Late 20th Century Tests of the
Continental Drift Hypothesis
6 – Characteristics of the Continents
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What to look for:
• All continents comprise a stable central part (craton)
surrounded on the edges by structurally complex ones
(mobile belt)
• The cratons include exposed ancient rocks plus regions
where these are overlain by thin flat sedimentary rocks.
• The mobile belts include much thicker packages of
structurally deformed sediments.
• Evidence from both sedimentology (detrital/carbonate
cycles) and structural geology (unconformities) indicate
that the mobile belts have been repeatedly deformed,
not just once.
Continental Structure
Sedimentologic, stratigraphic, and
structural oddities at the continental edge.
Coastal Plain
Piedmont
Appalachian Mts
Interior Lowlands
Great Plains
Rocky Mts
Cascades/Sierra Nevada
Basin and Range
10’s of thousands of feet of
complexly deformed and
metamorphosed sediments
with igneous intrusions and
volcanics.
A few hundred feet of
essentially undeformed and
unmetamorphosed
sediment with practically no
igneous rocks.
10’s of thousands of
feet of complexly
deformed and
metamorphosed
sediments with
igneous intrusions
and volcanics.
Western or Cordilleran
Mobile Belt.
Stable Craton.
Eastern or
Appalachian Mobile
Belt.
Continents all have mobile belts around their edges and stable cratons in the middle.
Details of the strata in the Appalachian Mobile Belt are typical:
Pay attention to the thicknesses and rocktypes as you look “upsection”.
A major source area
existed nearby
No source area existed
A major source area
existed nearby
No source area existed
A major source area
existed nearby
No source area existed
A major source area
existed nearby
From the latter part of
the Precambrian through
the Paleozoic the
sediments flip-flop from
detrital shales and
sandstones (which mean
lots of particulate matter
was weathering and
eroding off the land) and
limestone (which needs
a clear, turbidity-free
environment to form).
Sediment from
Appalachian Mountains
(continues in Coastal Plain)
Acadians eroded away
Sediment from
Acadian Mountains
Taconics eroded away
Sediment from
Taconic Mountains
Grenvilles eroded away
Sediment from
Grenville Mountains
In the words of the great
hippie bard Donovan,
“First there is a mountain,
then there is no mountain,
then there is a mountain,
then there is no
mountain …”
Deposition
Erosion
Tilt
Deposition
Erosion
Tilt
Deposition
Erosion
tilt
Depoisition
Erosion
Tilt
…
Remember the
Appalachian
cross-section from
the first test.
Subtle angular
unconformities
occur above the
limestone units.
The “tilting” they
imply records the
compressional
deformation that
went with the
mountain-building
(orogenic)
events!
Take-home message
• All continents comprise a stable central part (craton)
surrounded on the edges by structurally complex ones
(mobile belt)
• The cratons include exposed ancient rocks plus regions
where these are overlain by thin flat sedimentary rocks.
• The mobile belts include much thicker packages of
structurally deformed sediments.
• Evidence from both sedimentology (detrital/carbonate
cycles) and structural geology (unconformities) indicate
that the mobile belts have been repeatedly deformed,
not just once.