How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 50

40

30

PP

20

LW

PL

10

0

-10

20 30 40 50 60 70 80 90 100

Age (Ma)

Fig. 5.4 Paleolatitudinal offset of California
tectonostratigraphic terranes shown by red circles as a
function of age in Ma. Offsets are calculated from the
paleomagnetic data of sedimentary rocks for different
tectonostratigraphic terranes compared to the predicted
paleolatitude for cratonic North America for a particular
age. Blue squares show the uncorrected paleolatitudinal
offset for the Cretaceous Pigeon Point Formation (PP) and
the Cretaceous Ladd and Williams Formation (LW). The blue
arrows point to the corresponding anisotropy-corrected
paleolatitudinal offsets shown by the blue hexagons. In
these three cases, the paleomagnetic inclination for these
formations was corrected for inclination shallowing with
the remanence anisotropy measured for the rocks. (See
references for the terranes depicted by the red circles in
Butler et al . 1991 .) (See Colour Plate 5)

Fig. 5.3 A stereonet showing the magnetic fabric of the
Cretaceous Pigeon Point Formation rocks from Kodama &
Davi (1995). The minimum axes (solid dots) of the samples'
magnetic fabric ellipsoids are scattered from vertical to
horizontal while the maximum axes (open squares) are
horizontal and oriented north-south. This orientation
suggests a composite oblate and prolate fabric for the Pigeon
Point resulting from both deposition from a turbidity fl ow
current and from burial compaction. KP Kodama and JM
Davi, A compaction correction for the paleomagnetism of
the Cretaceous Pigeon Point Formation of California,
Tectonics , 14, 5, 1153-1164, 1995. Figure 7, page 1160.
Copyright 1995 American Geophysical Union. Reproduced
by kind permission of American Geophysical Union.

shallowing rather than from northward tectonic trans-
port (Fig. 5.4 ).
We continued our study of California tectonstrati-
graphic terranes with Tan & Kodama's (1998) study of
the Point Loma Formation and the Ladd and Williams
Formation from southern California. The work fol-
lowed the same general strategy of the Kodama and
Davi study with disaggregation of the marine sedimen-
tary rocks, laboratory compaction of the slurries
created from the sedimentary rock material and ARM
anisotropy and paleomagnetic measurements; this
time however the results indicated that all of the
anomalously shallow inclinations could be explained
by compaction-induced inclination shallowing. The
magnitude of the inclination fl attening ranged over
12 - 20 ° ( f = 0.56 - 0.65).
We fi nished up our California tectonostratigraphic
terrane studies in Baja California by collecting Creta-
ceous rocks from the Vizcaino Peninsula (Fig. 5.5),
where terranes outboard of the main tectonostrati-

Cretaceous marine sedimentary rocks that indicated
latitudinal offset of tectonostratigraphic terranes iden-
tifi ed along the California coast. The paleomagnetism
of these terranes showed c. 1000 km or c. 10 ° of north-
ward latitudinal transport since the Cretaceous, and
this amount of offset suspiciously resulted from incli-
nations that were c. 10 - 15 ° too shallow in marine
sedimentary rocks that were fi ne grained and rich in
clay. Laboratory compaction work had revealed that
clay was a major cause of inclination fl attening as the
magnetite particles stuck to clay particles and rotated
into the horizontal with them as compaction pro-
ceeded. The magnitude of inclination shallowing in the
laboratory was always c. 10 - 15 ° , so it looked quite
plausible that the Cretaceous sedimentary rocks from
western North America had suffered from inclination