Experimental studies on seismic anisotropy enhanced by alignment of melt: Implication for the origin of abnormal anisotropy in Tibetan crust

Abstract

It is commonly agreed that seismic anisotropy, most likely caused by lattice preferred orientation (LPO) of major minerals, is a very important indicator of intracrustal deformation. Ultrasonic velocity measurements on the rocks from higher Himalayan crystallines (HHC) and Honghe (红河) strike-slip fault zone in Southwest China showed an average anisotropic magnitude of about 5%. However, a series of seismic measurements conducted in Tibet indicated marked anisotropy with a magnitude ranging from 8% to 18% within middle to lower crust. What causes the anomalously strong anisotropy within Tibetan crust? Parts of HHC rocks, to some extent, had undergone granulitic-grade metamorphism, the temperature and pressure of which were in excess of their solidus. Additionally, oriented leucocratic portions, which are accepted to be products crystallized from localized melt bands and aligned melt pocket (AMP), are present in HHC pervasively. If melt is oriented, it is expected to be an extremely important factor to influence anisotropy behavior. Experiments performed on analogue materials composed of plexiglass matrix and chocolate demonstrated that aligned melt could result in an extra anisotropy whose magnitude might increase two to three times. The contribution of AMP on anisotropy is likely comparable to or larger than that induced by LPO of major minerals, possibly amphiboles and micas, in middle to lower crust. It is implied that aligned melt may be a potential factor to induce anomalously strong anisotropy within Tibetan middle to lower crust.

Keywords

partial melting / aligned melt pocket / anisotropy / experimental measurement / Tibetan crust

References

Babuska V.. Anisotropy of V _p and V _s in Rock-Forming Minerals. J. Geophys., 1981, 50: 1-6.

Cheng C. H.. Crack Models for a Transversely Isotropic Medium. J. Geophys. Res., 1993, 98(B1): 675-684.

Dell’Angelo L. N., Tullis J., Yund R. A.. Transition from Dislocation Creep to Melt-Enhanced Diffusion Creep in Fine-Grained Granitic Aggregates. Tectonophysics, 1987, 139(3–4): 325-332.

Eshelby J. D.. The Determination of the Elastic Field of an Ellipsoidal Inclusion, and Related Problems. Proc. R. Soc., London, Ser. A, 1957, 241: 376-396.

Katz R. F., Spiegelman M., Holtzman B.. The Dynamics of Melt and Shear Localization in Partially Molten Aggregates. Nature, 2006, 442(7103): 676-679.

Kern H., Ivankina T. I., Nikitin A. N., . The Effect of Oriented Microcracks and Crystallographic and Shape Preferred Orientation on Bulk Elastic Anisotropy of a Foliated Biotite Gneiss from Outokumpu. Tectonophysics, 2008, 457(3–4): 143-149.

Kind R., Ni J. F., Zhao W. J., . Evidence from Earthquake Data or a Partially Molten Crustal Layer in Southern Tibet. Science, 1996, 274(5293): 1692-1694.

Kohlstedt D. L., Zimmerman M. E.. Rheology of Partially Molten Mantle Rocks. Annu. Rev. Earth Planet. Sci., 1996, 24: 41-62.

Liao Z. J., Zhao P.. Yunnan-Tibet Geothermal Belt-Geothermal Resources and Case Histories, 1999, Beijing: Science Press

Lloyd G. E., Butler R. W. H., Casey M., . Mica, Deformation Fabrics and the Seismic Properties of the Continental Crust. Earth and Planetary Science Letters, 2009, 288(1–2): 320-328.

Makovsky Y., Klemperer S. L., Ratschbacher L., . INDEPTH Wide-Angle Reflection Observation of PWave-to-S-Wave Conversion from Crustal Bright Spots in Tibet. Science, 1996, 274(5293): 1690-1691.

Maluski H., Matte P., Brunel M., . Argon 39-Argon 40 Dating of Metamorphic and Plutonic Events in the North and High Himalaya Belts (Southern Tibet, China). Tectonics, 1988, 7(2): 299-326.

McKenna L. W., Walker J. D.. Geochemistry of Crustally Derived Leucocratic Igneous Rocks from the Ulugh Muztagh Area, Northern Tibet and Their Implications for the Formation of the Tibetan Plateau. J. Geophys. Res., 1990, 95(B13): 21483-21502.

Mecklenburgh J., Rutter E. H.. On the Rheology of Partially Molten Synthetic Granite. J. Structural Geol., 2003, 25(10): 1575-1585.

Meissner R., Rabbel W., Kern H.. Seismic Lamination and Anisotropy of the Lower Continental Crust. Tectonophysics, 2006, 416(1–4): 81-99.

Nelson K. D., Zhao W. J., Brown L. D., . Partially Molten Middle Crust beneath Southern Tibet: Synthesis of Project INDEPTH Results. Science, 1996, 274(5293): 1684-1688.

Neogi S., Dasgupta S., Fukuoka M.. High P-T Polymetamorphism, Dehydration Melting, and Generation of Migmatites and Granites in the Higher Himalayan Crystalline Complex, Sikkim, India. J. Petrol., 1998, 39(1): 61-99.

Owens T. J., Zandt G.. Implications of Crustal Property Variations for Models of Tibetan Plateau Evolution. Nature, 1997, 387(6628): 37-43.

Ozacar A. A., Zandt G.. Crustal Seismic Anisotropy in Central Tibet: Implications for Deformational Style and Flow in the Crust. Geophys. Res. Lett., 2004, 31 23 L23601

Rodgers A. J., Schwarts S. Y.. Low Crustal Velocities and Mantle Lithospheric Variations in Southern Tibet from Regional Pnl Waveforms. Geophys. Res. Lett., 1997, 24(1): 9-12.

Rutter E. H., Brodie K. H., Irving D. H.. Flow of Synthetic, Wet, Partially Molten “Granite” under Undrained Conditions: An Experimental Study. J. Geophys. Res., 2006, 111 B6 B06407

Rutter E. H., Neumann D. H. K.. Experimental Deformation of Partially Molten Westerly Granite under Fluid-Absent Conditions with Implications for the Extraction of Granitic Magmas. J. Geophys. Res., 1995, 100(B8): 15697-15715.

Sapin M., Hirn A.. Seismic Structure and Evidence for Eclogitization during the Himalayan Convergence. Tectonophysics, 1997, 273(1–2): 1-16.

Schaerer U., Xu R. H., Allegre C. J.. U-(Th)-Pb Systematics and Ages of Himalayan Leucogranites, South Tibet. Earth Planet. Sci. Lett., 1986, 77(1): 35-48.

Shapiro N. M., Ritzwoller M. H., Molnar P., . Thinning and Flow of Tibetan Crust Constrained by Seismic Anisotropy. Science, 2004, 305(5681): 233-236.

Shen X. J., Zhong W. R., Guan Y., . Heat Flow Profile from Yadong to Qaidam Running through the Tibetan Plateau. Chinese Science Bulletin, 1990, 35(4): 314-316.

Sherrington H. F., Zandt G., Frederiksen A.. Crustal Fabric in the Tibetan Plateau Based on Waveform Inversions for Seismic Anisotropy Parameters. J. Geophys. Res., 2004, 109 B2 B02312

Siegesmund S., Takeshita T., Kern H.. Anisotropy of V _p and V _s in an Amphibolite of the Deeper Crust and Its Relationship to the Mineralogical, Microstructural and Textural Characteristics of the Rock. Tectonophysics, 1989, 157(1–3): 25-38.

Teng J. W.. Physical and Dynamics of Kangding Lithosphere, 1994, Science Press: Beijing

Wang J. Y., Huang S. P.. Compilation of Heat Flow Data in the China Continental Area. Seismology and Geology, 1990, 12(4): 351-363.

Wang Q., McDermott F., Xu J. F., . Cenozoic K-Rich Adakitic Volcanic Rocks in the Hohxil Area, Northern Tibet: Lower-Crustal Melting in an Intracontinental Setting. Geology, 2005, 33: 465-468.

Weiss T., Siegesmund S., Rabbel W., . Seismic Velocities and Anisotropy of the Lower Continental Crust: A Review. Pure and Applied Geophysics, 1999, 156(12): 97-122.

Yang X. S., Jin Z. M.. Studies on Rb-Sr and Sm-Nd Isotope of Yadong Leucogranite in Tibet: Constraint on Its Age and Source Material. Geological Review, 2001, 47(3): 294-300.

Yang X. S., Jin Z. M., Ma J.. Anatexis in Himalayan Crust: Evidence from Geochemical and Chronological Investigations of Higher Himalayan Crystallines. Science in China (Ser. D), 2005, 48(9): 1347-1356.

Yang X. S., Ma J., Jin Z. M., . Partial Melting and Its Implications for Understanding the Seismic Velocity Structure within the Southern Tibetan Crust. Acta Geol. Sinica, 2003, 77(1): 64-71.

Yang X. S., Zhou P., Ming Y. H.. Vp of Muscovite-Biotite Gneiss up to 950 °C at 400 MPa: Constraints on the Origin of Abnormal Seismic Layers in Continental Crust. Chinese Science Bulletin, 2007, 52(18): 2175-2179.

Yang Y., Chen J. Y., Yang X. S., . Does Alignment of Melt Enhance Seismic Anisotropy beneath Tibet?. Seismology and Geology, 2010, 32(1): 59-69.

Yin Z. X., Teng J. W., Liu H. B.. The 2-D Crustal Structure Study in the Yadong-Damxung Region of the Xizang Plateau. Bulletin of the Chinese Academy of Geological Sciences, 1990, 21: 239-245.

Yuan X. H., Ni J. F., Kind R., . Lithospheric and Upper Mantle Structure of Southern Tibet from a Seismological Passive Source Experiment. J. Geophys. Res., 1997, 102(B12): 27491-27500.

Zeitler P. K., Chamberlain C. P.. Petrogenetic and Tectonic Significance of Young Leucogranites from Northwestern Himalaya, Pakistan. Tectonics, 1991, 10(4): 729-741.

Zhang G. L., Yang X. S., Chen J. Y., . The Influencing Factor of Elastic Anisotropy in Middle to Lower Continental Crust. Seismology and Geology, 2010, 32(2): 327-337.

Zhao L. S., Sen M. K., Stoffa P., . Application of very Fast Simulated Annealing to the Determination of the Crustal Structure beneath Tibet. Geophys. J. Int., 1996, 125(2): 355-370.