From: karish@pangea.Stanford.EDU (Chuck Karish)
Subject: Re: Questions raised by SoCal Activity
Summary: Tectonics
Keywords: usgs refs, landers
Organization: Mindcraft, Inc.
Date: Thu, 20 Aug 1992 18:21:16 GMT

In article <51084@seismo.CSS.GOV> stead@skadi.CSS.GOV (Richard Stead) writes:
>In article <jester.713904311@crash>, jester@crash.cts.com (Ken Bibb) writes:
>> 2)  At the end of the California portion of "The Making of A
>>     Continent" there is a computer generated map with a 'clock'
>>     showing millions of years.  This map proposes that in about 3
>>     million years all of California west of the Sierra Nevada fault
>>     and our recent Desert Hot Springs-Yucca
>>     Valley-Landers-Barstow-Ridecrest line of epicenters will swing out
>>     on the Pacific Plate and drift off to the north.  Why, the, all of
>>     the media attention to northward movement just westerly of the
>>     currently-designated San Andreas Fault?
>
>What it shows is a theory - one that is in "vogue" and receiving a lot
>of interest from tectonicists.  It is certainly a good theory, but it is
>only that.  One must resist the temptation to view such graphics as
>depicting the actual future.

>> 3)  Can you give me references for the role of plate tectonics in the
>>     creation of the Sierra Nevada, Owens Valley, White and Panamint ranges
>>     and Death Valley?  Note:  The Front Range of the Rocky Mountains also
>>     lines up between the Mendocino and Murray fracture zones projected
>>     eastward.

The mechanism of the uplift of the Rocky Mountains has been
a subject of spirited debate for quite some time.  The problem
is that the structures involved are difficult to study, because
many of the important ones are faults that juxtapose rocks
with similar seismic properties, so there isn't sufficient
velocity contrast to provide good seismic "seeing".

>A good reference for the tectonics of the meeting of the Pacific and North
>American plates is "Implications of Plate Tectonics for the Cenozoic
>Tectonic Evolution of Western North America", Tanya Atwater, Bulletin of
>the Geological Society of America, vol 81, p. 3513-3536, 1970.  It's a
>technical paper, but well-written and I believe accessible to the layman.

This is a classic paper, but it's a "big picture" review that
doesn't consider second-order effects that constrain many
interesting local processes.  To understand what might happen
to California over the next few million years, we have to
account for the room problem: the San Andreas Fault curves out
to sea and dies out against the Mendocino Fracture Zone,
which means that the rocks west of the San Andreas can't
just slide northwestward indefinitely unless the Gorda/Juan de
Fuca Plate is slivered.

Several people are studying what's happening where the San
Andreas runs into the Mendocino transform.  Ken LaJoie
published a paper in about 1980 in which he coined the term
"flake tectonics", describing a process in which the rocks
west of the fault are slivered into near-horizontal slices
that override each other and are pushed upward.  Pat
McCrory also studied the neotectonics of this area; I
haven't seen her published work.  Gary Carver of Humboldt
State is also working to understand these processes and has
produced some exciting detailed local maps and reviews of
regional geology.

This all refers to recent and ongoing research.  It's
accessible in scientific journals, but as far as I
know it hasn't been synthesized for a popular audience.
This shouldn't be an obstacle to an interested reader;
while the technical details of the papers may be
difficult, they usually include introductions to
the issues and conclusion/synthesis sections in
plain language.  Look for papers by these authors
in journals like _Geology_, which has a broad focus and
is meant to be read by people who are not specialists
in the authors' fields.

>> 	b) The North American Plate overrides the Pacific Plate and
>> 	   then is carried back eastward on the Pacific Plate from
>> 	   Cape Mendocino to Point Concepcion
>
>North America is not "carried back eastward on the Pacific Plate".

There are rocks with North American continental affinities
west of the San Andreas, which were, presumably, formerly
underridden by subducting Pacific Plate rocks and which
are currently being carried northwestward by the pacific Plate.

There is a component of eastward-directed compression
associated with the North America-Pacific plate
interactions.  That compression is the cause of folding,
thrust faulting, and uplift on both sides of the San
Andreas.  It does involve eastward transport of some rocks,
though only over small distances.

>> 	c) From Point Concepcion to the Salton Basin is
>> 	   faulting--sliding along a subducted fracture zone
>
>No - strike-slip faulting has nothing to do with subduction.  It is a
>fundamentally different type of plate boundary.

There are those who would argue that weakening of the
North American crust by subduction of hot young sea
floor helped in the initiation of the San Andreas faulting.

>Also the relative
>motion between North America and the Pacific makes the SanAndreas
>a transform fault, but the orientation of NA-PAC transforms is
>very different from PAC-JDF transforms (JDF fro Juan De Fuca plate).
>I could go into spherical geometry and velocity vectors at this point,
>but I'll spare you.

For detailed discussions of the geometric issues, try McKenzie
and Morgan's paper, Nature, v. 224, p. 125-133, 1969 or McKenzie's
chapter in _The Nature of the Solid Earth_, McGraw-Hill, 1972.

>>     A reference, please, for the location of the East Pacific Rise
>>     under California.
>
>No such reference - mainly because the East Pacific Rise is not under
>California.  the East Pacific Rise is a manifestation of the relative
>motion of the Cocos and Pacific plates.

This argument relies on an arbitrary definition of a spreading
center rather than on consideration of the geological implications
of the interaction of a spreading center with a continental
margin.

The answer to the question depends on whether
a spreading center (mid-ocean ridge) is an active or a passive
feature.  If it's passive, and ridge activity is a result of
welling up of mantle rocks where oceanic plates are pulled
apart, the effect of subducting a ridge is to shut off
its heat production and the effects on continental tectonics
will be just those of having somewhat warmer than usual
rocks underplating the continent.  If the spreading
center is an expression of a persistent heat source
deeper in the mantle, perhaps the edge of a mantle
convection cell, it may be meaningful to consider ongoing
thermal effects after a ridge meets the edge of a continent.

Alan Glazner worked on this problem for a while, and
presented some speculative interpretations that suggested
that the heat from the subducted East Pacific Rise had
some topographical expression that could be followed
eastward with time across the Mojave Desert.  Warren
Hamilton, another fearless synthesist, has published
numerous sketch maps in his papers that show a subducted
East Pacific Rise.  The DNAG volume that Andy recommended
is probably the best starting point for some fairly
recent thoughts on this issue.

On a somewhat related issue, Clark Johnson's MS thesis
at Stanford and papers based on that work document
the northward progression of volcanism in California
as the Mendocino Triple Junction moved northward.

>The meeting of the three plates is called a "triple junction".  triple
>junctions are special features of plate tectonics and invaluable for
>plate reconstruction and plate motion determination - they have well-defined
>geometric constraints.

Again, the approximation that lithospheric plates are rigid
spherical shells is convenient for calculations and very
useful for big-picture tectonics.  The questions raised
here can't be adequately addressed, though, unless we use a model of
the crust that includes more detail.

The crust is three dimensional.  It's fractured near the
surface and plastic at depth, so it can deform in ways that
contradict the rigid-plate approximation.  This extra
complexity has to be taken into account if, for example, we
ask what will happen at the northern end of Roy Dokka's
Eastern California Shear Zone, which made the news as
people pointed out that the Landers earthquake seems to
help delineate the zone's southern end.
--

    Chuck Karish          karish@mindcraft.com
    (415) 323-9000 x117   karish@forel.stanford.edu