from 1976 to 2016 with depths shallower than 50 km and Mw > 5.0. Symbol size increases with magnitude, and color indicates mechanism, based on which of the principal axes of the moment tensor is nearest to vertical: white for normal, red for strike slip, and black for thrust (www.globalcmt.org).

We assume that the Malpelo plate extends only as far north as ≈6°N where seismicity marks another boundary with a previously recognized microplate, the Coiba plate [Pennington, 1981; Adamek et al., 1988] (Figure 3).

Figure 4 shows a velocity space representation of the Nazca, Cocos, and Malpelo plates at a point (4.15°N, 82.6°W) along the Panama transform fault, which separates the Cocos and Malpelo plates. The Cocos-Nazca velocity is determined from the best fitting angular velocity described above. The strike of the Panama transform fault is known. To estimate the direction of motion between the Malpelo and Nazca plates, we assume that their pole of relative rotation lies where it is shown in Figure 3. With these assumptions, the speed of the Malpelo plate relative to the Nazca plate at this point is 5.9 mm a−1.

Details are in the caption following the image
Velocity space diagram at a point (4.15°N, 82.6°W) along the Panama transform fault. The solid line segments indicate the relative velocity between plates. The dashed line segment represents the strike of the Panama transform fault. The Cocos-Nazca relative speed is 61.7 mm a−1, the Cocos-Malpelo speed is 58.5 mm a−1, and the Malpelo-Nazca speed is 5.9 mm a−1.

4 Nonclosure of the Cocos-Nazca-Pacific Plate Motion Circuit

We re-estimate the nonclosure of the Cocos-Nazca-Pacific plate motion circuit with our new best- fitting angular velocity for Cocos-Nazca plate motion. If the plates were rigid and if there were no errors in the data, then the sum of the three best-fitting angular velocities would be zero. When we sum the Cocos-Pacific, Pacific-Nazca, and Nazca-Cocos best-fitting angular velocities of DeMets et al. [2010], however, we obtain an angular velocity of nonclosure of 0.34° Ma−1 (± 0.12° Ma−1; 95% confidence limits) about a pole at 24.8°N, 96.5°W (Figure 5a), which differs significantly from zero. Evaluated at the approximate location of the triple junction of 2.3°N, 102.0°W (Figures 5a and S3), we obtain a linear velocity of nonclosure of 15.0 mm a−1 (± 3.8 mm a−1; 95% confidence limits) 283° clockwise of north (Figure 5b). (This differs slightly from that found by DeMets et al. [2010], perhaps because we use a different reference point.) In comparison, our new angular velocity of nonclosure is 0.33° Ma−1 (± 0.12° Ma−1; 95% confidence limits) about a pole at 19.8°N, 96.9°W (Figure 5a) and the new linear velocity of nonclosure is 11.6 mm a−1 (± 3.8 mm a−1; 95% confidence limits) toward 286° clockwise of north (Figure 5b). By replacing the two transform fault azimuths from DeMets et al. [2010] with the improved set of three new transform fault azimuths, the nonclosure is reduced by 3.4 mm a−1.

Details are in the caption following the image
(a) Map of the eastern Pacific from Global Multiresolution Topography [Ryan et al., 2009]. Red-filled circle: Pole for the angular velocity of nonclosure for the Cocos-Pacific-Nazca plate circuit (this study). Blue-filled circle: Pole for the angular velocity of nonclosure of DeMets et al. [2010]. Red- and blue-filled ellipses: corresponding 95% error ellipses. Black-filled circle: reference point (i.e., the approximate location of the Galapagos triple junction) used for calculating the velocity of nonclosure. (b) Velocity space diagram for the Cocos-Pacific-Nazca plate circuit evaluated at the Galapagos triple junction. Color-filled circles with 95% confidence ellipses: Velocities determined from best fitting angular velocities of DeMets et al. [2010] (blue) and from this study (red). The velocity of nonclosure is the velocity of Cocos (final) minus the velocity of Cocos (initial).

The new angular velocity of nonclosure still differs significantly from zero, however. Using the F ratio test for plate circuit closure [Gordon et al., 1987], we obtain a value of F of 19.5 with 3 versus 205 degrees of freedom from the best-fitting angular velocities of DeMets et al. [2010] and a value of F of 16.9 with 3 versus 206 degrees of freedom from our new best-fitting angular velocities (Table S2). Reference values of F are F0.05 = 2.7 (5% significance level of 95% confidence level) and F0.01 = 3.9 (1% significance level of 99% confidence level); thus, the nonclosure, while reduced, remains significant.

5 Discussion

When we reanalyze the closure of the Cocos-Nazca-Pacific plate circuit using our new set of Cocos-Nazca transform fault azimuths and make no other changes, the nonclosure of the circuit is reduced from 15.0 ± 3.8 mm a−1 to 11.6 ± 3.8 mm a−1, thus reducing but not eliminating the nonclosure of the Pacific-Cocos-Nazca plate circuit. The sense (i.e., sign) of the velocity of nonclosure seems consistent with an explanation in terms of horizontal thermal contraction of oceanic lithosphere [Kumar and Gordon, 2009], but the magnitude of nonclosure is almost surely too large to be caused only by known processes of intraplate deformation including thermal contraction.

Specifically, the work of Kreemer and Gordon [2014] indicates that the displacement rates across the Pacific plate due to thermal contraction are 1–2 mm a−1, which are consistent with intraplate strain rates due to horizontal thermal contraction inferred by Mishra and Gordon [2016]. If all three of the Cocos, Nazca, and Pacific plates are characterized by intraplate displacements of 1–2 mm a−1 due to horizontal thermal contraction, and if we assume that the orientations and magnitudes of these are uncorrelated between plates, a nonclosure of ≈2–4 mm a−1 (1–2 mm a−1 × √3) might be expected, which is much smaller than the 11.6 ± 3.8 mm a−1 (95% confidence limits) of nonclosure that we find. Thus, the cause of at least part of the nonclosure remains unknown and we suggest that one or more plate boundaries remain to be discovered.

If the nonclosure is due to deformation of one of the plates or to an undiscovered plate boundary within that same plate, the Cocos plate seems the best candidate because of its proximity to the pole of rotation of nonclosure. Larger displacement rates would be required for an undiscovered plate boundary in the Pacific or Nazca plate simply because any hypothetical deformation zone would lie farther from the pole of rotation of nonclosure. Furthermore, the absence of significant nonclosure about the Nazca-Pacific-Antarctica plate motion circuit suggests that the Pacific and Nazca plates are not highly nonrigid [DeMets et al., 2010].

6 Conclusions

The lithosphere east of the Panama transform fault moves independently of the Nazca plate, constituting a microplate that we term the Malpelo microplate. The new transform fault azimuths result in nonclosure about the Galapagos triple junction that is 3.4 mm a−1 smaller than that found by DeMets et al. [2010], but remains large (11.6 ± 4 mm a−1). While the sense of the observed nonclosure is consistent with horizontal thermal contraction of oceanic lithosphere, the indicated magnitude of deformation remains too large to be explained only by thermal contraction. Thus, we suggest that one or more plate boundaries remain to be discovered.

Acknowledgments

This work was supported by NSF grant OCE-1559316. Some of the figures were made with Generic Mapping Tools software [Wessel and Smith, 1991]. The data supporting the conclusions can be obtained in the references, tables, and supporting information.