Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga

Abstract

The mode and rates of tectonic processes and lithospheric growth during the Archean [4.0 to 2.5 billion years (Ga) ago] are subjects of considerable debate. Paleomagnetism may contribute to the discussion by quantifying past plate velocities. We report a paleomagnetic pole for the ~3180 million year (Ma) old Honeyeater Basalt of the East Pilbara Craton, Western Australia, supported by a positive fold test and micromagnetic imaging. Comparison of the 44°±15° Honeyeater Basalt paleolatitude with previously reported paleolatitudes requires that the average latitudinal drift rate of the East Pilbara was ≥2.5 cm/year during the ~170 Ma preceding 3180 Ma ago, a velocity comparable with those of modern plates. This result is the earliest unambiguous evidence yet uncovered for long-range lithospheric motion. Assuming this motion is due primarily to plate motion instead of true polar wander, the result is consistent with uniformitarian or episodic tectonic processes in place by 3.2 Ga ago.

Fig. 1. Map of sampling localities.

The HEB (light green) is especially well preserved in the Soanesville Syncline [SVS; see inset Map (B)] and East Strelley greenstone belt [ESGB; top of Map (A)], along with the underlying Soanesville Group sediments (brown) and Dalton Suite mafic intrusives (dark gray). The age of the HEB (3192 to 3176 Ma) in this area is derived from the age of the coeval Dalton Suite intrusives [e.g., one sample dated to 3182 ± 2 Ma in the ESGB (18)].

Fig. 2. Orthogonal projection diagrams of representative samples.

All are in in situ coordinates and show four components; AF, light blue; L, dark blue; M, orange; and H, magenta (see the main text for discussion). (A) Sample SVA4. (B) SVC10. (C) SVD3. (D) SVE3. (E) ESB8A.

Fig. 3. Fold test on the H component.

Equal-angle stereonet projections show measured directions in (A) in situ and (B) 100%-fold–corrected coordinates. Mean H component directions from each site (small points) are color-coded by locality means (large points with error circles; SVA, red; SVC, light green; SVD, light purple; SVE, blue; ESB, orange). The large error circle is omitted from the ESB mean direction for clarity. L and M component in situ means are gray. The tilt-corrected mean of all sites in SVA, SVC, SVD, and SVE is shown in black. Tilt correction in (B) includes correction for the plunge of the SVS fold axis. (C) Histogram (orange bars) and cumulative distribution (bold curve) of maximum post-unfolding fit in a fold test on the SVC, SVD, and SVE directions after Tauxe and Watson (36), using the 069°/41° mapped axis of the SVS. The fold test passes with best-fit unfolding 100 ± 15%. (D) A fold test on the SVA, SVC, SVD, and SVE directions with best fit at 79 ± 11% unfolding.

Fig. 4. Paleopoles from the Pilbara Craton.

Stratigraphic coordinates apply except for HEBm and HEBm-R overprints in in situ coordinates. Numbers are ages in million years. Poles on the hemisphere opposite the viewer have dashed outlines. Poles reported in this study are red-filled circles and ellipses, locality-mean poles are black hollow circles, and site VGPs are red points (ESB sites in purple; ESB locality-mean pole not shown for clarity). Poles from other studies in the Pilbara are orange.

Fig. 5. Tests for Archean plate motion.

(A) Meso-Paleoarchean paleolatitude records from the Pilbara Craton (Western Australia, orange) and Kaapvaal Craton (South Africa, blue), including this study (HEBh). Paleolatitude error bars are 2σ; age error bars are 2σ or the full range allowable given available constraints. The orange shaded region is a sample of the range of minimum-velocity paths allowed by existing poles. Gray arrows at the bottom are constraints on the onset date of mobile-lid plate tectonics from a sample of previous studies, with letters referencing studies listed in the Supplementary Materials. (B) Distributions of time-averaged latitudinal velocities on the continents (24) recorded at N = 3736 randomly selected points from the present to 1 Ma (yellow with dashed lines) and over 170 Ma windows throughout the Phanerozoic (blue with solid lines) as both histograms and cumulative distribution frequencies (line overlays). The 170 Ma distribution tends toward slower rates because it often samples reversals in plate velocity. For comparison, the minimum motion rate implied by the EBm and HEBh poles is shown by the black point and error bars. This rate, which is averaged over a 170 Ma interval, is comparable with geologically modern rates.