Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 44

target because of their importance in helping to time
the northward motion of India and its collision with
Asia. Tauxe & Kent ' s (1984) early investigation
into red bed inclination shallowing was with the red
sediment collected from the Siwalik Group in Pakistan
and re-deposited in the laboratory. Ojha et al . ' s (2000)
magnetostratigraphic study of the Siwaliks in Nepal
observed 29.4° of shallowing for an expected inclina-
tion of about 50° ( f = 0.31). Most of the paleomagnetic
data were derived from laminated siltstones.
Perhaps the largest group of red bed studies that
show inclination shallowing come from Cretaceous
and Early Tertiary rocks of central Asia. Table 4.2 lists
six studies from western China that report observa-
tions of a low inclination anomaly. The initial observa-
tions of the central Asian inclination anomaly in the
1990s were interpreted to indicate geologically unrea-
sonable post-Tertiary northward motion of central
Asia. Various scenarios were used to explain the data:
some of the explanations included long-term non-
dipole fi eld contributions to the geocentric axial dipole
or large geologically unrecognized shear zones. As
inclination shallowing corrections were applied to the
red rocks carrying the inclination anomaly (e.g. Tan et

al . 2003) and sedimentary rock directions could be
compared to local volcanic directions (Gilder et al .
2003 ; Tan et al . 2010), it became clear that inclination
shallowing in red beds was the most likely explanation
for the low inclination anomaly. One reason that
inclination-shallowing anomalies were so glaring for
these rocks is that the expected inclination for central
Asia in the Cretaceous and Early Tertiary was close to
45 ° , where the tan I f = f tan I 0 relationship will give the
maximum amount of shallowing.
The studies showing inclination shallowing in red
beds from western China include Oligo-Miocene red
beds from Subei (Gilder et al . 2001), Miocene red beds
from the Maza Tagh mountains in the Tarim Basin
(Dupont - Nivet et al . 2002 ), Jurassic - Cretaceous red
beds from western Xinjiang province (Gilder et al .
2003), Tertiary red beds (20-60 Ma) from the Hoh Xil
basin of northern Tibet (Liu et al . 2003 ), Cretaceous
red beds from southeast China (Wang & Yang 2007)
and the Lhasa block (Tan et al . 2010 ). Tan et al . ' s
(2003) inclination-shallowing correction for the Cre-
taceous red beds of the Kapusaliang Formation in the
northern Tarim Basin will be discussed in more detail
in Chapter 5. Two studies of Cretaceous and Tertiary

Table 4.2 Hematite - bearing sedimentary rocks
Locality Lithology f factor Reference

Neogene basins in Catalan, Spain Miocene laminated red

fi ne-grained sediments

0.21-0.48 Garces et al . (1996)

Siwalik Group, Nepal Miocene red beds: siltstones,

sandstones, paleosols

0.31 Ojha et al . (2000)

Subei, China Oligo-Miocene red beds 0.49 Gilder et al . (2001)

Maza Tagh range, Tarim Basin, China Miocene red beds 0.32 Dupont-Nivet et al . (2002)

Western Xinjiang, China Jurassic-Cretaceous red beds 0.64 Gilder et al . (2003)

Hoh Xil basin, northern Tibet Tertiary red beds; 20-60 Ma 0.47 Liu et al . (2003)

Sierra de los Barrientos, Argentina Ediacaran red claystones 0.62 Rapalini (2006)

Jishui and Ganzhou, SE China Cretaceous red beds 0.65 Wang & Yang (2007)

Cis-Urals, Russia Permian-Triassic red beds 0.7 Taylor et al . (2009)

Lhasa block, China Cretaceous red beds 0.49 Tan et al . (2010)

Ukraine Permo/Carboniferous red and

gray sedimentary rocks

0.58 Iosifi di et al . (2010)

Average for hematite 0.51 ± 0.14 Bilardello & Kodama (2010b)

N = 11 f = 0.59 + 0.24/-0.19, N = 15

Note: Three additional studies of hematite-bearing rocks did not fi nd evidence of shallowing (see text).