Chapter 1 - Plate Tectonics

plate 2 traces a figure of rotation around two axes known 
as a spherical cycloid. 
14 Plate Tectonics and Cmstal Evolution 
Figure 1.13 Stress provinces in the United States. Arrows are directions of least (outward-directed) or greatest (inward-
directed) principal horizontal compression. Province abbreviations: SA, San Andreas; SBR, southern Basin and Range province; 
RGR, Rio Grande rift; CP, Colorado Plateau; NRM, northern Rocky Mountains; SOP, southern Great Plains. From Zoback and 
Zoback (1980). 
Figure 1.14 Spherical cycloid motion of plate 2 as its pole 
of rotation P processes along a line that represents a small 
circle around PI, the pole of rotation of plate 1. M is a 
point on plate 2. Time progresses from t^ to tg. Modified 
after Cronin (1987). 
Plate Velocities in the last 150 My 
Rates of plate motion can be quantified for the last 150 
My by correlating seafioor magnetic anomalies with the 
Geomagnetic Time Scale (Figure 1.15). For convenience, 
magnetic anomalies are numbered beginning with 1 
at ridge axes. From the Geomagnetic Time Scale, each 
anomaly is assigned an age (for instance anomaly 30 
corresponds to an age of 72 Ma, and anomaly 7 to 28 
Ma). If the spreading rate has been constant in one ocean 
basin, it is possible to extend the Geomagnetic Time Scale 
to more than 4.5 Ma using the magnetic anomaly pat-
terns. Although data indicate that a constant spreading 
rate is unlikely in any ocean basin, the South Atlantic 
most closely approaches constancy (~ 1.9 cm/y) and is 
commonly chosen as a reference to extrapolate the time 
scale. Paleontologic dates from sediment cores retrieved 
by the Deep Sea Drilling Project and isotopic ages of 
basalts dredged or drilled from the ocean floor sub-
stantiate an approximately constant spreading rate in the 
South Atlantic and allow the extension of the magnetic 
time scale to about 80 Ma. Correlations of magnetic 
anomalies with distance from ridge axes indicate that 
spreading rates in the South Indian and North Pacific 
basins have been more variable and, on the average, 
faster than the spreading rate of the South Atlantic (Fig-
ure 1.15). 
Plate velocities also can be estimated from disloca-
tion theory, using data derived from first-motion studies 
of earthquakes and from observed dip-lengths of sub-
duction zones, if these lengths are assumed to be a mea-
sure of the amount of underthrusting during the last 10 
Plate tectonics 15 
South 
Atlantic 
South 
Indian 
North 
Pacific 
South 
Pacific 
Ridge Figure 1.15 Magnetic profiles from the 
Axis 
^ ^-^'y\/'^/^^^/yA^ 
I • • l i i i i i w i i i i ' i i i i i i i i m i i i i i i 11101 
80 70 60 50 40 30 
AGE (Ma) 
20 10 
Atlantic, Indian, and Pacific Ocean basins. 
Geomagnetic Time Scale given beneath the 
profiles with normal (black) and reversed (white) 
magnetized bands. Proposed correlations of 
anomalies are shown with dashed lines. Numbers 
refer to specific anomalies. Modified after 
Heirtzler et al. (1968). 
My. Still another method used to estimate plate veloci-
ties is by using transform faults. Rates and directions of 
motion can be estimated from the azimuths and amount 
of offset along transform faults, provided the azimuth 
and timing of motion can be estimated accurately. 
Estimates of plate velocities commonly range within 
a factor of two of one another using the above methods 
averaged over several to many millions of years. Rates 
range from about 1 to 20 cm/y, averaging a few centi-
metres per year for most plates. Typical velocities of 
major plates (in cm/y) are as follows: North American, 
1.5-2; Eurasian, 2.4-2.7; African, 2.9-3.2; Australian, 
6-7.5; and Pacific, 5-7 (DeMets et al., 1990). 
From spreading rates estimated from seafloor mag-
netic anomalies, it is possible to contour the age of the 
sea floor and several such maps have been published. 
From these maps, we see that the rate of spreading has 
varied between crustal segments bounded by transform 
faults, and has even varied on opposite flanks of the 
same ridge. The oldest oceanic crust (Jurassic) occurs 
immediately adjacent to the Izu-Bonin subduction zone 
south of Japan. Since the rate at which oceanic crust has 
been produced at ridges during the past several hundred 
million years is of the order of a few centimetres per 
year, it is unlikely that crust much older than Jurassic 
will be found on the ocean floors today. The average 
age of oceanic crust is about 60 My and the average age 
it begins to subduct is about 120 My. Fragments of 
oceanic crust older than Jurassic (ophiolites. Chapter 3) 
are found in continental orogenic belts where they were 
tectonically emplaced during orogeny. 
From calculated seafloor spreading directions and rates, 
it is possible to reconstruct plate positions and to esti-
mate rates of plate separation for the last 200 My. One 
way of illustrating such reconstructions is by the use of 
flow or drift lines, as shown in Figure 1.16 for the open-
ing of the North Atlantic. The arrows indicate the rela-
tive directions of movement. Earlier positions of Africa 
and Europe relative to North America are also shown, 
with the corresponding ages in millions of years. The 
reconstruction agrees well with geometric fits across the 
North Atlantic. It is clear from the lines of motion that 
Europe and Africa have been on two different plates for 
the last 160 My. Changes in spreading rates and direc-
tions, however, occur on both the African and Eurasian 
plates at the same time, at about 60 and 80 Ma. The 
separation of Africa and North America occurred prima-
rily between 80 and 180 Ma, whereas separation of 
Eurasia from North America occurred chiefly in the last 
80 My. 
Plate velocities from paleomagnetism 
If true polar wander has been small compared with the 
rate of plate motions in the geologic past, it is possible 
to estimate minimum plate velocities even before 200 
Ma from plate motion rates (Bryan and Gordon, 1986; 
Jurdy et al., 1995). Also, if at least some hotspots have 
remained relatively fixed, it is possible to estimate plate 
motions relative to these hotspots (Hartnady and leRoex, 
1985). Compared with typical modem continental plate 
velocities of 2-3 cm/y, during the last 350 My most 
continental plates have excursions to much faster rates 
(Figure 1.17). Episodes of rapid plate motion are re-
corded during the Triassic-Early Jurassic (250-200 Ma) 
on most continents, and Australia and India show peak 
velocities at about 150 and 50 Ma, respectively, after 
they fragmented from Gondwana. Results indicate that 
in the past continental plates have moved as fast as 
modem oceanic plates for intervals of 30—70 My. It is 
noteworthy that maxima in some continental plate vel-
ocities occur just after fragmentation from a super-
continent. For instance, the velocity maxima in the early 
Mesozoic follow the beginning of rifting in Pangea, and 
the peak velocities for Australia and India follow separ-
ation of these continents from Gondwana (Figure 1.17). 
Paleomagnetic data from Archean rocks in southem 
Africa suggest that plates were moving at comparatively 
slow rates of about 2 cm/y between 3.5 and 2.4 Ga in 
this region, near the low end of the range of speeds of 
16 Plate Tectonics and Crustal Evolution 
- 9 0 ' -75 ' -60* 
Figure 1.16 Seafloor spreading reconstruction of the opening of the North Atlantic. Black continents are present positions; 
dates for earlier positions are indicated in millions of years ago. Arrows are flow lines. After Pitman and Talwani (1972). 
Phanerozoic plates (Kroner and Layer, 1992). How rep-
resentative this rate is of the Archean is not known, but 
it is surprising in that a hotter Archean mantle would 
seem to be more consistent with faster plate motions. 
Space geodetic measurements of plate