LIFESPANS OF PASSIVE MARGINS PRIOR TO ARC COLLISION, LATE ARCHEAN TO PRESENT

Between 2650 and 1780 Ma, and again between 1000 Ma and the present, most passive margins met their demise by entering a subduction zone, i.e. colliding with an arc. The lifespans of these margins depended on spreading rates, total length of the global plate boundary (therefore, size of plates), and chance. In this study, lifespans were estimated for 38 margins. The lifespan was taken as the time between the rift-drift transition and the passive-margin to foreland-basin transition.

The mean lifespan of ten passive margins in the Late Archean to Paleoproterozoic (2650-1780 Ma) interval is 195 m.y.; the mean lifespan for ten Neoproterozoic (~1000-542 Ma) margins is 192 Ma; the mean lifespan for seventeen Phanerozoic (542-0 Ma) margins is only 141 m.y. The longest lifespans for these same age groups are ~350 m.y (Huronian), ~380 m.y. (Uralian), and ~220 m.y. (Himalayan), respectively. These findings are surprising because the exponential decline of global heat production (the "engine" behind plate tectonics) predicts that passive margins should last longer now than they did in the Precambrian, and not the opposite.

The lengthy gap from ~1780 to 1000 Ma in arc-passive margin collisions is also puzzling. Does the gap exist because there weren't many passive margins around to collide with anything? The answer depends on the interpretation of Proterozoic miogeoclinal prisms in the Urals, Verkhoyansk, and the northern Canadian Cordillera. These might either be true passive margins, or halves of failed rift-sag sequences. Whatever the case, their immediate fate was re-rifting, such that an older miogeocline is overlain by a younger passive margin that did collide with an arc. If re-rifting of long-established passive margins was a relatively common occurrence during the mid-Proterozoic, it could explain the coincident shortage of arc-passive margin collisions.