Mapping seismic anisotropy using harmonic decomposition of receiver functions: An application to Northern Apennines, Italy

Abstract

[1] Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zone are imaged using a new method for the analysis of teleseismic receiver functions (RFs). More than 13,000 P-wave coda of teleseismic records from the [2003][2004][2005][2006][2007] Retreating-Trench, Extension, and Accretion Tectonics (RETREAT) experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of Mediterranean evolution. A new methodology for the analysis of receiver functions is developed, which combines both migration and harmonic decomposition of the receiver function data set. The migration technique follows a classical "Common Conversion Point" scheme and helps to focus on a crucial depth range (20-70 km) where the mantle wedge develops. Harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of P-to-S converted phases. The separation of the back-azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures. The application of our method to the RETREAT data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the Northern Apennines chain, where previous studies failed to clearly retrieve the geometry of the subducted interfaces. We present our results in closely spaced profiles across and along the Northern Apennines chain to highlight the convergence of the Tyrrhenian and the Adriatic microplates which differ in their crustal structure where the Adriatic microplate subducts beneath Tuscany and the Tyrrhenian sea. A signature of the dipping Adriatic Moho is clearly observed beneath the Tyrrhenian Moho in a large portion of the forearc region. In the area where the two Mohos overlap, our new analysis reveals the presence of an anisotropic body above the subducted Moho. There is a strong Ps converted phase with anisotropic characteristics from the top of the Adriatic plate to a depth of at least 80 km. Because the Ps conversion occurs much deeper than similar Ps phases in Cascadia and Japan, dehydration of oceanic crust seems unlikely as a causative factor. Rather, the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting.

A New Seismic Data Set on the Depth of the Moho in the Alps

“…This difference compared to the reference study and scatter between adjacent stations could be related to the presence of anisotropy or furthermore a laminated lowermost crust. Examples of anisotropy in the lower crust have been published in recent years (e.g., BISCHOFF et al 2006;ENDRUN et al 2011;ROUX et al 2011); some studies state that anisotropy affects RF (e.g., ECKHARDT and RABBEL 2011;PORTER et al 2011;BIANCHI et al 2010), and forward modeling of RF cannot match the data if anisotropy is not considered (e.g., BIANCHI et al 2008;NAGAYA et al 2008;LIU and NIU 2012). A number of geophysical processes affect the possibility of retrieving the Moho depth beneath an orogen.…”

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…Our H-k stacking and the simple direct models we performed cannot unequivocally distinguish between those factors, given the complexity and numbers of interfaces involved in the observed RF signal. A thoroughly analyse of the signal through more sophisticated methods taking into account phase back-azimuthal variations or polarity change on the transverse component might resolve these ambiguities (Shiomi & Park 2008;Bianchi et al 2010;Park & Levin 2016). Moreover, when present, the transverse component does not systematically show a common behaviour over the whole network, hence being indicative or either different axis of anisotropy, or variation in the dip of the interfaces.…”

Theory and Observations - Seismic Anisotropy

“…The layer above the Apennines slab is evident at 40-80 km depths, whereas Ps conversions in Cascadia and Japan are typically strongest at 40 km depth or shallower. Bianchi et al (2010) interpreted this as evidence of a process peculiar to continental dynamics, in which the lowermost crust peels downward with the subducting mantle lithosphere but eventually rises to form an ultrahigh-pressure (UHP) terrane in a mature eroded orogen, as seen, for example, in the European Alps ( Jolivet et al, 2003).…”

“…The Apennines slab is often characterized as continental subduction, but Bianchi et al (2010) found evidence from P receiver functions for a 10 km anisotropic layer above the downgoing slab beneath central Italy, superficially similar to oceanic subduction zones. The layer above the Apennines slab is evident at 40-80 km depths, whereas Ps conversions in Cascadia and Japan are typically strongest at 40 km depth or shallower.…”