Passive margins and their terminal collisions through Earth history Dwight Bradley U.S. Geological Survey Dwight Bradley U.S. Geological Survey

Conclusions about passive margins Passive margins are not known before the Neoarchean As far back as the Neoarchean, most passive margins have ended their tenure by colliding with an arc Peaks in the passive margin population at 1900 Ma, 550 Ma, and today correspond to times of continental dispersal Lulls at Ma and 300 Ma correspond to known (Pangea) or inferred supercontinents Unexpectedly, the seven longest-lived margins are Precambrian. No evidence for short lifespans in Precambrian Age distribution of blueschists and foredeep magmatism attests to real secular change in style of arc-passive margin collision, but does not require a fundamentally different tectonic regime

Mechanisms for greater heat loss Longer ridge system ( = more plates) Longer ridge system ( = more plates) Faster spreading with modern-size plates Faster spreading with modern-size plates (after Pollack, 1997)

Modern passive margins 94,000 km aggregate length oldest one is ~175 m.y.

Ancient passive margins compilation still in progress 55 margins so far, Neoarchean to Neogene

Wopmay Orogen— Paleoproterozoic arc-passive margin collision Hoffman & Bowring, 1981

SELECTED PASSIVE MARGINS Craton, margin, orogen Start date (Ma) End date (Ma) Lifespan (m.y.) Quality Steep Rock Lake, Superior craton Around 2900 Ma unknownD Pilbara craton, S. margin A Superior craton, Huronian margin ~350B Siberia, Verkhoyansk margin # ~600C Baltica, Uralian margin # ~370B Laurentia, Appalachian margin A India, Himalayan margin # A Australia, Timor margin A

Karlstrom, 2005 Laurentian NE-trending Proterozoic accretionary provinces– one of the most voluminous accretionary orogens in Earth history

Conclusions about passive margins Passive margins are not known before the Neoarchean As far back as the Neoarchean, most passive margins have ended their tenure by colliding with an arc Peaks in the passive margin population at 1900 Ma, 550 Ma, and today correspond to times of continental dispersal Lulls at Ma and 300 Ma correspond to known (Pangea) or inferred (Columbia) supercontinents Unexpectedly, the seven longest-lived margins are Precambrian. No evidence for short lifespans in Precambrian Age distribution of blueschists and foredeep magmatism attests to real secular change in style of arc-passive margin collision, but does not require a fundamentally different tectonic regime

The End Taconic orogeny, forebulge unconformity, Newfoundland

The Mesoproterozoic gap in passive margin that collided with an arc is either: 1.An artifact a. compilation incomplete b. recognition problems 2. Real a. no plate tectonics b. plate tectonics but somehow different 1.An artifact a. compilation incomplete b. recognition problems 2. Real a. no plate tectonics b. plate tectonics but somehow different