J. Julia

The mantle transition zone beneath West Antarctica: Seismic evidence for hydration and thermal upwellings

Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high-quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 and ∼0.48 Hz, respectively. Single-station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210 ± 15 km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225 ± 25 km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic.

Crustal structure of the eastern Borborema Province, NE Brazil, from the joint inversion of receiver functions and surface wave dispersion: Implications for plateau uplift

We investigate the crustal structure of the Borborema Province of NE Brazil by developing 44 S wave velocity-depth profiles from the joint inversion of receiver functions and fundamental mode, Rayleigh wave group velocities. The Borborema Province is located in the northeasternmost corner of the South American continent and represents a portion of a larger Neoproterozoic mobile belt that formed during the Brasiliano-Pan African orogeny. Extensional processes in the Mesozoic—eventually leading to the separation of Africa and South America—left a number of aborted rift basins in the continental interiors, and episodes of diffuse intraplate volcanism and uplift marked the evolution of the Province after continental breakup. Our velocity-depth profiles reveal the existence of two crustal types in the Province: (i) the thin crustal type, which consists of 30–32.5 km thick crust, with an upper layer of 3.4–3.6 km/s overlying a lower layer of 3.7–3.8 km/s and (ii) the thick crustal type, which consists of a 35–37.5 km thick crust, with velocities between 3.5 and 3.9 km/s down to ∼30 km depth and a gradational increase in velocity (VS≥4.0 km/s) down to upper mantle depths. The crustal types correlate well with topography, with the thick crustal type being mainly found in the high-standing southern Borborema Plateau and the thin crustal type being mostly found in the low-lying Sertaneja depression and coastal cuestas. Interestingly, the thin crustal type is also observed under the elevated topography of the northern Plateau. We argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes, as it is commonly believed. We propose that extensional processes in the Mesozoic stretched portions of the Brasiliano crust and formed the thin crustal type that is now observed in the regions of low-lying topography, leaving the rheologically strong thick crust of the southern Plateau at higher elevations. The crust making the northern Plateau would have thinned and subsided during Mesozoic extension as part of a greater Sertaneja depression, to then experience uplift in the Cenozoic and achieve its present elevation.

Upper and Middle Crustal Velocity Structure of the Colombian Andes From Ambient Noise Tomography: Investigating Subduction‐Related Magmatism in the Overriding Plate

E. P. acknowledges support from the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) through a 4 year scholarship to complete his Ph.D. J. J.thanks CNPq for his research fellowship (CNPq, grant 308171/2012-8). M. S.acknowledges support by Brazilian Science Without Border Program, grant 40.2174/2012-7.

Deviatoric Moment Tensor Solutions from Spectral Amplitudes in Surface Network Recordings: Case Study in São Caetano, Pernambuco, Brazil

Abstract We investigate the portability of a spectral amplitude method in determining moment tensor solutions for mine‐related seismicity recorded by deep mining networks to tectonic earthquakes recorded by surface aftershock networks. The original methodology inverts spectral amplitudes—with polarity attached—for direct P , SV , and SH waves recorded at deep‐seated geophones, so surface recordings must first be cleaned from free‐surface effects to recover the incident P and S waves. For precritical incidence, the correction is easily achieved by dividing the vertical component of the P and SV waveforms by the corresponding free‐surface reflection coefficients; for postcritical incidence, a more sophisticated correction that accounts for waveform distortion introduced by the coefficient’s phase shift is needed. Correction of SH components is achieved through division by a factor of 2. The proposed corrections are applied to 16 earthquakes recorded at local distances ( S ‐to‐ P critical reflection prevented the use of many SV amplitude measurements, inversion of the remaining spectral amplitudes allowed the recovery of deviatoric moment tensors for most of the events. Comparison with an independent first‐motion fault‐plane mechanism developed for the area shows consistency with our moment tensor solutions. Additionally, the ported methodology allows estimation of moment magnitudes for the selected events.