Download Vietnam - Continued Flooding October 16, 2000

So, why ARE the
Rocky Mountains
in the middle of
the continent?
The Canadian Rockies are
close to the plate boundary
But in the U.S., the Rocky
Mountain front is a long
way from the plate
boundary.
When 2 continents collide, they trap
mountains between them, as in the
Himalayas, Urals, and Alps
Marked by andesitic volcanoes on the overriding plate
and an ophiolite where the ocean closed up
But the
Rockies don’t
have either
one
North America’s Cordilleran
mountain belt is made of
accreted terranes
These exotic
crustal fragments
were added to the
continent during
the Triassic,
Jurassic, and Early
Cretaceous,
- before the Rocky
Mts. formed.
The Rocky Mountains formed
in the Late Cretaceous to Early
Tertiary
Parts of the craton
were uplifted along
steep reverse faults
whose cause is not
well understood.
The plate boundary
was closer than
today, but still
relatively far away.
The Rocky Mountains formed
during the Late K to Early T
Magmatism and
volcanism during
the Laramide
Orogeny were weak
and widespread.
This suggests that
other causes than
simple subduction
were important.
The Basin and Range Province
formed in Late Tertiary/Quaternary
The crust heated up
and thinned over a
broad region.
Normal faults,
horsts, and grabens
created scattered
uplifts with basins
between them.
Extension widened
the continent to 3 or
4 times its original
width.
WHY?
One suggestion is that migration
of triple junctions along the
Cascadia Trench changed the
stress field and caused the
extension
Another factor probably was
migration of the continent across
a hot spot (Columbia River,
Snake River, and Yellowstone
Plateaus)
- weakening the crust by heating