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TitlePatterns of Change in Earth Evolution: Report of the Dahlem Workshop Patterns of Change in Earth Evolution Berlin 1983, May 1–6
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Dahlem Workshop Reports
Physical, Chemical, and Earth Sciences Research Report 5
Patterns of Change in Earth Evolution

The goal of this Dahlem Workshop is:
to identify temporal variations in the nature
and intensity of important Earth processes,
and to assess the effects of these variations
on the evolution of the Earth and its biota

Page 2

Physical, Chemical, and Earth Sciences Research Reports
Editor: Silke Bernhard

Held and published on behalf of the
Stifterverband fUr die Deutsche Wissenschaft

Sponsored by:
Senat der Stadt Benin
Stifterverband fUr die Deutsche Wissenschaft

Page 215

Patterns and Geological Significance of Age Determinations 213

time intervals have been very widely quoted in the scientific literature.
For example, the author of a recent model (15) for the origin of "hot-
spot" volcanism linked to mantle plumes states that " ... one may correlate
periods of intense continental magmatism at 3.6, 2.7, 1.8, 1.0 Ga ago
with times of plume-triggered mantle convection and the intervening
quiet periods with times of mostly stagnant conditions in the mantle,
when the newly accumulated subducted crust was gradually heating up
but had not reached the point of instability." In this model intermittent
mantle convection alternates with stagnant conditions. Another model
(28) postulates that these same episodes of rapid continental growth
can be explained in terms of the interaction between major convective
overturns in the lower mantle and "megaliths" of subducted oceanic
lithosphere, suspended near the 650 km discontinuity.

Condie (5) surveyed global dates briefly and concluded that "existing
data indicate that world-wide Precambrian orogenic periods are episodic
averaging 200 to 400 million years in length and occurring about every
500 to 600 million years. Major orogenic periods occur at 3.0-3.8, 2.5-2.7,
1.5-2.0, 0.9-1.2, and 0.5-0.7 billion years. Major North American
Phanerozoic orogenies average about 50 million years in length and occur
at irregular spacings."

CONCLUDING DISCUSSION
The time is clearly not yet ripe for an authoritative answer to the main
problem under discussion, but any future attempts must take into account
the following observations:

1. The date ranges given by Condie (5) and Moorbath (21) are almost
certainly too generalized and oversimplified, since they are based on
data that are too limited. These, as well as previous, compilations have
been widely quoted in support of the proposition that major periods of
global tectonism were episodic and synchronous, and that they were
separated by intervening periods of global tectonic "recession."

2. The occurrence of episodic crustal "accretion superevents" is
established beyond reasonable doubt, but their global synchronicity
throughout geological time is in doubt. In one of the best-stUdied regions,
West Greenland, accretion superevents in adjacent terrains can be clearly
demonstrated at ca. 3700 - 3600, 3000 - 2700, and 1900 - 1700 Ma ago
(23, 32); the overlap is minor (31). A broadly similar range of dates has
been found on the adjacent North American craton, although the oldest

Page 216

214 s. Moorbath

group of ages in Greenland has not yet been positively identified in the
remainder of the North Atlantic craton or on the Baltic Shield. In the
Zimbabwean craton of southern Africa, accretion superevents occurred
ca. 3500 - 3400 and 2900 - 2700 Ma ago. Preliminary evidence is
accumulating «1), and Oxford, unpublished work) that the earliest accretion
superevent on the Indian subcontinent occurred about 3400 - 3200 Ma
ago; this event is not synchronous with any others that have been
recognized elsewhere. Several other major crust-forming events which
do not fit previously proposed intervals occurred in the Arabian Shield
and in southern Norway. Age and isoto~e studies have demonstrated
that in the Arabian Shield (ca. 600,000 km ) crustal accretion of mantle-
derived granitoids was intense between ca. 850 and 550 Ma ago. (10);
this event was partly contemporaneous with the major Pan-African orogeny
in other parts of Africa, which was dominated by the reworking of older
continental crust. In southern Norway, major crust-forming events
commenced at ca. 1600 - 1500 Ma ago and were completed several hundred
million years later (12).

3. The temporal and regional distribution of episodic accretion superevents
throughout Earth history must be viewed in conjunction with our
increasingly sophisticated understanding of the Earth's changing thermal
regime (6, 17, 18, 33). Much recent· geochemical and isotopic evidence
from continental and oceanic rocks is compatible with the hypothesis
that the evolution of the lithosphere is dominated by irreversible chemical
differentiation of the mantle (and particularly of the ~ mantle) and
by the permanence of the greater part of the continental crust once
it has accreted, differentiated, and stabilized. There have been several
notable recent efforts, based mainly on isotopic data, to model continental
growth by irreversible differentiation of the mantle through geologic
time (8, 16, 26, 27). In the light of global thermal and isotopic models,
I consider that the production of continental crust was strongly inhibited
prior to about 3800 Ma ago, and that before this time the Earth had an
essentially globe-encircling, impermanent, mainly mafic or ultramafic
protocrust most of which was recycled through the mantle by disruption
and foundering. This crust may have been akin to the skin of solids on
a lava lake or slag furnace. Heat production was undoubtedly much more
rapid and mantle convection more intense more than 3800 Ma ago.
Relatively small amounts of acid (granitic) differentiates may already
have existed at this early stage. Such rocks are certainly known from
the ca. 3800-Ma-old Isua supracrustal succession of West Greenland (22).
The creation of the first true, thick, calc-alkaline continental crust at

Page 429

Dahlem Workshop Reports
Life Sciences LS31 Microbial Adhesion and Aggregation.
Research Reports

Editor: KG. Marshall (1984, in press)

LS 30 Leukemia. (LS) Editor: 1. L. Weissman (1984, in press)
LS29 The Biology of Learning.

Editors: P. Marler, H. S. Terrace (1984, in press)

LS 28 Changing Metal Cycles and Human Health.
Editor: l.O. Nriagu (1984)

LS27 Minorities: Community and Identity.
Editor: C. Fried (1983)

LS26 The Origins of Depression: Current Concepts
and Approaches.
Editor: 1. Angst (1983)

LS 25 Population Biology ofInfectious Diseases.
Editors: R. M. Anderson, R. M. May (1982)

LS24 Repair and Regeneration of the Nervous System.
Editor: l.G.Nicholls (1982)

LS23 Biological Mineralization and Demineralization.
Editor: G.H.Nancollas (1982)

LS22 Evolution and Development.
Editor: 1. T. Bonner (1982)

LS21 Animal Mind - Human Mind.
Editor: D. R. Griffin (1982)

LS20 Neuronal-glial Cell Interrelationships.
Editor: T. A. Sears (1982)

Physical, Chemical PC5 Patterns of Change in Earth Evolution.
and Earth Sciences

Editors: H. D. Holland, A. F. Trendall
(1984)

Research Reports PC4 Atmospheric Chemistry.
(PC) Editor: E. D. Goldberg (1982)

PC3 Mineral Deposits and the Evolution of the Biosphere.
Editors: H. D. Holland, M. Schidlowski (1982)

Springer-Verlag Berlin Heidelberg New York Tokyo

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Dahlem Workshop Reports
Life Sciences
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Physical and
Chemical Sciences
Research Reports
(PC)

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LS 19

The Molecular Basis of Circadian Rhythms.
Editors: 1. W. Hastings, H.-G. Schweiger
Appetite and Food Intake.
Editor: T. Silverstone
Hormone and Antihormone Action at the Target Cell.
Editors: 1. H. Clark et al.
Organization and Expression of Chromosomes.
Editors: V. G. Allfreyet al.
Recognition of Complex Acoustic Signals.
Editor: T. H. Bullock
Function and Formation of Neural Systems.
Editor: G. S. Stent
Neoplastic Transformation: Mechanisms and
Consequences.
Editor: H. Koprowski
The Bases of Addiction.
Editor: J. Fishman
Morality as a Biological Phenomenon.
Editor: G. S. Stent
Abnormal Fetal Growth: Biological Bases and
Consequences.
Editor: F. Naftolin
Transport of Macromolecules in Cellular Systems.
Editor: S. C. Silverstein
Light-Induced Charge Separation in Biology and Chemistry.
Editors: H. Genscher, 1. 1. Katz
Strategies of Microbial Life in Extreme Environments.
Editor: M. Shilo
The Role ofintercellular Signals: Navigation, Encounter,
Outcome.
Editor: J. G. Nicholls
Biomedical Pattern Recognition and Image Processing.
Editors: K. S. Fu, T. Pavlidis
The Molecular Basis of Microbial Pathogenicity.
Editors: H. Schmith et al.
Pain and Society.
Editors: H. W. Kosterlitz, L. Y. Terenius
Evolution of Social Behavior: Hypotheses and Empirical
Tests.
Editor: H. Markl
Signed and Spoken Language: Biological Constraints on
Linguistic Form.
Editors: U. Bellugi, M. Studdert-Kennedy

PC I The Nature of Seawater (o:.Jt of print)
PC 2 Global Chemical Cycles and Their Alteration by Man

Editor: W. Stumm

Distributor for LS 1-19 and PC 1+2:
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