Transcript Passive margins and their terminal collisions through Earth history

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