Improved precision of modern 
Ar/
Ar ages and U-Pb zircon
ages has begun to bring tighter absolute age control to the Late
Jurassic-Early Cretaceous part of the GPTS. There are several
Ar/
Ar ages for M-sequence polarity chrons close to CM0. At
ODP Site 807 (Ontong Java Plateau) and Site 878 (MIT Guyot),
basalts erupted in the CM0-CM1 interval give ages in the 122-123 Ma
range [ Mahoney et al., 1993; Pringle et al., 1993].
Reversely magnetized granitic plutons from Canada, possibly
correlative to CM3, give ages close to 124 Ma [ Foland et al.,
1986]. A U-Pb zircon age of 137 Ma from the Great Valley Sequence
of Northern California is correlated through the nannofossil
biostratigraphy to the CM15-CM16 interval [ Bralower et al.,
1990]. The age control for the older end of the M-sequence is from
the Sierra Nevada [see Schweickert et al., 1984] and from the
Klamath Mountains [see Pessagno and Blome, 1990]. Although
the tie between ammonite zones and radiometric ages is not very
tight, 154 
2 Ma is the concensus age for the
Oxfordian-Kimmeridgian boundary, which correlates to M25 [ Ogg et al.,
1984].
The age estimates mentioned above for the base of CM0 (
121
Ma), CM16-CM15 (137 Ma) and the top CM25 (
154 Ma) are not
consistent with constant spreading rate in Larson and Hilde
[1975] Hawaiian oceanic magnetic anomaly block model (LH75).
Obradovich [1993] used the LH75 block model to interpolate between
the same three age estimates: CM0 (121 Ma), CM16/16n (137 Ma),
CM25n (156 Ma) (Table 3). The resulting timescale implies an
abrupt spreading rate change at CM16 in LH75 and other Pacific
block models. Gradstein et al. [1994] have presented an
integrated timescale for Triassic to Cretaceous time. For the Late
Jurassic--Early Cretaceous interval (Table 3), constant spreading
rates were assumed for parts of the LH75 block model with an
increase and then decrease in spreading rate in Valanginian and
Berriasian in order to satisfy the three principal age constraints.
A new Hawaiian block model [ Channell et al., 1994] may
represent an improved estimate of a constant spreading rate record
for the M0-M29 interval. The three popular ages for CM0, CM16/15
and CM25 are consistent with constant spreading in the new Hawaiian
block model and the resulting timescale (Table 3) does not imply
abrupt or synchronous changes in spreading rate in the Pacific
block models.