P. Molnar et Jm. Gipson, VERY LONG-BASE-LINE INTERFEROMETRY AND ACTIVE ROTATIONS OF CRUSTAL BLOCKS IN THE WESTERN TRANSVERSE RANGES, CALIFORNIA, Geological Society of America bulletin, 106(5), 1994, pp. 594-606
Changes in baseline vectors between very long baseline interferometry
(VLBI) receiving stations in the Western Transverse Ranges imply that
east-west blocks of crust in this region rotate clockwise about vertic
al axes with respect to the Pacific and North American plates. Minimum
apparent rotations, given by the ratios between components of velocit
y perpendicular to baseline vectors and the lengths of the baselines,
imply minimum current rotation rates of a few degrees per million year
s. The relevant VLBI receivers lie on different crustal blocks that ar
e separated by major active faults. Both geologic and other geodetic o
bservations imply north-south convergence between such blocks at sever
al millimeters per year. Corrections to perpendicular components of ve
locity for such relative movements between blocks yield likely clockwi
se rotation rates of 6-degrees/m.y. +/- 2-degrees m.y., which are indi
stinguishable from the average rate inferred from paleomagnetic declin
ations of rocks in the Western Transverse Ranges with ages less than 1
5 m.y. Thus, rotation seems to have occurred continuously and apparent
ly with only small variations in rate during a period when the tectoni
cs of southern California changed dramatically. This apparent independ
ence of the rotation rate on the changing surface kinematics is consis
tent (1) with such rotation being a manifestation of continuous deform
ation at depth in the lower crust and upper mantle, (2) with weak faul
ts separating upper-crustal blocks, and (3) with the important resista
nce to continental deformation lying in the upper mantle and/or lower
crust.