Ten kilometer vertical Moho offset and shallow velocity contrast along the Denali fault zone from double-difference tomography, receiver functions, and fault zone head waves

Abstract

Please cite this article as: A.A. Allam, V. Schulte-Pelkum, Y. Ben-Zion, C. Tape, N. Ruppert, Z.E. Ross , Ten kilometer vertical Moho offset and shallow velocity contrast along the Denali fault zone from double-difference tomography, receiver functions, and fault zone head waves, Tectonophysics (2017Tectonophysics ( ), doi: 10.1016Tectonophysics ( /j.tecto.2017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.A C C E P T E D M A N U S C R I P T AbstractWe examine the structure of the Denali fault system in the crust and upper mantle using double-difference tomography, P-wave receiver functions, and analysis (spatial distribution and moveout) of fault zone head waves. The three methods have complementary sensitivity; tomography is sensitive to 3D seismic velocity structure but smooths sharp boundaries, receiver functions are sensitive to (quasi) horizontal interfaces, and fault zone head waves are sensitive to (quasi) vertical interfaces. The results indicate that the Mohorovičić discontinuity is vertically offset by 10 to 15km along the central 600km of the Denali fault in the imaged region, with the northern side having shallower Moho depths around 30km. An automated phase picker algorithm is used to identify ~1400 events that generate fault zone head waves only at near-fault stations.At shorter hypocentral distances head waves are observed at stations on the northern side of the fault, while longer propagation distances and deeper events produce head waves on the southern side. These results suggest a reversal of the velocity contrast polarity with depth, which we confirm by computing average 1D velocity models separately north and south of the fault. Using teleseismic events with M>=5.1, we obtain 31,400 P receiver functions and apply common- km to the north. To the east, this offset follows the Totschunda fault, which ruptured during the M7.9 2002 earthquake, rather than the Denali fault itself. The combined results suggest that the Denali fault zone separates two distinct crustal blocks, and that the Totschunda and Hines Creeks segments are important components of the fault and Cretaceous-aged suture zone structure.

Crustal Structure in Alaska From Receiver Function Analysis

“…The crustal structure in western Alaska is newly reported here. Crustal thickness decreases suddenly from~40 km under the Alaska Range to~28 km in the Tanana Basin across the Denali Fault in south central Alaska (Figures 2a and 3), agreeing with the results of other receiver function studies (Allam et al, 2017;Brennan et al, 2011;Miller et al, 2018;Veenstra et al, 2006). The gently varying crust in western Alaska significantly contrasts to the sharp and substantial changes of crustal thickness in central Alaska, which have been observed in many previous studies (Brennan et al, 2011;Fuis et al, 2008;Miller et al, 2018;Veenstra et al, 2006).…”

Lithospheric Structure Across the Alaskan Cordillera From the Joint Inversion of Surface Waves and Receiver Functions

“…The results of an active USArray Flex Array experiment (Christensen & Abers, ) in the area at the time of our study with denser station coverage might further refine this ambiguity. However, based on our current tomography results and previous studies, we favor a shallower crust‐mantle transition as seen by existing Moho picks especially since several (>10) stations from multiple studies (Allam et al, ; IRIS DMC, ; Miller et al, ) image a similar Moho depth over the mantle wedge. Assuming the existing Moho picks are accurately locating the crust‐mantle transition, their integration with our tomography results suggests that the nature of the mantle wedge is significantly different from the rest of the sub‐Moho mantle across the Alaskan Cordillera.…”

“…The presence of a thin mantle wedge under the Wrangellia Composite Terrane violates the simple assumption of a layer over a half‐space, and the inversion effectively splits the difference between the continental Moho and the top of the subducting slab. The deviation of the continental Moho picks (Allam et al, ; IRIS DMC, ; Miller et al, ) from the 4.2‐km/s velocity contour is, however, not as easily explained unless the nature of the crust‐mantle transition above the wedge is significantly different from the rest of our study area.…”

“…(a–c) Moho depth estimates from three studies (Allam et al, ; IRIS DMC, ; Miller et al, ). (d) The colored circles show the depth where our shear wave velocity model exceeds 4.2 km/s from the 1‐D joint receiver function and surface wave inversions results, whereas the map shows the depth where the final 3‐D surface wave only shear wave inversion results exceeds 4.2 km/s (see main text for difference).…”

“…Our detailed 3‐D velocity model allows us to track the lateral extent of a low‐velocity, low‐density zone interpreted by previous studies (Arkle et al, ; Mankhemthong et al, ) as underplated sediments responsible for the rapid uplift of the Chugach Mountains. Integration of our velocity model with existing Moho estimates (Allam et al, ; Haney et al, ; IRIS DMC, ; Miller et al, ; Tarayoun et al, ) allows us to directly evaluate these models while simultaneously validating anomalous and unexpected features in our velocity model. One such notably anomalous and unexpected feature in our results is a laterally extensive low‐velocity mantle wedge.…”