Kelly H. Liu

Southern African crustal evolution and composition: Constraints from receiver function studies

[1] Stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern Africa reveals systematic spatial variations in the ratio of crustal P and S wave velocities (Φ), crustal thickness (H), and the amplitude of the converted Moho phases (R). The eastern Zimbabwe and the southern Kaapvaal cratons are characterized by small H (∼38 km), small Φ (∼1.73), and large R (∼0.15) values, suggesting that the relatively undisturbed Archean crust beneath southern Africa is separated from the mantle by a sharp Moho and is felsic in composition. The Limpopo belt, which was created by a collisional event at 2.7 Ga, displays large H (∼43 km) but similar Φ and R values relative to the cratonic areas. The Bushveld Mafic Intrusion Complex and its surrounding areas show large Φ (∼1.78), large H (∼43 km), and small R (∼0.11) values, reflecting the intrusion of mafic material into the original crust as a result of the Bushveld event at 2.05 Ga. Excluding the Bushveld, the spatially consistent and age-independent low Φ accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material. Within such an island arc model, our data, combined with xenolith data excluding mantle delamination in cratonic environments, suggest that the modification to a felsic composition (e.g., by the partial melting of basalt and removal of residue by delamination) is restricted to have occurred during the collision between the arcs and the continent.

Mantle deformation beneath southern Africa

Seismic anisotropy from the southern African mantle has been inferred from shear-wave splitting mea- sured at 79 sites of the Southern African Seismic Experi- ment. Thesedataprovidethemostdramaticsupporttodate thatArcheanmantledeformation ispreservedasfossil man- tle anisotropy. Fast polarization directions systematically follow the trend of Archean structures and splitting delay times exhibitgeologic control. Themost anisotropic regions are Late-Archean in age (Zimbabwe craton, Limpopo belt, western Kaapvaal craton), with delay times reduced dra- matically in o-craton regions to the southwest and Early- Archean regions to the southeast. While thin lithosphere can account for weak o-craton splitting, small or vertically incoherent anisotropy is a more likely explanation for the Early-Archean region. We speculate that this dierence in on-craton anisotropic structure is the result of two dierent continent-forming processes operating.

Shear wave splitting and mantle flow associated with the deflected Pacific slab beneath northeast Asia

[1] A total of 361 SKS and five local S wave splitting measurements obtained at global and regional seismic network stations in NE China and Mongolia are used to infer the characteristics of mantle fabrics beneath northeast Asia. Fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features. The dominant fast directions at the eastern part, beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone (MTZ), are about 100 from north, which are almost exactly the same as the motion direction of the Eurasian plate relative to the Pacific plate, and are independent of the direction of local geological features. The splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3% anisotropy. The shear wave splitting observations, complemented by the well-established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the Paleozoic era, can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected Pacific slab in the MTZ, or by back-arc asthenospheric flow in the mantle wedge above the slab.

Seismic anisotropy beneath the Afar Depression and adjacent areas: Implications for mantle flow

[1] Shear wave splitting is a robust tool to infer the direction and strength of seismic anisotropy in the lithosphere and underlying asthenosphere. Previous shear wave splitting studies in the Afar Depression and adjacent areas concluded that either Precambrian sutures or vertical magmatic dikes are mostly responsible for the observed anisotropy. Here we report results of a systematic analysis of teleseismic shear wave splitting using all the available broadband seismic data recorded in the Afar Depression, Main Ethiopian Rift (MER), and Ethiopian Plateau. We found that while the ∼450 measurements on the Ethiopian Plateau and in the MER show insignificant azimuthal variations with MER‐parallel fast directions and thus can be explained by a single layer of anisotropy, the ∼150 measurements in the Afar Depression reveal a systematic azimuthal dependence of splitting parameters with a p/2 periodicity, suggesting a two‐layer model of anisotropy. The top layer is characterized by a relatively small (0.65 s) splitting delay time and a WNW fast direction that can be attributed to magmatic dikes within the lithosphere, and the lower layer has a larger (2.0 s) delay time and a NE fast direction. Using the spatial coherency of the splitting parameters obtained in the MER and on the Ethiopian Plateau, we estimated that the optimal depth of the source of anisotropy is centered at about 300 km, i.e., in the asthenosphere. The spatial and azimuthal variations of the observed anisotropy can best be explained by a NE directed flow in the asthenosphere beneath the MER and the Afar Depression.

Making Reliable Shear-Wave Splitting Measurements

Shear-wave splitting (SWS) analysis using SKS, SKKS, and PKS (here- after collectively called XKS) phases is one of the most commonly used techniques in structural seismology. In spite of the apparent simplicity in performing SWS measure- ments, large discrepancies in published SWS parameters (fast direction and splitting time) suggest that a significant portion of splitting parameters has been incorrectly determined. Here, based on the popularly used minimization of transverse energy technique, we present a procedure that combines automatic data processing and care- ful manual screening, which includes adjusting the XKS window used for splitting analysis, modifying band-pass filtering corner frequencies, and verifying and (if nec- essary) changing the quality ranking of the measurements. Using real and synthetic data, we discuss causes and diagnostics of a number of common problems in perform- ing SWS analysis, and suggest possible remedies. Those problems include noise in the XKS window being mistaken as signal, non-XKS seismic arrivals in the XKS window, excessive use of null ranking, measurements from misoriented sensors and from sen- sors with mechanical problems, and inappropriate dismissal of usable measurements.

Significant crustal thinning beneath the Baikal rift zone: New constraints from receiver function analysis

[1] Thinning of the crust of more than 10 km is a major feature of typical continental rifts such as the East African (EAR) and Rio Grande (RGR) rifts. However, numerous previous studies across the Baikal rift zone (BRZ), which has similar surface expressions and tectonic history, and more active seismicity relative to EAR and RGR, have resulted in contradicting amount of thinning, ranging from almost none to more than 10 km. We measure crustal thickness by stacking teleseismic receiver functions beneath 51 sites on the southern and central parts of the BRZ and adjacent Siberian Platform and Sayan-Baikal-Mongolian Foldbelt. Our measurements reveal that beneath the southern part of the Platform, the average crustal thickness is about 38 km, which is about 7 km thinner than that beneath the Foldbelt and the un-rifted part of the BRZ. The thinnest crust, 35 km, is found beneath the central part of the rift, and represents a significant thinning of about 10 km relative to the un-rifted parts of the BRZ. INDEX TERMS: 7200 Seismology; 7203 Seismology: Body wave propagation; 7205 Seismology: Continental crust (1242). Citation: Gao, S. S., K. H. Liu, and C. Chen (2004), Significant crustal thinning beneath the Baikal rift zone: New constraints from receiver function analysis, Geophys. Res. Lett., 31, L20610, doi:10.1029/2004GL020813.

Spatial variations of crustal characteristics beneath the Hoggar swell, Algeria, revealed by systematic analyses of receiver functions from a single seismic station

The Hoggar swell in Algeria is one of the significant massifs of northwest Africa. The paucity of high-resolution geophysical studies of the crust and mantle beneath the massifs is mostly responsible for the heated debates about the depth of the source region of the Cenozoic volcanism and the closely related uncertainty about the mechanism that formed and maintains the high elevation of the swells. Here we report results from a systematic study of 1386 high-quality receiver functions (RFs) recorded by station TAM, the only permanent broadband seismic station on the Hoggar swell. The resulting crustal thickness is about 34 km and the Vp/Vs is 1.77 when all the RFs from the station are stacked. Our study reveals a sharp contrast in the amplitude of the P-to-S converted phases between the volcanic, highly-fractured Tefedest terrane and the non-volcanic, less fractured Laouni terrane. The former has a stacking amplitude that is comparable to typical cratonic areas, and the latter has an amplitude that is only about 25% as large. Spatially consistent crustal thickness and an intermediate-mafic crust are inferred on the Tefedest terrane, while spatially variable crustal thickness and a felsic crust is inferred beneath the Laouni terrane. The observations can be best explained by a mantle-derived underplated magmatic layer beneath the mechanically-stronger Laouni terrane, and magmatic diking and resultant volcanism associated with the mechanically weaker Tefedest crust. The study demonstrated the significance of a long-running station in the investigation of spatial variations of crustal characteristics.

Crustal Structure and Evolution Beneath the Colorado Plateau and the Southern Basin and Range Province: Results from Receiver Function and Gravity Studies

Over the past several decades, contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long-term stability of the Colorado Plateau (CP) and crustal thinning beneath the Basin and Range Province (BRP) in the southwestern United States. Here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations on the southwestern CP and the southern BRP. Our results reveal that the BRP is characterized by a thin crust (28.2 ± 0.5 km), a mean Vp/Vs of 1.761 ± 0.014 and a mean amplitude (R) of P-to-S converted wave (relative to that of the direct P wave) of 0.181 ± 0.014 that are similar to a typical continental crust, consistent with the model that the thin crust was the consequence of lithospheric stretching during the Cenozoic. The CP is characterized by the thickest crust (42.3 ± 0.8 km), largest Vp/Vs (1.825 ± 0.009) and smallest R (0.105 ± 0.007) values in the study area. In addition, many stations on the CP exhibit a clear arrival before the P-to-S converted phase from the Moho, corresponding to a lower crustal layer of about 12 km thick with a mafic composition. We hypothesize that the lower crustal layer, which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines, contributed to the long-term stability and preuplift low elevation of the Colorado Plateau.

Mantle transition zone discontinuities beneath the contiguous United States

Using over 310,000 high-quality radial receiver functions recorded by the USArray and other seismic stations in the contiguous United States, the depths of the 410 km and 660 km discontinuities (d410 and d660) are mapped in over 1,000 consecutive overlapping circles with a radius of 1°. The average mantle transition zone (MTZ) thickness for both the western and central/eastern U.S. is within 3 km from the global average of 250 km, suggesting an overall normal MTZ temperature beneath both areas. The Pacific Coast Ranges and the southern Basin and Range Province are underlain by a depressed d410, indicating higher-than-normal temperature in the upper MTZ. The proposed Yellowstone and Raton hot spots are not associated with clear undulations of the MTZ discontinuities, but d410 beneath another proposed hot spot, Bermuda, is depressed significantly and d660 has a normal depth. Low-temperature regions are found in the upper MTZ associated with the subducted Juan de Fuca slab beneath the northern Rocky Mountains and in two circular areas beneath the northern Basin and Range Province and the southern Colorado Plateau. Part of the Great Plains is characterized by a depressed d660. This observation, when combined with results from seismic tomography, suggests the existence of a cold region in the lower MTZ, probably associated with subducted Farallon slab segments.

A uniform database of teleseismic shear wave splitting measurements for the western and central United States

We present a shear wave splitting (SWS) database for the western and central United States as part of a lasting effort to build a uniform SWS database for the entire North America. The SWS measurements were obtained by minimizing the energy on the transverse component of the PKS, SKKS, and SKS phases. Each of the individual measurements was visually checked to ensure quality. This version of the database contains 16,105 pairs of splitting parameters. The data used to generate the parameters were recorded by 1774 digital broadband seismic stations over the period of 1989–2012, and represented all the available data from both permanent and portable seismic networks archived at the Incorporated Research Institutions for Seismology Data Management Center in the area of 26.00°N to 50.00°N and 125.00°W to 90.00°W. About 10,000 pairs of the measurements were from the 1092 USArray Transportable Array stations. The results show that approximately 2/3 of the fast orientations are within 30° from the absolute plate motion (APM) direction of the North American plate, and most of the largest departures with the APM are located along the eastern boundary of the western US orogenic zone and in the central Great Basins. The splitting times observed in the western US are larger than, and those in the central US are comparable with the global average of 1.0 s. The uniform database has an unprecedented spatial coverage and can be used for various investigations of the structure and dynamics of the Earth.