Schmucker, 1970 (Scripps) - MTNet

Schmucker: Geomagnetic Variations 103
The theoretical Zp- and hp-curves of the illustrations are normalized with
the normal plus anomalous horizontal variations at the reference stations
Auburn (figs. 40, 43), Fresno (figs. 41, 42, 44), and Cameron (fig. 45) to
facilitate the comparison of model calculations and observations (cf. com w
ment in sec. 4.4 regarding this point).
7.4 Inland Anomalies in California and Nevada
Some of the inland anomalies mentioned in section 4.5 can be explained by
nonuniformities near the surface but others calUlot. Let us begin with the
San Joaquin val.ley in central California (fig. 21). Its sediments have been
represented in the models by a conductive slab, 100 km wide and having a
total conductivity of 1.2 • 10-6 emu· cm. This is three times more than the
continental average value of equation 7.2b and equivalent to 6000 m of sediments
with 5 Qm resistivity. Since the valley is 1000 km long but less than
150 km in Width, we may treat it as a 2-dimensional structure (App. III).
The Sierra Nevada and its igneous rock formations form an adjoining slab of
zero conductivity.
The computed zp-curves on the basis of these values explain rather well
the anomalous behavior of Z -variations near the edges of the San Joaquin
valley. Particularly striking is the agreement between observations and
model calculations for the out-of-phase zp-profiles at low frequencies (except
for a constant displacement). Hence, this inland anomaly can be derived
from the superficial conductivity contrast between the San Joaquin valley
and the Sierra Nevada, using reasonable values for their total conductivities.
If there should exist an additional deep-seated change in conductivity,
its effect would be masked by the coincident superficial gradient.
The model calculations do not explain the highly anomalous behavior of Zvariations
between Napa and Davis at 2 and 4 cph (fig. 40). It seems to be
connected with a local rise in mantle conductivity which has not been encountered
on the profile Monterey-Bridgeport further south.
There remains also the consistent misfit of the observed and computed
zp-profiles along the eastern slope of the Sierra Nevada which can be recognized
on all profiles. We observe that the in-phase zp-values drop off more
rapidly toward inland than expected from the computed curves and that they
reverse their sign eastward of the Sierra Nevada (CAC, BRI, BIS). This suggests
that an anomalous concentration of surface or subsurface induction currents
exists parallel to the Sierra. Since zp is positive again at Fallon, Nevada,
which is just 100 km east of Carson City, the internal current concentration
must be rather narrow.
There is no geological evidence for a marked change in surface conductivity
between the Sierra Nevada and the adjacent Basin and Range province.
We conclude therefore that subsurface currents are involved which are drawn
into a bulge of high mantle conductivity eastward of the Sierra (fig. 47). If
'We compare the inland portions of the profiles San Clemente;"Yuma (fig. 45)
and Farallon Islands-Fallon (fig. 40), we notice a certain similarity between