Intracontinental deformation occurrence and the processes and physical parameters that control the rates and styles of deformation were examined. Studies addressing specific mechanical aspects of deformation were reviewed and the studies of deformation and of the structure of specific areas were studied considering the strength of the material and the gravitational effect

Molnar, Peter

Toksoz, M. Nafi

English

NASA Technical Reports Server

b 
/3/ 2302, 
SP 
Earth Resources Laboratory 
Department of Earth, Atmospheric, and Planetary Sciences 
Massachusetts Institute of Technology 
Cambridge, Massachusetts 02139 
I 
FINAL TECHNICAL REPORT TO: National Aeronautics and Space Administration 
Geodynamics Program 
TITLE Geophysical Study of the Structure and Processes of the Continental 
Convergence Zones - Alpine-Himalayan Belt 
GRANT NUMBER NAG 5-41 
INVESTIGATORS: M. Nafi Toksoz, Peter Molnar 
PERIOD: 1 May 1980 - 31 December 1983 
REPORT DATE: 19 April 1988 
t 
(IiASa-a-182647) GEOPHYSICAL S T U D Y  OF THE ta 08- I 98 42 
S911UCTURE A N D  PROCESSES OF THE CONTIHEYTAL 
CONVEBGENCE ZONES:  ALPINE-HIHZILAY AN BELT 
Final Technical  Report, 1 Hay 1980 - 31 Uec. Unclas 
1983 (Hassachusetts Z n d .  of T e c h , )  6 p G3/46 0131802 
https://ntrs.nasa.gov/search.jsp?R=19880010458 2020-03-20T08:13:03+00:00Z
2. 
Introduction 
8 The focus of our work has been toward understanding how intracontinental 
deformation occurs and what are the processes and physical parameters that 
control the rates and styles of deformation. This work has required both the 
selected regions and numerical analyses of processes and phenomenon thought 
to occur. 
processes resist deformation in continents. One obviously is the strength of 
material, which depends upon the constituent minerals, the ambient 
temperature, and perhaps other properties such as the presence of water. The 
second is gravity, for to build mountain ranges or to thicken the crust work 
must be done against gravity. Our work has addressed both of these aspects in 
diflerent ways, but in all cases we have tried to isolate them from one another. 
The logic here has been that it is better to isolate specific phonemena and 
understand them than to try to include simultaneously too many processes and 
not understand which are important. 
specific mechanical aspects of deformation, then consider studies of 
deformation and of the structure of specific areas in l i h t  of these ideas. 
Underlying our work has been the presumption that two entirely diflerent 
In summarizing our work, let us begin with a review of studies addressing 
Mechanical Aspects of Continental Deformation 
to deform than weaker rock. The heterogeneous distributions of active 
tectonics and of relief in Asia imply normal lateral variations in the strength of 
the lithosphere, and Molnar and Tapponnier (1981) argued that older (less 
recently deformed) crust was less susceptible to deformation when India 
collided with Eurasia, because older crust tends to be colder than younger 
crust. 
One major difference between continental tectonics and plate tectonics is 
that crustal thickening is important only in continents. Concurrent with such 
thickening, the mantle lithosphere should thicken, but such thickening should, 
in turn, induce large horizontal temperature gradients that can convectively 
destroy such a lithospheric root. Houseman et  al. (1981) carried out a series of 
numerical experiments to  examine this process and concluded that the initial 
temperature structure of mantle lithosphere might be restored only a fewto 10- 
20 Ma after crustal thickening began. Such a reheating of the mantle 
lithosphere beneath thickened crust is likely to make it hot and weak. 
elevations and thick crustal root store gravitational potential energy that can be 
extracted during later crustal thinning. As a result. thrust faulting and crustal 
thickening on the margins of mountain belts can occur simultaneously with 
normal faulting and crustal extension at high altitudes (Dalmayrac and Molnar, 
Obviously stronger rock requires greater stress to deform and is less likely 
One important manifestation of crustal thickening is that the high 
1981). 
Deep Structure of Asia 
plateaus and of mountain chains to determine both the extent to which the 
crust is thick and the degree to which the mantle is hot. W e  focussed on the 
Himalaya and Tibet. 
A major effort was devoted toward determining the deep structure of high 
3 .  
The crust of Tibet is indeed very thick (Chen and Molnar, 1981a), but several 
arguments suggest that the upper mantle is quite hot (Chen and Molnar. 1 9 8 1 ~  
Lyon-Caen, 1986; Molnar and Chen, 1984). Surface wave dispersion cannot be 
accounted for with crust thinner than 55 km. and together with measured Sn 
velocities, a thickness of 70 5 5-10 km seems required. S-wave delays from 
'IEbetan earthquakes, however, are large, and they imply a thick zone of low 
velocities beneath Tibet. A careful study of Sh phases recorded at regional 
distances (Lyon-Caen, 1988) proves that this low velocity zone must underlie the 
upper 250 km of the mantle beneath Tibet. Hence, it seems likely that 
conveclive processes have remuved a lhickenad lilhuspheric roo1 Lenaalh Tibal. 
A brief study of the high plateaus of Iran and Turkey suggests that they too 
are underlain by hot material (Chen et al., 1980). 
A convective process beneath Tibet might reveal itself in the structure of 
the Himalaya. Gravity anomalies can be explained by a simple model in which 
the Indian plate is flexed down beneath the Himalaya and loaded by the weight of 
the Himalaya, only if an additional force acts on that plate (Lyon-Caen and 
Molnar, 1983, 1985). That force seems most likely to be due to convective 
downwelling of the leading edge of the Indian plate, which induces a bending 
moment in the plate just north of the Himalaya. 
A similar analysis for the underthrusting of the Tamir Basin beneath the 
Kunlun (Lyon-Caen and Molnar, 1984) does not require an additional force, but it 
too calls for a substantial amount of underthrusting beneath the range. 
Deformation 
earthquakes in two areas. Earthquakes in the Himalaya seem to occur on the 
top surface of the Indian plate as it is underthrust, intact, beneath the Himalaya 
(Baranowski et al., 1984). Hence, the Himalaya seem to comprise a giant 
crystalline nappe scraped off the leading edge of the Indian plate. 
Earthquakes in Tibet consistently indicate east-west extension of the 
plateau. Moreover, these events occur either at very shallow depths in the crust 
or in the uppermost mantle. Hence, they seem to d e h e  a zone of low strength 
in the lower crust, a zone that could decouple upper and lower crustal 
deformation. 
Using synthetic seismograms, we have studied the faulting associated with 
Reviews 
We have periodically written review articles to summarize our work at 
various stages. Molnar and Chen (1982) summarized the seismic evidence for 
differences in how mountain ranges are built in different regions, and more 
recently Molnar and Lyon-Caen (1988) considered a number of different physical 
processes in mountain belts. Chen and Molnar (1981b) briefly summarized data 
on the structure of Tibet and Molnar (1988) more recently wrote a 
comprehensive review of the deep structure of the Himalaya, Tibet and the 
Karakorum. In addition, summaries, both at a technical level (Molnar, 1984) and 
at a popular level (Molnar, 1988a) have been written on the tectonic evolution of 
the Himalaya. Finally, a paper in Scientific American (Molnar, 1986b) gives a 
simple interpretation of the results of our work and that of others. 
. 
4, 
PUBLICATIONS 
Chen, C.-Y., W.-P. Chen and P. Molnar, The uppermost mantle P wave velocities 
beneath Turkey and Iran, Geophys. Res. Lett.,  7, 77-80, 1980. 
Chen, W.-P. and P. Molnar, Constraints on the seismic wave velocity structure 
beneath the Tibetan Plateau and their tectonic implications, J.  Geophys. 
Res. ,  86, 5937-5962, 1981a. 
Chen, W.-P. and P. Molnar, Constraints on the seismic wave velocity structure 
beneath the Tibetan Plateau, in Geological and Ecological Studies of 
Qinghai-Xizang Plateau, vo l .  1, Science Press, Beijirq, 763-770, 1981b. 
Toksoz, M.N., W.R Buck and A.T. Hsui; Crustal evolution and thermal state of 
Tibet, in Geological and Ecological Studies of Qinghai-Xizang Rateau,  vo l .  
1, Science Press, Beijing. 847-857, 1981. 
Dalmayrac, E. and P. Molnar. Parallel thrust and normal faulting in Peru and 
constraints on the state of stress, Earth Planet. Sei. Lett.,  55, 473-481, 
198 1. 
Toksoz, M.N. and D.M. Davis, Plate tectonics and earthquakes, in: R e n d s  in 
P@sics 1981, Fifth General Conf. of the European Physical Society, 
Istanbul, Turkey, 7-11 September, 1981. 
Houseman, G.A., D.P. McKenzie and P. Molnar, Convective instability of a 
thickened boundary layer and its relevance for  the thermal evolution of 
continental convergent belts, J.  Geophys. Res. ,  86, 61 15-6132, 1981. 
Molnar, P. and P. Tapponnier, A possible dependence of tectonic strength on the 
age of the crust in Asia, Earth Planet. Sci. Lett.,  52, 107-114, 1981. 
Sawyer, D.S., B.A. Swift, J.G. Sclater and M.N. Toksoz, Extensional model for the 
subsidence of the Northern United States Atlantic continental margin, 
Ge~logy ,  IO, 134-140, 1982. 
Sawyer, D.S., B.A. Swift, M.N. Toksoz and J.G. Sclater, Thermal evolution of the 
Baltimore Canyon Trough and Georges Bank Basin, in J.S. Watkins (ed.), 
Stutides in Cbntinental Margin Ceology, AAPG Memoir 34, 743-764, 1982. 
Molnar, P. and W.-P. Chen, Seismicity and mountain building, in Mmntdn 
Building Processes, K.J. Hsu (ed.). 41-57, Academic Press, 1982. 
Chen, W.-P., and P. Molnar, Focal depths of inhacontinental and intraplate 
earthquakes and their implications for thermal and mechanical 
properties of the lithosphere, J.  Ceophys. Res. ,  88, 4183-4214, 1983. 
Hsui, AT., B.D. Marsh and M.N. Toksiiz, On melting of the subducted oceanic 
crust: Effects of subduction induced mantle flow, Tectonophysics, 99, 
207-220, 1983. 
Kasapoglu, ICE. and M.N. Toksoz, Tectonic consequences of the collision of the 
5. 
Arabian and Eurasian plates: finite element models, Tectonophysics, 100, 
71-95, 1983. 
Buck, W.R and M.N. Toksoz, Thermal eflects of continental collisions: Thickening 
a variable viscosity lithosphere, Tectonophysics, 100, 53-69, 1983. 
Wang, Q. and M.N. Toksoz, A Anite element model for sluggish accumulation and 
relaxation of the stress along the fault zone, Seismology and Geology, 5, 
1983 (in Chinese). 
Molnar, P. and W.-P. Chen, Focal depths and fault plane solutions of earthquakes 
under the Tibetan Plateau, J .  Geophys. Res., 88, 1180-1196, 1983. 
Lyon-Caen, H. and P. Molnar, Constraints on the structure of the Himalaya from 
analysis of gravity anomalies and a flexural model of the lithosphere, J .  
Gkophys. Res. ,  88, 817143191. 1983. 
deformation related to large earthquake development, R o c .  Internat. 
Gkol. Conf.l Moscow, 1984. 
Loo, H.-Y., Three-dimensional numerical analysis of continental marginal basin 
Molnar, P. and W.-P. Chen, S-P wave travel time residuals and lateral 
inhomogeneity in the mantle beneath Tibet and the Himalaya, J.  Geophys. 
Res., 89, 6911-6917, 1984. 
Molnar, P., Structure and tectonics of the Himalaya: Constraints and 
implications of geophysical data, Ann. Rev. Earth Rane t .  Sci. Lett.,  12, 
489-5 18, 1984. 
Baranowski, J., J. Armbruster, L. Seeber and P. Molnar, Focal depths and fault 
plane solutions of earthquakes and active tectonics of the Himalaya, J. 
Geophys. Res. ,  89, 6918-6928, 1984. 
Lyon-Caen, H. and P. Molnar, Gravity anomalies and the structure of western 
Tibet and the southern Tarim Basin, Geophys. Res. Le t t . ,  1 1 ,  1251-1254. 
1984. 
Lyon-Caen, H. and P. Molnar, Gravity anomalies, flexure of the Indian plate, and 
the structure, support and evolution of the Himalaya and Ganga Basin, 
Tectonics, 4, 513-538, 1985. 
Buck. W.R., When does small-scale convection begin beneath oceanic 
lithosphere'?, N d u r e ,  313, 775-777, 1985. Lyon-Caen, H., Deep structure 
of the Himalaya and Tibet from gravity and seismological data, Ph.D. 
thesis, M.I.T., Cambridge, 1986. 
Molnar. P., The geologic history and structure of the Himalaya, Amer. Sci., 74, 
144- 154, 1986a. 
Molnar, P., The structure of mountain ranges, Sci. Amer., 254, 70-79,' 1986b. 
Molnar, P. and H. Lyon-Caen, Some simple physical aspects of the support, 
structure, and evolution of mountain belts, in Processes in Continental ! 
6. 
Ljthospheric Deformation, Geol. Soc. Am. Spec. Pap. 218, S.P. Clark, Jr. 
d (ea.), in press, 1988. 
Molnar, P. A review of geophysical constraints on the deep structure of the 
Tibetan Plateau, the Himalaya, and the Karakorum and their tectonic 
implications, Phd. Dam. Roy. SOC. Land., Ser. A , ,  in press, 1988. 


Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan Belt

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