However you get there, it is clear that the high country of the West
is one strip of volcanic rock after another and that similar geology
trends in a north south direction. We already knew that of course as
you could hardly not know that, but it does inform us that EW
geological change must come often and radically as is hinted at by
experience and strong mineral regimes that often extend hugely along
trend. A great example is the Idaho silver rich mineral regime which
can be traced into Mexico and deep into Canada. Step outside to east
or west and silver is not important anymore.
The missing idea has been to understand the continental nature of
these trends in the geology.
In the meantime we are having some luck here in determining what
remnant plates may exist and are now actually mapping them.
Constructive
collisions
by Staff Writers
Munich, Germany (SPX) Apr 09, 2013
One popular model for
the accretion process postulates that a huge oceanic plate - the
Farallon Plate - acted as a conveyor belt to sweep crustal fragments
eastwards to the margin of American Plate, to which they were
attached as the denser Farallon Plate was subducted under it.
The mountain ranges of
the North American Cordillera are made up of dozens of distinct
crustal blocks. A new study clarifies their mode of origin and
identifies a previously unknown oceanic plate that contributed to
their assembly.
The extensive area of
elevated topography that dominates the Western reaches of North
America is exceptionally broad, encompassing the coastal ranges, the
Rocky Mountains and the high plateaus in between.
In fact, this mountain
belt consists of dozens of crustal blocks of varying age and origin,
which have been welded onto the American continent over the past 200
million years.
"How these blocks
arrived in North America has long been a puzzle," says LMU
geophysicist Karin Sigloch, who has now taken a closer look at the
problem, in collaboration with the Canadian geologist Mitchell
Mihalynuk.
Collisions and
continental growth
One popular model for the accretion process postulates that a huge oceanic plate - the Farallon Plate - acted as a conveyor belt to sweep crustal fragments eastwards to the margin of American Plate, to which they were attached as the denser Farallon Plate was subducted under it.
However, this scenario
is at variance with several geological findings, and does not explain
why the same phenomenon is not observed on the west coast of South
America, the classical case of subduction of oceanic crust beneath a
continental plate.
The precise source of
the crustal blocks themselves has also remained enigmatic, although
geological studies suggest that they derive from several groups of
volcanic islands.
"The geological
strata in North America have been highly deformed over the course of
time, and are extremely difficult to interpret, so these findings
have not been followed up," says Sigloch.
Sigloch and Mihalynuk
have now succeeded in assembling a comprehensive picture of the
accretion process by incorporating geophysical findings obtained by
seismic tomography.
This technique makes
it possible to probe the geophysical structure of the Earth's
interior down to the level of the lower mantle by analyzing the
propagation velocities of seismic waves. The method can image the
remnants of ancient tectonic plates at great depths, ocean floor that
subducted, i.e., disappeared from the surface and sank back into
the mantle, long time ago.
Intra-oceanic
subduction of the Farallon Plate
Most surprisingly, the new data suggest that the Farallon Plate was far smaller than had been assumed, and underwent subduction well to the west of what was then the continental margin of North America.
Instead it collided
with, and subducted under, an intervening and previously unrecognized
oceanic plate. Sigloch and Mihalynuk were able to locate the remnants
of several deep-sea trenches that mark subduction sites at which
oceanic plates plunge at a steep angle into the mantle and are drawn
almost vertically into its depths.
"The volcanic
activity that accompanies the subduction process will have generated
lots of new crustal material, which emerged in the form of island
arcs along the line of the trenches, and provided the material for
the crustal blocks," Sigloch explains.
As these events were
going on, the American Plate was advancing steadily westwards, as
indicated by striped patterns of magnetized seafloor in the North
Atlantic.
The first to get
consumed was the previously unknown oceanic plate, which can be
detected seismologically beneath today's east coast of North America.
Only then did the continent begin to encounter the Farallon plate. On
its westward journey, North America overrode one intervening
island arc after another - annexing ever more of them for the
construction of its wide mountains of the West.
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