From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: More comments on Plate Tectonics and California
Date: 4 Sep 92 13:59:01 GMT

In article <jester.715314185@crash>, jester@crash.cts.com (Ken Bibb) writes:
> 1.  Clarification on the northward movement of volcanism in
> California:  since the source of volcanism of the Cascade Range is the
> offshore subduction of the Juan de Fuca Plate, it is the northwesterly
> movement of that plate and its subduction trench which determines
> where the accompanying volcanism will be found.  The Mendocino
> Fracture Zone, not the Mendocino Triple Junction (which is just a
> point on the MFZ), delimits the southern end of the subduction trench
> and, by extension, the limit of volcanism.  It is the absense of the
> trench which accounts for the absense of volcanism.

There is NO difference between referring to the MFZ or the MTJ when studying
Cascade volcanism.  In fact, the triple junction is the correct reference,
since, as you point out, the absense of the trench accounts for the absense
of volcanism and the triple junction marks the endpoint of the trench as
well as the endpoint of the MFZ and the San Andreas.

> 2.  The rise and oceanic plate movement:  an oceanic ridge is the rift
> where the plate spreads away from it in both directions.  The rate of
> movement varies between different fracture zones.  At the same time,
> the entire plate - the Pacific Plate - has a northwesterly movement.
> When the ridge was subducted into the trench and entinguished both
> along the Pacific coast from the Murray FZ to the Mendocino FZ the
> westerly movement of this portion of the plate ceased.  Thereafter,
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

NO NO NO!  Pacific plate motion is absolutely unchanged. It is only a
fortuitous combination of the shape of the North American west coast,
the motion of the Pacific and the motion of North America that results
in the San Andreas being a transform fault.  Actually, it is incorrect
to give compass directions to refer to plate motion - such directions
would only be locally applicable.  Plate motions are measured as rotation
rates about poles of rotation.  Usually, plate motions are measured in
a relative sense - for example how the Pacific plate moves relative
to North America, where the pole of rotation is at 53N 53W.  (If you
made a globe with the pole at that location, the San Andreas would be a
line of latitude.)  Determining absolute motions (as in "the Pacific
plate is moving NW") is much more difficult.  The primary difficulty
is establishing a reference frame - researchers have generally agreed
that the "hot spot" reference frame is the inertial frame, but it really
can't be proven (and there are known problems with it).  Since this is
the reference frame, it is particularly convenient for determining Pacific
motion, which has multiple hot spot tracks - the most notable being
the Hawaiian Island and seamount chain.  So it's NW absolute motion
is pretty well documented (actually the absolute motion is more WNW
than NW over most of the Pacific).  North America's absolute motion
can be resolved from its relative motion to the Pacific or from the
Yellowstone hot spot.  North America is moving almost due west.

> its only movement is that of the entire plate.  How does the absense
> of a rise and that westerly movement of the plate tectonically affect
> Central California?  Does the San Andreas fault mark the location of
> the extinguished trench (as the edge of the North American Plate?)

To a certain extent, but the trench was actually off shore, and the
trench would not have been as straight as the San Andreas, in fact
trenches tend to be arc-shaped.

> 3.  I propose that the Gulf of California/Salton Trough rift be names
> "Cortez Rift" (from the Sea of Cortez) to simplify nomenclature.  When

Fine, but you alone would be referring to it by this name.

> this rift was forming it should have entended to the Murray Fracture
> Zone.  But the next portion of the rift, north of the Murray kFZ, is

Where do you get this from?  The Murray Fracture Zone is a passive remnant
of a transform fault that existed across the original Pacific rift before
it was subducted.  It was never active!  Fracture Zones are not generally
active features - only the transform segments of them are active, such
as the segment of the Mendocino Fracture Zone between the Juan de Fuca
rift and the San Andreas.

> offset over 100 miles to the west.  This offset caused a diagonal
> break across the northerly end of the Baja California Block from one
> rift to the other.  This break became what is known as the San Andreas
> Fault system (SAFS).  What became of the portion of that portion of

No, the San Andreas formed in response to the subduction of the Farallon
Plate.

> the nascent Cortez Rift that was north of the SAFS?  I suggest that

The environment is one of the death of a rift (the old Farallon rift of which
the Juan de Fuca rift and Rivera rift and Cocos rift are just remnants of the
once continuous rift - likewise the Juan de Fuca, Rivera and Cocos plates
are just fragments of the old Farallon Plate).  It is not the environment
of the "birth" of a rift.  The rifting in the Gulf of California and
Salton trough is in response to a mis-alignment of geometry.  The boundary
between North America and Pacific does not form a perfect line of latitude
about the relative pole of rotation.  It is off a bit in the vicinity of
the Gulf of California.  As the motion of the plates proceed, this causes
a gap to form betwene the Pacific and North America - gaps in plate tectonics
are always satisfied by rifting.  Some transforms are occassionally only
a little bit off - these are called "leaky transforms" instead of rifts.

> the series of northwest trending faults in the Mojave Desert from the
> Helendale fault to the Pisgah fault are the remains of the Cortez Rift
> that were bypassed and never further developed.  Some of these later
> appear to have become transform faults.

These faults do not show extensional motion - they are strike-slip like the
San Andreas.  They have no history of extension.  None are transforms since
they do not "transform" plate boundaries (the only definition of transform
fault).  They are just ordinary strike slip faults.



I would suggest you do some more background reading in Plate Tectonics and
Geology before expounding on your personal theories to the general public.
I would recommend an excellent collection of the original papers on Plate
Tectonics - "Plate Tectonics and Geomagnetic Reversals", 1973, editted by
Allan Cox, publisher Freeman.  These are the original papers which defined
Plate Tectonics.  The AGU also has a good collection of papers editted
by John M. Bird and just titled "Plate Tectonics".  Try to understand
the science that has already been done before striking out on your own.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov