Hrafnkell Karason

4-D evolution of SE Asia's mantle from geological reconstructions and seismic tomography

Abstract How the collision between India and Asia is related to processes deeper in the mantle is unclear. Here we compare geological reconstructions of block motions within Asia since ≈50 Ma with the tomographically imaged three-dimensional (3-D) morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure. Past positions of the convergent margin show remarkable similarities with slab geometry at specific depths. The striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision. The slab contours match the progressive deformation of Asia’s margin, including India’s indentation and Sundaland’s extrusion. Ever since the onset of collision, the Indian plate appears to have overridden its own sinking mantle and it does not seem, at present, to underthrust Tibet significantly north of the Zangbo suture. If correct, this observation would provide further evidence against models of plateau build-up involving Indian lithosphere. The tomographic images beneath India confirm that Asian deformation has absorbed at least ≈1500 km of convergence since collision began. From the match between the southeastward motion of Sundaland between 40 and 20 Ma and the principal change in slab structure between 700 and 1100 km depths, we infer that lateral advection in the mantle is small and that the sinking rate beneath Sunda was ∼2 cm/yr in the lower mantle and ∼5 cm/yr above the transition zone.

Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff)

The mapping of variations in P wave speed in the deep mantle is restricted by the uneven sampling of P waves, in particular beneath the Southern Hemisphere. To enhance data coverage, we augmented the ∼1.6 million summary rays of P, pP, and pwP that we used in previous studies with differential travel times of diffracted and refracted core phases. For the core-refracted differential travel time residuals (PKPAB-PKPDF and PKPAB-PKPBC) we used 1383 cross-correlated digital waveforms as well as ∼27,000 routinely processed bulletin data. We used the waveform data to define quality criteria for the selection and reduction of the bulletin PKP data. For PKPDF-Pdiff we only considered 543 records derived from waveform cross correlation. No PcP data were used in this study. We used optical ray theory to calculate the ray paths associated with the P, pP, pwP, and PKP data, which are measured at 1 Hz. However, to account for the large Fresnel zones of the low-frequency (∼50 mHz) PKPDF-Pdiff data we estimated the three-dimensional shape of the Frechet sensitivity kernels from kernels calculated by normal mode summation. The use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high-frequency data to constrain small-scale structure. The differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers, and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary (CMB). However, images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle. Here, we do not make a priori assumptions about the mantle source of anomalous time differentials. From test inversions we conclude that (both upper and lower) mantle structures that are poorly resolved by P data can be mapped into the core along PKP paths but that the effect of outer core structures, if any, on the mantle model is small. Compared to the inversion of the P, pP, and pwP alone, the inclusion of the PKPAB-PKPDF and PKPAB-PKPBC and PKPDF-Pdiff data improves the resolution of structure beneath 2200 km depth. In particular, the joint inversion puts better constraints on the long-wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the CMB that is in agreement with but still smaller than inferences from shear wave studies. Resolution tests indicate that in some regions the enhanced definition of structure is significant, but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle.

Epeirogenic uplift above a detached slab in northern Central America

P-wave tomographic images reveal that the northern Central America highlands east of the modern volcanic arc overlie a detached slab. Hypsometric analysis of the highlands in Honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the North American–Caribbean plate margin. The dissected Central American plateau contains a network of superimposed rivers with meanders cut into bedrock; such a geomorphic character indicates that the regional uplift occurred in the absence of tilting. We propose that the epeirogenic uplift of northern Central America is the buoyant upper-plate response to the influx of mantle asthenosphere following the break-off and sinking of the slab.

Comparing P and S wave heterogeneity in the mantle

From the reprocessed data set of Engdahl and co-workers we have carefully selected matching P and S data for tomographic imaging. We assess data and model error and conclude that our S model uncertainty is twice that of the P model. We account for this in our comparison of the perturbations in P and S-wavespeed. In accord with previous studies we find that P and S perturbations are positively correlated at all depths. However, in the deep mantle systematic differences occur between regions that have undergone subduction in the last 120 million years and those that have not. In particular, below 1500 km depth ∂ln Vs/∂ln Vp is significantly larger in mantle regions away from subduction than in mantle beneath convergent margins. This inference is substantiated by wavespeed analyses with random realizations of the slab/non-slab distribution. Through much of the mantle there is no significant correlation between bulk sound and S-wave perturbations, but they appear to be negatively correlated between 1700 and 2100 km depth, which is also where the largest differences in ∂ln Vs/∂ln Vp occur. This finding supports convection models with compositional heterogeneity in the lowermost mantle.