Regional lineaments in continental evolution were studied. It was concluded that lineaments within continents are attributed to a small number of processes of continent formation and modification, but interactions among these processes may produce complex patterns

Burke, Kevin

English

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WILSON CYCLE STUDIES 
NASA GRANT NAG5-585 
Semi -Annual Report  
J u l y  15, 1986 - January 15, 1987 
A c t i v i t y  concentrated on the  study o f  reg ional  1 ineaments i n  con t inen ta l  
e v o l u t i o n  and an extended a b s t r a c t  (Appendix 1 a t tached)  was prepared t h a t  
w i l l  be presented a t  a for thcoming US-India workshop on con t inen ta l  1 ineaments 
t o  be h e l d  i n  Bangalore i n  A p r i l  1987 (attendance i s  a t  no cos t  t o  the  p r o j e c t ) .  
(6ASB-CH-180353) WILSCN C Y C l E  SPUDIES ~ a 1 - 1 5 a 2 4  
Semiannual Regort ,  15 J u l .  19E6 - 15 Jan. 
1587 (Houston Univ.) 7 F C S C L  OBG 
Unclas 
63/46 43555 
https://ntrs.nasa.gov/search.jsp?R=19870010391 2020-03-20T11:11:49+00:00Z
. 
REGIONAL LINEAMENTS AND CONTINENTAL EVOLUTION 
Kevin Burke, Lunar and Planetary Institute, 3303 Nasa Road I,  Houston, TX 77058 and 
Department of Geosciences, University of Houston, Houston, T X  77001 r n d  Cel:l Sen&, 
Lunar Planetary Institute and Istanbul Technical University. 
1. SUMMARY 
Lineaments within continents are attributable to a small number of processes of conti- 
nent formation and modification, but interactions among these processes may produce 
complex patterns. 
2. FORMATlON AND MODIFICATION OF THE CONTINENTAL CRUST 
Continents originate by the sweeping together of island arcs (Burke and SengEr, 1986). 
Continental and island arc crusts are  dominated by rocks of comparable compositional 
and density ranges ( S O z  50-7096; 2.6-2.9 gm/cc), evidence of the operation of this process 
is abundant in the geological record, and can be seen in action today in Indonesia. 
Misgivings have been voiced about the simple idea of assembling continents by colliding 
arcs, because some arcs do not closely resemble average continental material and because 
much continental material is very old (> 2.5 Ga) and the world is likely to have operated 
in a different way in  the remote past. 
The world must have operated in a different way in the very remote past, but it is 
increasingly being recognized that the record of the rocks as f a r  back as 3.8 Ga ago, is 
consistent with the idea "that many processes have apparently not changed at  all" 
(Thompson et al, 1984, p. 404). For this reason, we here employ the hypothesis that as f a r  
back as the record goes, continents have been assembled by processes fundamentally com- 
parable to  those acting today. 
There are within the oceans shallow-water objects distinct from active arcs, extinct 
arcs and micro-continents which have been suggested capable of contributing to the conti- 
nental crust. One such population is that formed at  intra-oceanic hot-spots, e. g., Hawaii 
and Iceland, but although small objects representative of oceanic islands have long been 
repoited from mountain belts of all ages, there are no large Iceland or Hawaii like objects 
with volumes in excess of a thousand cubic kilometers in the mountain belts of the world. 
We therefore infer that i t  is the fate of such material to be largely removed from t h e  
surface of the earth by either subduction or obduction. The alternative idea, that older 
oceans did not contain hotspot volcanics seems less likely. A similar argument applies to 
the oceanic plateaus (e. g., Caribbean, Manihiki, Ontong Java, Shatsky Rise, Hess Rise) 
which extend over areas of millions of square kilometers. Evidence both from places 
where slivers have been caught up in accretionary prisms and from their relationship to 
old spreading centers shows that they do not represent continental fragments (as has been 
suggested), but are very thick accumulations of basalt interbedded with pelagic sediments. 
The  mountain belts of the world contain slivers of material of this type, but no large 
volumes, and we therefore conclude that, oceanic plateaus do not contribute significant 
volumes of material to the continents. 
Materials that contribute substantial volumes to the continents are parts of Island arc  
systems, including sediments of accretionary wedges which in some areas, for  example, 
the Songpan-Ganzi, the Makran and the Barbados wedge may be of enormous volume. 
jected a re  limited in  both number and extent. We distinguish six main processes by which 
continents are  modified and summarize them thus: 
(1) The construction of a n  Andean arc a t  the continental margin involves a variety of 
phenomena, including: (a) addition of magmatic material from the mantle; (b) partial 
melting of older continental crust and related vertical fractionation; (c) thickening of 
crust by up  to a factor of 2 (to 70km) and consequent elevation; (d) compressional 
tectonism (especially on the borders of the arc) with formation of both a forearc and a 
foreland trough; (e) extensional tectonism (especially a t  the crest of the arc). 
(2) Continental collision, as now in Tibet and the Himalayas, produces effects similar to 
those of Andean collision, but over a much larger area. Indian workers have long 
described the Himalaya and, thanks to recent pioneering efforts by Chinese geologists 
and their French and British colleagues, we are beginning to appreciate some of the ways 
in which continental crust is modified at  collision in Tibet. 
Once a continent is assembled by arc collision, the vicissitudes to which it can be sub- 
~~~~ 
Tremendous thrusting of the kind first described from the Alps, Scandinavia, the  
Himalaya and from the North of Scotland about 100 years ago, has accommodated the  
extensive doubling of continental thickness that characterizes both Andean and collisional 
mountain belts. Partial melting of the thickened crust at  collision produces compositional 
differentiation with shallow "minimal-melt" granites and deep anhydrous residues. Thick- 
ened continents, are unstable being subject to rapid erosion as well as to gravitational 
collapse, two processes that restore continental thicknesses to normal values (30-40 km) i n  
8 few tens of millions of years after the cessation of convergence. 
(3) Flooding of the continents by the waters of the ocean produces a sediment thickness 
typically of a few hundred meters and reflects sea level changes dominated by the varying 
average age of the ocean floor and the volume of ice-caps. 
(4) The mature continent records bombardment from outer space, but  since the beginning 
ORIGIPSA PAGE rs 
OY: POOR QUALITY 
of the rock record (at 3.8 Ga), the flux has been low, and the most complete record is on 
the oldest continents with Vredefort in South Africa and Sudbury on the Canadian Shield. 
(5) Sporadic non-plate mrrglo (or hot-spot) volcanlrm will modify the continental crust 
from time to time, but these effects generally produce only local modification. 
(6) Rifting (the formation of elongate depressions related to rupture of the lithosphere in 
extension) is one of the dominant ways in which continents are  modified. Rif ts  form in 
diverse tectonic environments: Andean arcs may split along their crestal volcanoes to form 
marginal basins; continents like Africa now and about 200 Ma ago may come to rest over 
the convective circulation of the mantle and rupture. Rifts also form in association 
with continental collision (as in the Rhine and Baikal). 
Continental assembly from island arcs, Andean arc  formation, continental collision and 
rifting are the dominant processes of continental evolution which leave their mark in the 
deep structure of the continents. Continental regional geophysical lineaments can be 
related to one or other of all of these processes. 
3. REGIONAL LINEAMENTS WITHIN THE CONTINENTS 
Lineaments related to the various processes that generate and modify continental 
crust are widespread, and I here describe a few examples: 
(1) The almost North-South trend of the greenstone belts of Southern India (Fig. 1) is now 
TbO 78" BOe 
Figure 1. Numerous Archean arc-collisions 
a re  suggested responsible for  the Greenstone 
belt trends (horizontal lines) north of a line 
through Kolsr. Bangalore r n d  Mysore. 
The dash-dot line close to  the East coast is a 
lineament marking the site of a possible Late 
Proterozoic collision to which the Cuddapah 
Basin is 8 foreland-basin. Late Proterozoic 
strike-slip faults are perhaps associated 
with this collision. From Drury r n d  others. 
Journal of Geology, 1984. 
. 
OWEC?~,"iL PAGE IS 
OF POOR QUALITY 
(2) The characteristic trend of an Andean arc can be recognized in the Gangdise intrusive 
and volcanic belt of Tibet (Fig. 2) which represents the arc  system associated with north- 
ward subduction of Neotethyan ocean floor that preceded the collision of India with the 
rest of Asia in Paleogene times. Ancient examples of arcs of this kind probably exist 
within the Indian Shield, perhaps, for example, in the Aravallis. 
I Mop of -y. 
Figure 2. Structural map of 
the Eastern Himalaya modified 
from Gansser (1980) showing 
parallel lineaments attributable 
to the Indus-Yarlung Tsangpo 
suture (black) and the Gangdise 
intrusives (short dashes) 
representing the roots of a n  
Andean arc. 
(3) The Indus-Yarlung Tsangpo suture zone is a familiar feature (Fig. 2) marking the site 
of the collision between India and the rest of Asia. Similar lineaments are  recognized in  
many parts of the world as recording collisions between continental and arc blocks and 
fragments. A map of such suture zones in Asia forms Figure 3. 
Figure 3. Sutu res of 
& along which 
blocks and fragments 
(sometimes confusingly 
called terranes) have 
been joined, mainly 
during the Phanerozoic. 
(4) Many of the world's most spectacular lineaments mark the sites of major strike-slip 
fault-zones and some of the most prominent of these faults are  related to continental 
collision. New understanding of these features has followed from appreciation of their 
role in the process of "tectonic escape" by which material caught in  a collision attempts to 
escape toward an  oceanic "free-face". Faults in China and southeast Asia, such as the 
Altyn Tagh fault  (Fig. 4) are the best examples of active faults of this kind, although 
more ancient features, such as the New York to Alabama lineament, have also been asso- 
ciated with collisional events. 
Examples of strike-slip related linear features shown in Figure 1 are  possibly associated 
with a late Proterozoic collisional event roughly along the site of the present East Coast 
of India. The Cuddapah basin may be a Precambrian foreland basin related to such a 
collision. 
(5) Although rifts  occur in a variety of different tectonic environments, many of the most 
familiar are related to collision. For example, in Figure 4 the Baikal and Shansi rifts 
inferred from their ages to be associated with the current Indian-Asian collision. 
Figure 4. Tectonic escape in South-East 
Asia. Modified and simplified from Tapponnicr 
e t  al. Note that evidence of timing indicates 
an association with the Himalayan collision 
for  rifts as f a r  apar t  as  Baik2l a n t  the 
Sumatran coal basins (at 103 E. 3 S). A-T is 
the Altyn Tagh fault zone I200 km in length. 
7Q W W J U V  l l Q  
Fjgure 5 is a representation of Gondwana-Land in Late Permian times in which i t  is 
indicated, following a suggestion of Peter Lehner, that the "Gondwana" rifts of India, 
Madagascar and Australia can be attributed to an arc-collision event recorded in the 
'Sa mf ra u'. 
Although virtually all of the major lineaments of the continents can be ascribed to 
one or other of the processes outlined here, deciphering exactly how the processes have 
operated is generally difficult and may not always be possible. An illustrative example of 
0RiGiPJ.U PAGE IS 
OF POOR QUALITV 
Figure 5. End Paleozoic Collision of 
arc systems with Gondwana induced 
rifting in Africa, India. Australia and 
Antarctica. The Cimmerian continent 
left Gondwana and an extensive fore- 
land basin (K) was filled by 
Karroo sediments. 
how rifting, strike-slip motion and suturing may interact to yield complex geometric 
patterns is shown in Figure 6. 
Figure 6. Complex lineament patterns within continents (as 
in IV and V) may be produced by rifting (I  and 11) and 
strike-slip motion (IIIa) before suturing (IVa) or suturing 
(IIlb) followed by strike-slip motion (IVb). 
Burke, Kevin and CelAl ScngEr, 1986. Tectonic escape in the evolution of the continental 
crust in Reflection Se ismolonv: The Cont inental Crus1 . Geodynamics Series Vol. 14, 
pp. 41-53, American Geophysical Union. 
Thompson, A. B., F. M Richter, H. Ahrendt, M. J. Bickle, K. Burke, R. F. Dymek, H'. Frisch, 
R. D. Gee, A. Kroner, R. K. O'Nions, E. R. Oxburgh, and K. Weber, 1984. The long-term 
evolution of the crust and mantle in Patterns of Channe of Earth EvolutioL edited by 
Holland and Trendall, pp. 389-406. 


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