Electrically Conductive Crust in Southern Tibet from INDEPTH

Electrically Conductive Crust in Southern Tibet from INDEPTH

ence that this feature may represent an dial) around the angle of incidence of the P wave. The analysis we first extracted the group velocity disper­disper- evolving intrusive boundary at the top of a Qo component is perpendicular to the incoming P sion curve of the fundamental mode by applying a phase in the plane of incidence and contains mostly Gabor Transform to the recorded Rayleigh wave partial-melt zone. SV energy and little P-wave energy. Source equal­equal- train. The fundamental mode Rayleigh waveforms ization was accomplished by deconvolution of the Q0 were isolated from the recorded Rayleigh waves by component with the P wave of the L component. applying a phase match filter. REFERENCES AND NOTES 3. R. Kind, G. L. Kosarev, N. V. Peterson, Geophys. 8. J. Wu et al., Eos 76, F392 (1995). J. Int. 121,121,191 191 (1995). 9. L.-S. Zhao and C. Frohlich, Geophy. J. Int. 124,525124, 525 1.1.K. K. D. Nelson etef al., ScienceSCienceSc/ence 274, 16841684 (1996). 4. N. A. Haskell, J. Geophys. Res. 67,475167, 4751 (1990). (1996); L.-S. Zhao, M. K. Sen, P. Stoffa, C. Frohlich, 2. Before processing the data the ground displacement 5. Trace A36 results from averaging receiver functions ibid. 125,355125, 355 (1996). was restored at all broadbroadband band and short period sta­sta- 10. We thank I.1. Billings, M. Brunner, A. Fabritius, S. tions. Each seismogram was rotated from the stan­stan- from BB10,BBlO, BB36,BB36. and SP12 and trace A18 is an averaging of receiver functions from stations BB34, Klemperer, Y. Makovksy, K. Wu, and Chinese scien­scien- dard vertical, north and east coordinate system into a tists in helping to collect the seismic data used in this ray system [Longitudinal (L) and T componentslcomponents] us­ BB18, and BB20. ray system [Longitudinal (L) and T components] us- study. This research was funded by the National ing the eigenvalues of the covariance matrix for the 6. L. D. Brown et al., Science 274, 1688 (1996). Science Foundation Grant NSF EAR-9316814. computation of the rotation angles at a time window 7. The two-station phase velocity dispersion of the fun­fun- following the P-wave arrival. The LandL and Q0 compo­compo- damental mode Rayleigh wave was computed using nents result from a rotation of Z (vertical) and R (ra- programs developed by R. Herrmann. In the data 2 August 1996; accepted 5 November 1996 Electrically Conductive Crust in Southern Tibet For both lines, the phase difference be-be­ Electrically Conductive Crust in Southern Tibet tween the electric and magnetic fields, in from INDEPTH Magnetotelluric Surveying both polarizations of induction (9), was greater than 45°450 at all but a few sites and Leshou Chen, John R. Booker, Alan G. Jones,* Nong Wu, increased with increasing period. This result Martyn J. Unsworth, Wenbo Wei, Handong Tan indicates that conductivity increases in the crust (resistivity decreases) with increasing The crust north of the Himalaya is generally electrically conductive below depths of 10 depth (10). Apparent resistivities were low, to 20 km. This conductive zone approaches the surface beneath the Kangmar dome <<10 10Ion· nfQ . m, at frequencies less than 0.1 Hz, (dipping north) and extends beneath the Zangbo suture. A profile crossing the northern and for some sites the apparent resistivities Yadong-Gulu rift shows that the high conductivity region extends outside the rift, and its were'were below 1 m for frequencies less than Yadong-Gulu rift shows that the high conductivity region extends outside the rift, and its were below 1 nl .* m for frequencies less than on June 7, 2010 top within the rift coincides with a bright spot horizon imaged on the INDEPTH CMP 0.01 Hz. Such pervasively low values are (common midpoint) profiles. The high conductivity ofofthe the middle crust is atypical of stable atypical in the continents; shield regions continental regions and suggests that there is a regionally interconnected fluid phase in have apparent resistivities that are more the crust of the region. than two orders of magnitude higher. We modeled the MT data using 202D in­in- version algorithms that simultaneously searched for the smoothest as well as best­best- The INDEPTHIN DEPTH magnetotelluric (MT) in­in- terns:tems: a five-component commercial wide­wide- fitting models ((11) 11) (Figs. 1 and 2). The data vestigation undertaken during April to July band system (Phoenix VS) for shallow prob­ptob­ can be fit by more complex models, but vestigation undertaken during April to July band system (Phoenix V5) for shallow prob- can be fit by more complex models, but www.sciencemag.org of 1995 was designed to study the electrical ing, and 20 five-component recording sys­sys- models with less structure result in unac­unac- structure of the lithosphere beneath south­south- tems (GSC LIMS) for deeper penetration ceptable misfits. ern Tibet. Two earlier MT studies were (5, 6). During the period of LIMS acquisi­acquisi- The model for the lOO-line100-line is based on carried out in the region: A Sino-French tion, 24 March to 31 July 1995, sunspot the MT and vertical-field transfer-function group collected MT data in southern Tibet activity was low, resulting in poor signal. We data at 13 frequencies, from 80 to 0.0015 in the early 1980s (I),(1), and, subsequently, compensated for this by recording for a long­long- Hz, from all sites, and fits the data with an MT data were collected by Chinese inves­inves- er interval at each location than originally average RMS misfit of 10°100 in phase, 5% in tigators as part of the Golmud to Yadong planned (4 to 5 weeks instead of 2 to 3 apparent resistivity, and 0.10 in GDS trans­trans- Downloaded from Global Geoscience Transect activities (2). weeks), resulting in fewer total sites than fer function. These misfits are high, but The ININDEPTH DEPTH MT study provided closely planned for the experiment. most misfit is concentrated on data from spaced sites and a wide frequency band­band- Distortion analysis (7) was applied to the the southernmost four sites, which were width compared to these surveys, and sub­sub- MT response estimates, both in single-fre­single-fre- found impossible to model because their stantiated several of their conclusions.conclusions, quency, single-site and in multi-frequency, responses were inconsistent between the We acquired MT data along two lines-a multi-site modes, to determine the dimen­dimen- two modes. main north-south line that extended ftomfrom sionality of the data and derive the domi­domi- The model exhibits the following first­first- the crest of the Himalaya to near Yangbajain nant electric strike direction. The distor­distor- order features that are apparent in the data, in the Lhasa block [lOO-line,[100-line, figure 1 of (3)], tion models fit the data well, which implies appeared in all inversions for different fre­fre- and a northwest-southeast trending line that that a two-dimensional (20)(2D) description of quency and site subsets and different assump­assump- obliquely crossed the northern Yadong-Gulu regional structures is a reasonable assump­assump- tions about static shifts (12), and were not rift near Damxung (200-line) (4). The MT tion.tion, The electric strike directions at fre­fre- influenced by the abherent responses just data were recorded with the use of two sys- quencies sampling the thick Tibetan crust mentioned: (i) From approximately 150 km beneath the lOO-line100-lineWO-line are frequency-inde­frequency-inde- south of the Zanbo suture to the north end of Leshou Chen, Wenbo Wei, Handong Tan, China Univer­Univer- pendent and east-west for most sites-par­sites-par- the line, the crust is generally electrically sity of Geosciences, Beijing, China. allel to the regional surface geologic strike. conductive below depths of 10 to 20 km J. R. Booker, Nong Wu, M. J. Unsworth, School of Geo­Geo- This observation suggests that the surface «200(<200 n·fl m). (ii)(iil North of Kangmar the physics, University of Washington,Washin9ton, Seattle, WA 98195, USA. geologic strike in the region is representa­representa- crust becomes extremely conductive at A. G. Jones, Geological Survey of Canada, 1 Observatory tive of structure throughout the thickness of depth (generally much less than 30 n·5i * m; Crescent, Ottawa, Ontario, Canada K1AK1 A OY3.0Y3. the crust sensed by the MT survey, which is "3" in Fig. 1). (iii) A north-dipping zone of •* To whom correspondence should be addressed. not always the case in orogenicomgenic belts (8)(8). high conductivity extends upward to the 16941 694 SCIENCE •* VOL. 274 •* 6 DECEMBER 1996 I;JAUlniS S ..i near-surfacenear-surface beneathbeneath the Kangmar dome tivity with depthdepth ooverver mostmost ooff thethe survey,susurvrvey.vey, dodomeme suggests thatthat partially moltenmolten mmateri­materi-ateri­ ("Z"("2" in Fig.Fig. 1)1).. At depthdepth thisthis zonezone appears to and in particular the midmidcrustalcrustal resistiviiresistivitiesresistivitiesties al has been trtransportedansported relativelyrelatively upward coconnectnnect with the midcrustalmidcrustal zonelOne of high ooff about 1I to 10to fln· m nnorthorth ofof KangnKangmar,Kangmar,nar, beneabeneathth the north flank ooff thethe Kangmar coconductivitynductivity toto the north.north. (iv)(i v) The upper-upper­ ssubstantiatesubsmmiates the oobservationbservation tthathat thisthis re­re- antidinoriumanticlinorium.. This might eieitherther be asas an most ccrustrust beneath the ZangboZangbo suturesuture cocon­con-n­ giongion hashas high crustacrustalcrustal I conductivityconductivity (J(1(1, ,,1),2).2).. intrusionintrusion or transpotransportrt in the hanging wallwall ooff tainstains a nanarrownarrow eleelectricallycnically conductive body The midcrustal coconductivitynductivity is so hhighigh a thrust.thrust. Alternatively,Alternatively, sasalineline fluid trapped ("I("1"" in Fig.Fig.

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