Crustal Thickness Variation In South Central Alaska: Results From Broadband Experiment Across The Alaska Range

Abstract

The Broadband Experiment Across the Alaska Range (BEAAR) was a passive source seismic study in which 36 three-component broadband seismic stations were deployed over a period of 27 months to collect high quality data to study the Alaska Range and perhaps elucidate tectonic processes. The wavetrain of a teleseismic body wave may be interpreted in terms of reflection and transmission of waves converted at discontinuities. The recorded signal may be regarded as a convolution of the source-time function, the receiver function, and the instrument response. A receiver function is the contribution to the seismic waveform recorded at a single station due to the response of local crustal structure, and can be inverted for vertical velocity structure beneath the three-component broadband seismic station. Receiver function analyses reveal typical crust beneath the lowlands north of the Alaska Range is 26 km thick, while beneath the mountains typical crust is 35--45 km thick. Receiver function analysis of ~15,000 teleseismic waveforms recorded by BEAAR broadband seismometers provided over 100 crustal thickness data points. Similarity between crustal thicknesses determined from receiver function analysis and crustal thicknesses predicted from topography assuming Airy isostasy indicate the observed crustal root is sufficient to support the Alaska Range. North of the range, however, the crust is systematically thinner than predicted by simple Airy isostasy. A crustal density contrast of 4.6% across the Hines Creek Fault 2700 kgm-3 to the north and 2830 kgm-3 to the south, explains the observed difference between the crustal thicknesses predicted by simple Airy isostasy and the crustal thicknesses determined by receiver function analysis. Our results indicate that variations in both crustal thickness and density are required to explain the seismic and gravity data. Crustal thicknesses across the central Alaska Range suggest that these mountains are supported by a crustal root developed due to contractional thickening. Crustal thickness data reveal differences in terrane thickness: a thin Yukon-Tanana terrane north of the Hines Creek fault and thicker Kahiltna/Wrangellia terranes to the south. Finally, the pattern of thin crust to the north and thicker crust to the south appears to be modified by late Cenozoic structures such as the Denali fault, with contractional thickening in the Alaska Range, including areas north of the Hines Creek fault in the northern foothills fold and thrust belt. BEAAR crustal thickness data suggest that major faults extend to the base of the crust.