From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Questions raised by SoCal Activity
Keywords: usgs refs, landers
Date: 21 Aug 92 21:16:03 GMT

In article <1992Aug20.182116.10567@morrow.stanford.edu>, karish@pangea.Stanford.EDU (Chuck Karish) writes:
> In article <51084@seismo.CSS.GOV> stead@skadi.CSS.GOV (Richard Stead) writes:
> >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
> 
> 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.

This is incorrect.  A quick glance at Atwater's paper or some playing with
triple junctions shows why.  Where the San Andreas meets the Mendocino
Fracture Zone is a triple junction - the third fault is the subduction
zone for the Gorda/Juan de Fuca Plate.  The Mendocino Fracture Zone is
moving northwest relative to North America.  This movement is as inevitable
as that of the San Andreas.  The path of least resistance to resolving
the change in geometry that occurs where the angle of the coast line
changes would be spreading between Pacific and North America.  Not
much would be needed - less than that required to form the Gulf of
California when the angle of that coast didn't quite work out a few
million years ago.

> >> 	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 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.

The point still stands.  Once a North American rock has been incorporated
onto the Pacific Plate, it is a Pacific Rock.  Also, compression on the
San Andreas depends entirely on the local direction of the fault.  In
some areas, sure there is "transpression".  The most stunning example
is along the big bend in the San Andreas through the transverse ranges.
Current thrusting and mountain building in that region is due to
compression across the San Andreas.  Gene Humphries has shown that the
compression has also resulted in a great "root" of sub-crustal material
which is being scraped off the underside of the crust and is hanging deep
into the mantle.  However, the Imperial Valley shows a "leaky transform"
where the angle of the fault has changed such that there is a component
of spreading across the fault.  As I mentioned elsewhere, there is only
one direction that allows for perfect strike-slip motion (for the
spherical geometrically inclined, this direction is that of small
circles relative to the pole of rotation between the Pacific and North America).
The San Andreas is not perfectly straight, so along some sections you
get a bit of compression and on others, extension.  (It works on all scales,
too, so that you can get little areas of local extension in big areas
of general compression - an example of this is the Palmdale reservoir.)

> >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.

First, most triple junction do not permit the subduction of a ridge.
That is, the ridge never goes under the overriding plate.  Even if you
mark exactly where the ridge is at one point, then let the plates go for
awhile, then look again, none of your mark is under the overriding plate.
But as to the argument that ridges are the edges of convection cells -
most have given up that concept.  The problem is, you can't devise a
system that creates cells with such long, coninuous edges.  Anyway,
mantle convection is more likely in a chaotic regime due to the very
high Rayleigh number of the flow.  The flow is dominated by plumes
and sinks in that case.  The plates act as integrators - their motion
is the ingral of the drag forces exerted beneath.  They in turn can
exert entraining forces on the flow, so it starts to become a chicken
and egg argument - are the plates moving one way because all the
mantle under them is moving that way, or is the all the mantle moving
that way becuase the plates are.  Given how well plate tectonics
gives into the demands of spherical geometry, I tend to go for
the position that plate boundaries are due to such demands.  I
especially like the intersecting ridge stuff that's come out recently,
showing how the geometry and progression of ridge intersections is
predictable and constrained.

> 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.

This confuses me.  Volcanism turns _off_ historically south of the MTJ as
it moves north.  Did Clark Johnson find something different?

> 
> >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.
> 
> 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

Well, the crust needn't be three dimensional to violate rigid plates, but that's
nit-picking.

> 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.

Indeed, this is the biggest problem with the hypothesis.  It can't end and
it can't take up anything near plate tectonic rates of motion without some
sort of satisfaction (generally a triple junction) at the northern end.
Possibilities:  1) Mendocino fracture zone fractures northern CA, and
subduction jumps back from the coast of Oregon/Washington to the center
of these states.  This is geometrically the simplest, but geologically
and tectonically unsatisfactory.  2) subduction along Oregon/Washington
ceases as does spreading between Pacific and Gorda/Juan de Fuca.  Then
strike slip breaks from the North end of Dokka's zone past Vancouver
Island.  All that one can say here is that it's easier to justify than
the first try, but still not likely.  Any other options?


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