The
Growth and Evolution of Continents
The earth's lithosphere can be
divided into two major structural features: ocean basins and
continents. Ocean basins, as we discussed in another essay, are
formed at spreading ridges and consumed at subduction zones. The
continents are a different story. How did the continents
originate, and how have they changed over time? The origin of the
earliest continental crust is still relatively obscure, but the
evolution of the continents over time is not: the continents have
clearly grown laterally over time via the process of accretion.
To understand how this process works, we can consider the Pacific
plate and its interactions with surrounding oceanic and
continental margins.
The Pacific plate is created by
seafloor spreading at the Pacific rise and moves slowly northwest
in a conveyor belt fashion at about 8-11cm per year. As a result
of this movement, the Pacific plate converges with surrounding
plates. In areas where the Pacific plate is being subducted, the
overiding plate is characterized by periodic deep-focus
earthquakes and a ring of felsic/andesitic volcanoes a few
hundred kilometers inland. The vast majority of all earthquakes occur at the three major types of plate
boundaries: spreading ridges, subduction zones and transform
faults.
Scattered throughout the Pacific
plate are a variety of topographic features rising hundreds to
thousands of meters from the ocean floor. These include island
arcs and seamounts, such as the Hawaiian islands-Emporor Seamount
chain, extinct spreading ridges, uplifted regions of ocean crust,
and apparent continental fragments, such as the Ontong Java
plateau. Nur et al estimate that are approximately 100 such
features are present on the modern sea floor, covering
approximately 10% of its surface (p. 7). Reymer and Schubert (1984)
estimate that approximately 1.40km3 of new sialic crust is
created annually in the form of new volcanic arcs and seamounts.
As the Pacific plate continues
to move northwest, these structures arrive at subduction zones
located at the margins of the Pacific plate. There they are fated
to become detached from the subducting plate and accreted onto
the margin of the overiding plate. Often a portion of oceanic
crust will be sandwiched between the accreted terrane and margin.
This process of lateral accretion at subduction zones has
obviously played a major role in the growth and evolution of the
Circum Pacific margins. Ben-Avraham writes:
"Recent investigations
of the geology of the Pacific Ocean margins have shown that
many sectors of these margins are composed of hundreds of
pieces called tectonostratigraphic terranes . . . Terranes
are fault-bounded packages of of rocks of regional extent,
each characterized by a geological history that is different
from the histories of neighboring terranes (Jones et al.,
1983) . . . The Circum-Pacific terrane map (Howell et al.,
1983) defines more than 300 different terranes within the
Pacfic borderlands. Some of the terranes are truly exotic to
their present surroundings and came from remote locations,
while others came to their present location from nearby areas.
"The evolution of the
Pacific Ocean margins involved accretion -collision
tectonics, which prevailed along most of its sectors for a
long time. As a result of this process, lengthy sectors of
the margins of the continents surrounding the Pacific Ocean
were built from bits and pieces that came from the sea. The
collisonal events are quite complex and are usually
associated with intense folding, thrust and strike-slip
faulting, penetrative deformations, and recrystallization"
(1989, p. 3).
Accretionary processes have
added a significant volume of material to the margins of North
America and Eurasia during the Phanerozoic. Howell and Jones
estimate that the total area of accreted terranes added to Circum-Pacific
margins during the past 200my is approximately 33 x 10^6km2 (p.
38). Condie (1989, p. 256) writes:
"The conclusion that
much of the Cordillera in western North America is composed
of a collage of accretionary terranes is now well established
by geological, paleontological, and paleomagnetic evidence (Coney
et al., 1980; Jones et al., 1986). More than 200 terranes
that lie west of the Precambrian craton have been recognized
in the Cordillera. Most of these terranes have been added to
North America during the Mesozoic and Cenozoic, during which
time the continental margin was extended by as much as 800km.
. . Most Cordilleran accreted terranes appear to represent
fragments of continents, oceanic plateaus, or portions of arc
systems, some of which traveled greater than 5000km from
their sources."
Allocthonous terranes on the North
American margin. From the USGS server.
From late Proterozoic to early
Jurassic time, the western border of North America was a passive
margin somewhere in the vicinity of the Nevada-California border.
Sediments deposited during this time were in the form of westward-thickening
wedges of marine deposits (miogeocline). These typically grade
from open marine in the west to shallow marine in the east east.
Sediment sources were to the east. In overall structure the
Cordilleran miogeocline was very similar to the Phanerozoic
Atlantic continental margins.
Beginning in the late Paleozoic,
there was a reorganization of plate movement, leading to the
formation of a subduction zone along the western margin of North
America. By Jurassic time, several accretionary events led to the
emergence of highlands at the western margin, effectively
blocking the sea from transgressing eastward over the craton.
These events are reflected in changes in the geology of the
Cordilleran foreland, where Protorozoic through Paleozoic open
marine deposits with sediment sources to the east are replaced in
late Paleozoic/Early Mesozoic time by terrestrial, fluvial,
restricted marine and eolian deposits with sediment sources to
the west.
Paleomagnetic evidence shows
that, as expected, many of the terrances now found in the
Cordillera and the rest of the circum Pacific margins, including
China, Japan, Siberia, Alaska, and Canada, have traveled large
distances before reaching their present positions. In some case,
a previously unified terrane has become fragmented and its
fragments widely dispersed. One example is the Wrangellia
terrane, pieces of which are found in British Columbia, Oregon,
and southern Alaska. Though fragments of Wrangellia are currently
spread over about 24 degrees latitude, paleomagnetic evidence
shows that the original spread was probably less than 4 degrees (Howell
and Jones, p. 37).
How are paleomagnetic data
explained on the basis of Noah's flood theories? Since large
Phanerozoic basalts and plutons would have remained well above
their curie temperatures for "weeks or months"
after the flood catastrophe, they should possess remnant
magnetization consistent with their present latitudes.
Assuming that the dipole geomagnetic field has existed since
creation, and that the only significant latitudinal displacement
of continents occured during the "weeks or months"
of the flood, then it seems that all igneous rocks, Precambrian
or Phanerozoic, should document only a "creation latitude"
or a post-flood latitude.
There is ample evidence that
Phanerozoic style accretionary tectonic processes were operating
well before the Cambrian, at least since the early Proterozoic,
and that these processes played a dominant role in the growth and
evolution of the continents (see below). All continents document
an extensive Precambrian/Preflood tectonic history involving
rifts, transforms, and collisions, along with their
characteristic associations of rock types. Tectonic
processes occuring during the Phanerozoic were just a
continuation of tectonic processes which had already been in
operation for well over a billion years.
To illustrate this point, we can
compare the Phanerozoic evolution of Eurasia to the Proterozoic evolution of the North American
craton. It is evident in both cases that these landmasses were
formed by a process of lateral accretion of smaller landmasses.
In Eurasia this accretion occured in several stages thoughout the
Phanerozoic and latest Proterozoic, whereas in North America,
this process of accretion was largely completed during the
Proterozoic, the major exception being the Corilleran belt on the
western margin of North America, which is composed of terranes
which were accreted during the Phanerozoic, and a few Paleozoic
terranes accreted to to east coast. Regarding the structure of
Eurasia, Maruyama et al. note:
"The Eurasian plate is
the largest in the world but is not composed simply of a
large craton surrounded by younger orogenic belts like the
North American plate. Instead, the present Eurasian landmass
is a collage of six, once seperated, major cratons, cemented
by a number of Phanerozoic orogenic belts of various ages"
(p. 75).
"The major Asiatic
Phanerozoic foldbelts, of decreasing age, are termed
Caledonian (early to middle Paleozoic), Variscan (late
Paleozoic), Indosinian (early Mesozoic), Yenshanian (late
Mesozoic), and Himilayan (Cenozoic) . . . All of these
foldbelts contain ophiolite and/or high P/T metamorphic rocks
and are thought to be formed during accretion" (p. 78).
Wherever they are found,
ophiolites are evidence of convergent tectonic processes.
Ophiolites are slivers of oceanic crust which have been "scraped
off" or obducted from the top of a subducting oceanic plate
and accreted onto the margin of an overriding plate. From the top
down, they consist of oceanic sediments such as cherts, followed
by pillow basalts, sheeted dikes, gabbros, and certain ultramafic
rocks (e.g., serpentinized harzburgite and lherzolite). Average
thickness is about 5km. The distinctive sheeted dike complex [diabase
dikes, >30cm thick, chilled margins] of ophiolites are
generated by successive magma intrusions at spreading (divergent)
plate boundaries, which of course implies plate motion.
Ophiolites are often abundant in
foldbelts sandwiched between two seperate terranes, cratons or
continents. Each of the Asian cratons (the Russian, Siberian, Indosinian, Sino-Korean,
Tarim, and Yangtze, etc.) are flanked by ophiolites, and/or
accretionary prisms and island arc assemblages. The cratons
themselves are much older than the Phanerozoic foldbelts
seperating them. For instance, the Indian Platform contains rocks
well over 3bya, but the foldbelt seperating India from mainland
Asia is composed of Jurassic-Cretaceous age ophiolites,
ophiolitic melange,and deep-ocean sedimentary rocks (Gansser,
1980; Le Fort, 1997; Corfield et al.,1999). The Indian craton had
a long and varied history prior to its accretion to Eurasia.
Paleomagnetic evidence suggests that India, Australia and
Antarctica were part of the same supercontinent during most of
the Proterozoic (Condie, 1989, p. 324).
Plate Tectonics During
the Precambrian
Young-earth creationism attempts
to explain the large-scale structure of the earth's surface in
terms of two events described in the book of Genesis -- the 2nd
and 3rd "creation days," a less-than 48 hour event
which occured approximately 4500BC (e.g. R. V. Gentry, 1988; K.
Wise, 1992), and Noah's flood, which occurred about 2500BC, and
which is said to have lasted less than one year. Roughly
speaking, the boundary between "creation strata" and
"flood strata" corresponds to the conventional Cambrian/
Precambrian boundary. On this view, all Precambrian geologic
formations were created instantaneously and ex nihilo,
by direct divine intervention, and have no actual prior
sedimentological or tectonic history whatsoever.
This is actually an
overgeneralization, since there appears to be wide disagreement
amongst creationists as to which strata are flood deposits and
which aren't. Several different pre-flood/flood and flood/post-flood
boundary schemes have been proposed. Some creationists see many
Precambrian strata as flood deposits, whereas others believe, as
we said, that they were created instantaneously. Some
creationists see all Phanerozoic strata as flood
deposits, whereas others place the flood/post-flood boundary as
early as the Permian (Steven
J. Robinson, "Can Flood Geology Explain the Fossil Record?"
Creation Ex Nihilo Technical Journal, 10(1996):1:32-69). All creationists, however, seem to
agree that many Archaean and Proterozoic formations are of
preflood age, and that at least Paleozoic strata represent flood
deposits.]
The Precambrian cratons of all
continents are composed of numerous distinct terranes of
different isotopic ages, structural trends, lithologies, and
paleomagnetic and tectonic histories. These terranes are
typically bounded by collisional orogens. Stratigraphic
relationships can be used to show that North America (and the
rest of Laurentia), for example, underwent an extensive tectonic
evolution well before the Cambrian seas transgressed over the
North American craton in the early Phanerozoic.
The North American craton is
separated into seven such provinces. Examples of well studied
Proterozoic orogenic foldbelts in North America include the
Wopmay belt or orogen and the Trans-Hudson belt between the
Wyoming and Superior crustal provinces in the northern US. A
handy map of precambrian crustal provinces in the US can be seen here and here. Condie (1989) states:
"Field and geophysical
data from the Canadian Shield, as well as results from
boreholes in in platform sediment, indicate that North
America is an amalgamation of plates, recently referred to as
the 'United Plates of America' (Hoffman, 1988). The Archaean
crust is composed of of at least six seperate provinces
joined by early Proterozoic foldbelts. The systematic
asymmetry of stratigraphic sections, structure,
metamorphicism, and igneous rocks is consistent with an
origin by subduction and collision. Such asymmetry is
particularly well-displayed along the Trans Hudson, Labrador,
and Penokean orogenic belts. In these belts, zones of
foreland deformation are dominated by thrusts and recumbent
folds, whereas hinterlands typically show transcurrent faults.
Both features are characteristic of subduction zones. Some
Proterozoic orogens have large accretionary prisms . . .
" (p. 354).
Cratons, often thought of as the
stable "basement," have Archaean "subcratons"(?)
themselves, which are in turn surrounded by younger Proterozoic
folbelts. For instance, the Yangtze craton appears to have been
formed by collision/accretion processes operating during the
Proterozoic (Guo et al., 1985). It consists of a late Archaean (2.86
U-Pb) nucleus to the northwest, flanked by progressively younger
Proterozoic rock to the south.
Tectonically emplaced ophiolites
are found at least as far back as the mid-Proterozoic, and
disputed ophiolites or ophiolite analogs are found in much older
Archean crustal provinces. These demonstrate the existence of
spreading boundaries and plate collisions, and hence tectonic
plate motion, since at least early Proterozoic (the YEC preflood)
time.
"[O]phiolites occur in
several Proterozoic orogenic belts and provide strong
evidence of the existence of oceanic plates like those of
today. The oldest is an ophiolite in the Cape Smith belt on the south side of Hudson Bay in
Canada whose age has been firmly established at 1.999 billion
years. There is a 1.8-billion-year-old ophiolite in the
Svecofennian belt of southern Finland, but most Proterozoic
ophiolites are 1 billion to 570 million years old and occur
in the Pan-African belts of Saudi Arabia, Egypt, and The
Sudan, where they occur in sutures between a variety of
island arcs" (Britannica.com article Precambrian time).
Another line of evidence which
shows that plate tectonic processes were active during the
Proterozoic is based on paleomagnetic data. While much less
complete than Phanerozoic data, paleomagnetic data from
Proterozoic formations (such
as the massive Kaweenawan basalts [Halls et al., 1982] and the
Grand Canyon Supergroup [Elston et al., 1973], etc.) show the
"preflood" continents moving with respect to latitude
over time during the Proterozoic, at rates comparable to, though
slightly faster than, those measured today (<20cm/yr). Condie
states that "rates of Proterozoic plate motions can be
obtained from APW paths . . . results indicate that Proterozoic
continental plate velocities (3-10cm/y) often exceeded those of
present-day continental plates (which average about 5cm/y) and
were equivalent to those of present day oceanic plates"
(p. 219). Such gradual slowing is expected due to gradual
reduction in the amount of heat generated within the earth by
decaying radionuclides.
A characteristic feature of
Phanerozoic and Proterozoic APW paths is the presence of
loops with an average periodicty of roughly 200my. In Phanerozoic
APW paths, these loops can often be correlated with orogenic
episodes. Several loops in Proterozoic APW paths also appear to
be correlated with major orogenic episodes (1150, 1750, and 1850
in North America; 1100 and 2150 in Africa; Condie, 1989, p. 333).
For instance, the portion of the
mid to late Proterozoic APWP for the North American craton
constructed from the Kaweenawan basalts and the Grand Canyon
Supergroup "has yielded the temporally longest,
stratigraphically controlled polar path yet developed for North
America (~1250) to 800Ma)", and describes a prominent
"loop." P.K. Link et al. note:
"The polar path for the
Unkar Group, beginning at the level of the Shinumo Quartzite
and extending to the lower member of the Nankoweap Formation,
overlaps and coincides with the polar path reported from
lower, middle, and upper Keweenawan rocks of the Lake
Superior region" (p. 479).
"The composite polar
path . . . from the Unkar-Nankoweap and Kaweenawan-Chequamegon
poles, shows an overlapping, concordant, nonconflicting
progression of poles, and a single polar path for the two
regions. The concordant north- and then south-trending paths
form a single loop that is here called the Unkar- Kaweenawan
loop" (p. 480).
The apex of the Unkar-Keweenawan
loop appears to coincide with the end of the Grenville orogeny
and the cessation of Keweenawan rifting. Mafic sills of similar
age in both Unkar and Keweenawan strata appear to have been
emplaced at this time as well, indicating that whatever event is
indicated by the apex affected a large region of the North
American Craton.
Plate Tectonics Today
Accretionary continental growth
continues to occur at the Pacific margins today. For instance,
the Yakutat terrane and other allocthonous blocks are colliding
with southern Alaska. Baha California is moving north along a
transform boundary, and will probably eventually become accreted
to Alaska as well. The Nazca and Juan Fernandez ridges are
colliding with the western coast of South America, and the
Louisville and Marcus-Neker Rises are colliding with subduction
zones in the western Pacific (Nur and Ben-Avraham, p. 8). India
is still "colliding" with Eurasia, and the Himalayas
continue to rise. Numerous other terranes of diverse types are
slowly moving towards the margins of the Pacific Ocean, where
they are fated to be accreted to an already massive amount of
accreted allocthonous terranes.
"The Ontong Java
Oceanic plateau in the southwest Pacific Ocean has a crustal
thickness of 36km and may be a rifted and now submerged
continental fragment. In size, it is comparable to the
Yangtze continental terrane of Asia. Accreted against the
south side of the Ontong Java plateau is part of the Solomon
islands volcanic arc, and numerous other volcanic arcs lie
nearby. If one imagines the closing-up of the coral sea, this
inferred continental fragment wreathed with accreted volcanic
arcs would form a major addition to the Australian continent."
What might future
supercontinents look like? Long terrm extrapolation based on
present plate trajectories is likely to be less than perfectly
accurate, since we know that plate motions can change or be
reorganized over time. Nevertheless, some interesting conjectures
can be made. For instance, Australia is moving north, towards
Eurasia, at about 6cm per year. Between the two contienents are
numerous island arcs and microcontinents, such as New Guinea,
Indonesia and Malaysia. North America and Eurasia are converging
with each other at a rate of about 2cm per year. Extrapolating
from current trajectories, one could easily imagine a future
supercontinent consisting of a unified Eurasia, Australia, and
North America. Joe Meert's homepage has a graphic illustrating
what the continents might look like in 250my, based on current
plate trajectories. The graphic can be seen here.
