A JUPITER ORBIT -- LUNAR ORBIT RESONANCE MODEL: POSSIBLE CAUSE FOR THE BEGINNING OF THE MODERN STYLE OF PLATE TECTONICS

Since the Plate Tectonics Revolution in the earth sciences, there has been this lingering debate about when plate tectonics began. There are two end-member schools of thought: (1) that plate tectonics has been operating throughout geologic time (Shervais, 2006, GSA SP-405, p. 173) and (2) that plate tectonics, as we understand it today, can only occur when the Earth has cooled sufficiently to permit plates to attain negative buoyancy (Davies, 1992, Geology, 20, p. 963; Stern, 2005, Geology, 33, p. 556). Stern (2005) proposes that the modern style of plate tectonics began ~1.0 Ga ago.

The Late Proterozoic also appears to be a critical time in the history of the lunar orbit. Peale and Cassen (1978, Icarus, 36, p. 245) identified an orbital resonance state between Jupiter’s orbit and the lunar orbit when the lunar orbital radius is at 53.4 ER (earth radii). Orbital traceback calculations suggest that the earth-moon distance would be ~53.4 ER in Late Proterozoic time.

The key element in such an orbital resonance is the perigean cycle of the lunar orbit (the prograde progression of the perigee position of the lunar orbit). At present the perigean cycle is 8.85 years (60.3 ER). At 53.4 ER the perigean cycle would be ~12 years, the approximate period of Jupiter’s orbit. Under these conditions the major axis of the lunar orbit would be increased to the resonant value by a forced ECC (eccentricity) but the orbital angular momentum would remain near that of a 50 ER orbit and then slowly increase in time to that of a 53.4 ER orbit. As the lunar orbital ECC reaches a maximum of up to ~0.3, the rock and ocean tidal amplitudes would be ~2.5 times higher than that of a circular orbit of 53.4 ER. Such rock tidal activity could lead to the development of tidal vorticity induction cells in the upper mantle (Bostrom, 2000, Oxford Univ. Press) that would aid in the initiation of subduction.

This model can be tested via the tidal rhythmite record of the Late Proterozoic. The two predictable constants are that the semi-major axis of the lunar orbit should be near 53.4 ER and the number of sidereal months per year should be about 16 throughout this era. Assuming that the earth rotation rate at 1.1 Ga (before the forced ECC episode) is ~16.9 hr/d, the number of days per year would change from ~519 d/yr at ~1.1 Ga to 466 d/yr at the end of this orbital resonance era.