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(a)
(c)
(e)
ALL PLATES RIGID
FARALLON
FARALLON
NORTH
AMERICA
NORTH
AMERICA
FARALLON
NORTH
AMERICA
hole
PACIFIC
PACIFIC
PACIFIC
(b)
(d)
(f)
NORTH AMERICAN PLATE NON-RIGID
fa
fa
P
F
F
P
fp
fp
NORTH
AMERICA
FARALLON
A
A
pa
'back-arc'-type
extension fills hole
pa
PACIFIC
Figure 3.26.
Evolution of the Mendocino Triple Junction (prior to change in Juan de
Fuca rotation pole). (a) A stable Mendocino Triple Junction with the subduction zone
and the fault being collinear, as in Fig. 3.25. (b) Relative-velocity vectors for the
geometry in (a). (c) An unstable Mendocino Triple Junction; the subduction zone and
the fault are not collinear. (d) Relative-velocity vectors for the geometry shown in (c).
(e) Evolution of the triple junction shown in (c) when the three plates are assumed to
be rigid. (f) Alternative evolution of the triple junction shown in (c) if the two oceanic
plates are assumed to be rigid but the continental (North American) plate is allowed
to deform. (From Ingersoll (1982).)
a wide zone to the east of both the subduction zone and the fault. The deformations
which would be produced by this process are identical to those indicated by
continental palaeomagnetic data and would account for the regional extension in
the western U.S.A. (which began about 30 Ma ago when the Mendocino Triple
Junction was formed), as well as for the eastward stepping of the San Andreas
Fault with time, which has transferred parts of coastal California from the North
American plate to the Pacific plate. This eastward stepping of the plate boundary
has effectively been accomplished by the capture of sections of the partially
subducted Farallon plate by the Pacific plate. This capture of subducted parts
of oceanic plate subjected the overriding North American continental margin
to distributed shear and extensional forces. Utimately parts of the overriding
margin joined the Pacific plate and the plate boundary stepped eastwards, with
regions, such as the Western Transverse Ranges, being caused to rotate and zones
of extension developing as a result. Baja California has rotated clockwise and
