Passive margins and their terminal collisions through Earth

Passive margins and their terminal collisions through Earth

Passive margins and their terminal collisions through Earth history 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 1750-1000 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) (after Pollack, 1997) Faster spreading

with modern-size plates Modern passive margins 94,000 km aggregate length oldest one is ~175 m.y.

Ancient passive margins 55 margins so far, Neoarchean to Neogene compilation still in progress Wopmay Orogen Paleoproterozoic arc-passive margin collision

Hoffman & Bowring, 1981 SELECTED PASSIVE MARGINS Craton, margin, orogen Steep Rock Lake, Superior craton

Start date (Ma) End date (Ma) Around

2900 Ma Lifespan (m.y.) Quality

unknown D Pilbara craton, S. margin 2685

2445 240 A

Superior craton, Huronian margin 2225 1778 ~350

B Siberia, Verkhoyansk margin #1 1650

1050 ~600 C Baltica, Uralian margin #1

1000 628 ~370

B Laurentia, Appalachian margin 542 462

80 A India, Himalayan margin #2

245 52 193 A

Australia, Timor margin 170 4

166 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 1750-1000 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 Taconic orogeny, forebulge unconformity, Newfoundland The End

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

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