Crustal extension is accommodated by a wide range of structural styles ranging from high-angle normal faults to low-angle detachments. In some areas different structural styles are superposed by multiple extension events, and in other areas different structural styles are juxtaposed. As shown with examples from the Rio Grande rift and the Red Sea-Gulf of Aden rift system, high strains and low-angle faulting are commonly spatially and temporally associated with a hot and probably thin lithosphere as indicated by major magmatic activity. Theoretical studies of strength profiles in the lithosphere suggest that there may effectively be a critical range for the geotherm above which low-angle faulting and crustal decollement may be favored over high-angle faulting

Morgan, P.

English

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THERMAL CONTROL OF THE STYLE OF EXTENSIONAL TECTONICS; Paul Morgan, 
Department o f  Geosciences, Purdue Universi ty,  West Lafayette ,  I N  47907, USA. 
Crus ta l  extension i s  accommodated by a wide range of  s t r u c t u r a l  s t y l e s  
ranging from high-angle normal f a u l t s  t o  low-angle detachments. I n  some a reas  
d i f f e r e n t  s t r u c t u r a l  s t y l e s  a r e  superposed by mul t ip le  extension events,  and 
i n  o t h e r  a r e a s  d i f f e r e n t  s t r u c t u r a l  s t y l e s  are juxtaposed along s t r i k e  i n  t h e  
same extension event.  A s  shown below with examples from t h e  Rio Grande rift 
and t h e  Red Sea-Gulf of Aden rift system, high s t r a i n s  and low-angle f a u l t i n g  
a r e  commonly s p a t i a l l y  and temporally associa ted  with hot  and probably t h i n  
l i thosphere  a s  ind ica ted  by major coeval magmatic a c t i v i t y .  Theore t ica l  
s t u d i e s  o f  s t r e n g t h  p r o f i l e s  i n  t h e  l i thosphere  suggest t h a t  t h e r e  may 
e f f e c t i v e l y  be a c r i t i c a l  range f o r  t h e  geotherm above which low-angle 
f a u l t i n g  and c r u s t a l  decollement may be favored over high-angle f a u l t i n g .  
A t  least two phases o f  Cenozoic extension have been i d e n t i f i e d  i n  t h e  Rio 
Grande rift ( 1 ) .  Early phase extension began i n  mid-Oligocene (about 30 Ma) 
and may have continued t o  t h e  Ear ly  Miocene (about  18 Ma). This phase of 
extension is charac ter ized  by l o c a l  high-strain extension events  ( l o c a l l y  50- 
loo%, reg iona l ly  30-50$), low-angle f a u l t i n g  and t h e  development o f  broad, 
r e l a t i v e l y  shallow basins. Extension events  were not synchronous during e a r l y  
phase extens ion and were o f t e n  temporally and s p a t i a l l y  associa ted  with major 
magmatism. Late phase extension occurred pr imar i ly  i n  t h e  Late Miocene (10 t o  
5 Ma) wi th  minor extension continuing t o  t h e  present.  It is character ized  by 
apparently synchronous, high-angle f a u l t i n g  g iv ing l a r g e  v e r t i c a l  s t r a i n s  with 
r e l a t i v e l y  minor l a t e r a l  s t r a i n  (5-202) which produced t h e  modern Rio Grande 
rift morphology. Late phase graben o r  ha l f -  graben bas ins  cut ,  and o f t en  
obscure e a r l y  phase broad basins. A s i m i l a r  two-phase extension h i s t o r y  has 
been repor ted  f o r  o the r  a r e a s  of t h e  Basin and Range province (2) .  
Evidence has r ecen t ly  been presented f o r  t h e  Red Sea-Gulf of Aden rift 
system t o  suggest  t h a t  r o t a t i o n  of  t h e  Arabian peninsula away from north-east 
Africa has been accommodated synchronously by mechanisms ranging from 
sea-floor spreading i n  t h e  c e n t r a l  por t ion  o f  t h e  Red Sea t o  d i f f u s e  extens ion 
i n  t h e  Afar t r i a n g l e  ( 3 ) ,  C o u r t i l l o t  ( 3 )  has described t h i s  mechanism a s  the  
propagation of  two rifts from the  t h e  Red Sea and t h e  Gulf of Aden i n t o  t h e  
Afar region, t h e  eventual  in terconnect ion  o f  which w i l l  effect the  complete 
separa t ion  of southern Arabia from t h e  African continent .  O f  i n t e r e s t  t o  t h e  
present  s tudy is t h e  change i n  extens ional  s t y l e  along s t r i k e  i n  the  Red Sea- 
Gulf of Aden rift system from cont inenta l  sepa ra t ion  t o  d i f f u s e  extension f o r  
the  same t o t a l  extensional  s t r a i n .  The d i f f u s e  extension is s p a t i a l l y  
associa ted  with extens ive  volcanism (4). 
Experimental rock mechanics d a t a  can be used t o  es t imate  l i t h o s p h e r i c  
s t r eng th  p r o f i l e s  f o r  l i thosphere  of d i f f e r e n t  compositional and thermal 
s t r u c t u r e s  (5 ) ,  These p r o f i l e s  show an approximately l i n e a r  inc rease  i n  
b r i t t l e  s t r e n g t h  wi th  depth i n  t h e  l i thosphere  which is truncated by an 
approximately exponential  decrease i n  d u c t i l e  s t r eng th  a t  t h e  b r i t t l e -  d u c t i l e  
t r a n s i t i o n .  The depth of this t r a n s i t i o n  depends pr imar i ly  upon composition, 
temperature and s t r a i n  r a t e ,  and compositional l aye r ing  can r e s u l t  i n  "layersw 
of s t r e n g t h  wi th in  t h e  l i thosphere ,  t h e  most prevalent  of  which a r e  a l a y e r  of 
upper c r u s t a l  s t r eng th  and a l a y e r  o f  uppermost mantle s t rength .  The most 
fundamental r e s u l t  of  s t u d i e s  of  l i t h o s p h e r i c  s t r e n g t h  p r o f i l e s  is t h a t  t h e  
l i thosphere  is s i g n i f i c a n t l y  weaker i n  regions  where t h e  geotherm is elevated,.  
r e s u l t i n g  i n  shallow b r i t t l e - d u c t i l e  t r a n s i t i o n ( s ) ,  and t o  a l e s s e r  extent  
where t h e  c r u s t  is th ick ,  r e s u l t i n g  i n  t h e  replacement of  d u c t i l e l y  s t rong  
mantle l a y e r s  w i t h  r e l a t i v e l y  d u c t i l e l y  weak c r u s t a l  l a y e r s  a t  equivalent  
https://ntrs.nasa.gov/search.jsp?R=19860021676 2020-03-20T13:37:40+00:00Z
i CONTROL OF EXTENSIONAL TECTONICS 
98 Morgan, P.  
depths and temperatures. This  r e s u l t s  suggests  t h a t  t h e  geotherm and/or 
c r u s t a l  th ickness  may be important f a c t o r s  i n  t h e  l o c a l i z a t i o n  of  extens ional  
s t r a i n .  The second important r e s u l t  o f  these  s t u d i e s  is t h e  p red ic t ion  t h a t  
t h e  r e l a t i v e  importance of  t h e  c r u s t a l  and mantle s t r e n g t h  l a y e r s  may change 
as t h e  geotherm changes: f o r  low heat  flow ( s t a b l e  a reas )  most o f  t h e  s t r eng th  
i n  the  l i thosphere  is i n  t h e  uppermost mantle; a t  intermediate heat  flow 
values t h e  uppermost mantle and c r u s t a l  s t r e n g t h s  become s i m i l a r ;  f o r  high 
heat  flow, neg l ig ib le  mantle s t r e n g t h  is predicted,  wi th  a l l  t h e  s t r eng th  
res id ing  i n  t h e  uppermost c rus t .  
Uncer ta in t ies  i n  t h e  ex t rapo la t ion  of l abora to ry  rock mechanics d a t a  t o  
l i t h o s p h e r i c  condit ions l i m i t  t h e  q u a n t i t a t i v e  use  o f  the  l i t h o s p h e r i c  
s t r eng th  p r o f i l e s  ca lcula ted  from these  data.  However, q u a l i t a t i v e  mechanisms 
can be proposed which a r e  cons i s t en t  wi th  the  ca lcu la ted  s t r eng th  p r o f i l e s  and 
t h e  t e c t o n i c  observations i n  t h e  Rio Grande rift and Red Sea-Gulf of  Aden rift 
system. Major magmatic a c t i v i t y  leading up t o  e a r l y  phase extension i n  the  
Rio Grande rift i n d i c a t e s  very high hea t  flow from which a s t r eng th  p r o f i l e  
with only s t r e n g t h  i n  the  uppermost c r u s t  is predic ted ,  During t h i s  extension 
event t h e  l i thosphere  was dominated by its d u c t i l e  p roper t i e s  with shallow 
decollement a t  a shallow br i t t l e -duc  t i l e  t r a n s i t  i o n  r e s u l t i n g  i n  low-angle 
faultiulg i n  a t h i n  upper c r u s t a l  b r i t t l e  layer .  A reduction i n  magmatism 
p r i o r  t o  l a t e  phase extension suggests  lower heat  flow, f r o m  which a s t r e n g t h  
p r o f i l e  with some uppermost mantle s t r eng th  is predicted. Increased 
l i t h o s p h e r i c  s t r e n g t h  may be one f a c t o r  i n  producing smaller  l a t e r a l  s t r a i n  
during t h i s  event, but i t  is a l s o  poss ib le  t h a t  t h e  l a y e r  of mantle s t r eng th  
prevented shallow c r u s t a l  decollement r e s u l t i n g  i n  high-angle planar o r  l a r g e  
radius  l i s t r i c  normal f a u l t s  ( 1 ) .  
For t h e  Red Sea-Gulf of Aden system v a r i a t i o n  i n  t h e  volumes of  volcanics  
along t h e  s t r i k e  of t h e  rift system suggest v a r i a t i o n s  i n  heat  flow and the  
geotherm along s t r i k e .  Abundant volcanics  cons i s t en t  with high heat  flow i n  
t h e  Afar region suggests  mechanical condi t ions  s i m i l a r  t o  e a r l y  phase 
extension i n  the  Rio Grande rift, with a r e l a t i v e l y  d u c t i l e  l i thosphere  
r e s u l t i n g  i n  d i f f u s e  extension. Dis t r ibuted  d ike  i n t r u s i o n  has made an 
important cont r ibut ion  t o  t h i s  d i f f u s e  extension (9) .  I n  t h e  c e n t r a l  por t ion  
of t h e  Red Sea and t h e  Gulf o f  Aden smaller volumes of  volcanics cons i s t en t  
with lower heat  flow suggest  mechanical condi t ions  s i m i l a r  t o  l a t e  phase Rio 
Grande rift extension, favoring high-angle f au l t ing .  A s  v e r t i c a l  s t r a i n  is 
constrained by buoyancy considerat ions,  however, continued extension on high- 
angle f a u l t s  is l imi ted ,  and can only be accommodated by d ike  in t rus ion ,  
probably i n i t i a l l y  loca l i zed  by t h e  high-angle f a u l t s .  To accommodate a high 
t o t a l  s t r a i n  i n  t h e  lower heat  flow a r e a s  an e a r l y  t r a n s i t i o n  from c r u s t a l  
extension t o  organized sea-floor spreading is predicted.  
References: (1 )  Morgan, P. and Golombek, M. (1984) Factors  con t ro l l ing  
the  phases and s t y l e s  of extension i n  t h e  northern Rio Grande rift. Field 
Conf. Guidebook, &. H. GeoL. m., s, p. 13-20. (2)  Eaton, G. P. (1982) The 
Basin and Range province: Origin and t e c t o n i c  s igni f icance .  h. &Yo Earth 
Planet .  a., JQ, p. 409-440. (3)  C o u r t i l l o t ,  V. E. ( 1980) Opening of t h e  
Gulf of Aden by progressive tear ing .  Phvs. Earth P_lanet. U., a, P. 343- 
350. (4)  Barberi,  F., Santacroce, R. and Varet, J. (1982) Chemical a spec t s  of 
rift magmatism. Continentaa Oceanic Rifts, Geodvnamics m. 8, Am. 
Geopys. Un., p. 223-258. (5) Lynch, H. D. ( 1983) Numerical models of the  
formation o f  cont inenta l  rifts by processes o f  l i t h o s p h e r i c  necking. .Ph.D. 
d i s s e r t a t i o n ,  &. J$. S t a t e  U., Las Cruces, NM, 290 pp. 


Thermal control of the style of extensional tectonics

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