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Tectonics
E.M. Moores, Department of Geology, University of California,
Davis, California 95616
Ten years ago, tectonics was basking in the first flush of
success after the plate-tectonics revolution. People were still
applying modern models to whatever existing data they could
get their hands on. It seemed clear how geology reflected plate-
tectonic processes, and how broad patterns of evolution related
to continental drift. The first attempts at relating Alpine
tectonics to plate-tectonic processes were being hailed as the
greatest contribution since Argand by some Alpine geologists
and dismissed as more English language geo-trash by others.
Early attempts had been made to relate actualistic processes to
Precambrian terrains. Mantle plumes seemed to be a tectonic
“rosetta stone” relating absolute motions at the surface to deep
mantle or even core processes.
As for predictions, first it seemed that the then-popular
attempts to relate Cordilleran or Appalachian tectonics to
simplistic “single-stage” subduction zones were likely to undergo
revision as new information became available and ideas became
more sophisticated. Indeed, the evidence for collage tectonics
had been apparent since Warren Hamilton’s early plate-tectonic
papers in the late 60s and my “crashing island arc” idea,
published in 1970. No one would have ever predicted, however,
that “suspect terranes” would, as one wag has put it, “multiply
like rabbits,” or that some of the chief proponents of “noncolli-
sion tectonics” of 1973 would become the torch bearers a decade
later for “suspect terranes.”
Second, it seemed that the use of indirect means, such as pale-
ontology and biogeographic provinces, would make the working
out of Paleozoic plate-tectonic boundaries easy. This has proven
not to be the case. Despite great progress through the integration be a poor example of the classic “Andean-style” plate boundary.
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of biogeography, sedimentary environments, and ever more
precise and subtle paleomagnetic work, the task of working out
actual plate boundaries in the Paleozoic, as distinguished from
continental positions, has proven elusive. One complicating factor
is that the continents are made up of more separate pieces than we
originally thought.
Third, it seemed relatively easy to use ophiolite complexes as a
tool to calibrate the rate of spreading of ancient oceans. This also
has turned out not to be the case. The oceanic crust has itself
proven to be so complex that any simple comparison with ophio­
litic pseudostratigraphy is probably wrong. Ophiolites themselves
have proven to be complex and of uncertain significance.
Fourth, the prediction that plate-tectonic processes would be
easy to apply to the Precambrian has proven to be an oversimplifi-
cation. Paleomagnetic work on Proterozoic rocks has been a great
deterrent to simple application of our actualistic models, although
the latter clearly are applicable in some instances.
Hardly anyone predicted surprises such as the spectacular
success of slip-line field theory in explaining the active tectonics
of continental collision zones, or the COCORP results from the
southern Appalachians, and the resultant rapid progress toward
understanding the nonrigid tectonics of collisional regions.
Several predictions for the next ten years come to mind:
(1) The widespread and increasing availability of computer
programs will make the generation of structural models from
gravity, seismic, and magnetic data as widespread in academia and
government as they currently are in industry. Such models will
become an integral part of regional structural geology, a fact
that will be reflected in most student theses. (2) The crystalline
cores of all Phanerozoic mountain ranges will prove to be as
allochthonous as the Appalachians. (3) The Andes will prove to
(4) Precambrian orogenic belts will prove to be as allochthonous
as Phanerozoic ones. (5) Continental fragmentation and reassembly
will turn out to have been at least as frequent in the Proterozoic as
in the Phanerozoic.
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