Juan Carlos Afonso

Density structure and buoyancy of the oceanic lithosphere revisited

[1] The density structure of the lithospheric and sublithospheric oceanic mantle is assessed with an integrating methodology that incorporates mineral physics, geochemical, petrological, and geophysical data. Compressibility, partial melting, and compositional layering are considered in addition to the standard thermal modelling. The results indicate that due to differences in the degree of melt depletion and crust segregation, the depth-averaged density of old oceanic plates with thermal thicknesses of ∼ 105 ± 5 km is always lower than the density of the underlying sublithospheric mantle. Moreover, representative depth-averaged density contrasts between the plate and the adiabatic mantle, Ap, do not exceed values of ∼40 kg m -3 , in contrast to what is assumed (∇p > 70 kg m -3 ) in many geodynamic models. Thus, the role of ∇p in triggering/assisting processes such as subduction initiation may be less critical than previously thought.

Tibetan chromitites: Excavating the slab graveyard

Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (>400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages (T RD ) of 234 ± 3 Ma, while T RD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 ± 7 Ma, e Hf = 9.7 ± 4.6, and d 18 O = 4.8‰–8.2‰. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.

Arc-Continent Collision: The Making of an Orogen

There is no one model, no paradigm, that uniquely defines arc–continent collision. Natural examples and modelling of arc–continent collision show that there is a large degree of, and variation in, complexity that depend on a number of key first-order parameters and the nature of the main players; the continental margin and the arc–trench complex (the arc–trench complex includes the arc and the subduction zone). Although modelling techniques can be used to gain insights into these, they cannot and do not aim at reproducing the messiness of nature. In natural examples, identifying the nature of the main players involved, such as the age, physical properties, and pre-existing structure of the margin and the arc is just a beginning. Once this is done, parameters such as time, convergence velocity and vector need to be taken into account when determining the tectonic processes that were operative in any one arc–continent collision. In active examples, such as those in the southwest Pacific, some of these first-order parameters can be readily determined, and the nature of the main players easily assessed. Fossil arc–continent collisions, however, have commonly undergone post-collision deformation, erosion, and possibly partial dispersion to be left outcropping in the middle of a forest, with many of the key ingredients missing or hidden. This leaves the geologist to resort to comparison with other natural examples and with models that are mechanically constrained and simplified reproductions of the process to reconstruct and explain what may have been there and, importantly, what processes may have been operating and when. We attempt to show that this is not an easy task that can be put into one simple model. In this chapter we do not present a model for arc–continent collision. Instead, we begin with the main players involved, highlighting the characteristics of each that likely have a major influence on an arc–continent collision. Then, we investigate a range of possible processes that could take place once an intra-oceanic volcanic arc collides with a continental margin.

Lithospheric structure in the Baikal–central Mongolia region from integrated geophysical‐petrological inversion of surface‐wave data and topographic elevation

[1] Recent advances in computational petrological modeling provide accurate methods for computing seismic velocities and density within the lithospheric and sub-lithospheric mantle, given the bulk composition, temperature, and pressure within them. Here, we test an integrated geophysical-petrological inversion of Rayleigh- and Love-wave phase-velocity curves for fine-scale lithospheric structure. The main parameters of the grid-search inversion are the lithospheric and crustal thicknesses, mantle composition, and bulk density and seismic velocities within the crust. Conductive lithospheric geotherms are computed using P-T-dependent thermal conductivity. Radial anisotropy and seismic attenuation have a substantial effect on the results and are modeled explicitly. Surface topography provides information on the integrated density of the crust, poorly constrained by surface waves alone. Investigating parameter inter-dependencies, we show that accurate surface-wave data and topography can constrain robust lithospheric models. We apply the inversion to central Mongolia, south of the Baikal Rift Zone, a key area of deformation in Asia with debated lithosphere-asthenosphere structure and rifting mechanism, and detect an 80–90 km thick lithosphere with a dense, mafic lower crust and a relatively fertile mantle composition (Mg# < 90.2). Published measurements on crustal and mantle Miocene and Pleistocene xenoliths are consistent with both the geotherms and the crustal and lithospheric mantle composition derived from our inversion. Topography can be fully accounted for by local isostasy, with no dynamic support required. The mantle structure constrained by the inversion indicates no major thermal anomalies in the shallow sub-lithospheric mantle, consistent with passive rifting in the Baikal Rift Zone.

The Subductability of Continental Lithosphere: The Before and After Story

The temporal evolution of internal forces in a collision environment controls first-order characteristics such as convergence rate, slab dip, subduction stall, and slab breakoff, amongst others. Foremost among these forces are the positive buoyancy provided by the subduction of felsic continental material and the negative buoyancy associated with the slab. In this work we use fully dynamic thermomechanical models coupled with thermodynamic/petrological formalisms to study the evolution of these forces during a continent–arc/microcontinent collision and their influence on the large-scale dynamics of the system. Two distinctive features of our models that allow a self-consistent assessment of collision dynamics are: (1) the use of a new thermodynamic database valid up to ~25–30 GPa that includes most of the major phases relevant to continental subduction, and (2) a fully dynamic approach in which no velocities are imposed to either force or stop subduction. The former allows realistic computations of the buoyancy forces driving the system as a function of P-T-composition. The latter assures that computed velocities emerge self-consistently in our simulations in response to the balance between internal forces in our numerical domain.

The capacity of hydrous fluids to transport and fractionate incompatible elements and metals within the Earth's mantle

Both silicate melts and aqueous fluids are thought to play critical roles in the chemical differentiation of the Earths crust and mantle. Yet their relative effects are poorly constrained. We have addressed this issue by measuring partition coefficients for 50 trace and minor elements in experimentally produced aqueous fluids, coexisting basanite melts, and peridotite minerals. The experiments were conducted at 1.0–4.0 GPa and 950–1200°C in single capsules containing (either 40 or 50 wt %) H2O and trace element-enriched basanite glass. This allowed run products to be easily identified and analyzed by a combination of electron microprobe and LAM-ICP-MS. Fluid and melt compositions were reconstructed from mass balances and published solubility data for H2O in silicate melts. Relative to the basanite melt, the solutes from H2O-fluids are enriched in SiO2, alkalis, Ba, and Pb, but depleted in FeO, MgO, CaO, and REE. With increasing pressure, the mutual solubility of fluids and melts increases rapidly with complete miscibility between H2O and basanitic melts occurring between 3.0 and 4.0 GPa at 1100°C. Although LREE are favored over HREE in the fluid phase, they are less soluble than the HFSE (Nb, Ta, Zr, Hf, and Ti). Thus, the relative depletions of HFSE that are characteristic of arc magmas must be due to a residual phase that concentrates HFSE (e.g., rutile). Otherwise, H2O-fluids have the capacity to impart many of the geochemical characteristics that distinguish some rocks and melts from the deep mantle lithosphere (e.g., MARID and lamproites).

3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle: III. Thermochemical tomography in the Western-Central U.S.

We apply a novel 3-D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite-derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present-day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere-asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small-scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present-day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%. ©2016. American Geophysical Union. All Rights Reserved.

Geophysical‐petrological model of the crust and upper mantle in the India‐Eurasia collision zone

We present a new crust and upper mantle cross section of the western India-Eurasia collision zone by combining geological, geophysical, and petrological information within a self-consistent thermodynamic framework. We characterize the upper mantle structure down to 410 km depth from the thermal, compositional, and seismological viewpoints along a profile crossing western Himalayan orogen and Tibetan Plateau, Tarim Basin, Tian Shan, and Junggar Basin, ending in the Chinese Altai Range. Our results show that the Moho deepens from the Himalayan foreland basin (~40 km depth) to the Kunlun Shan (~90 km depth), and it shallows to less than 50 km beneath the Tarim Basin. Crustal thickness between the Tian Shan and Altai mountains varies from ~66 km to ~62 km. The depth of the lithosphere-asthenosphere boundary (LAB) increases from 230 km below the Himalayan foreland basin to 295 km below the Kunlun Shan. To NE the LAB shallows to ~230 km below the Tarim Basin and increases again to ~260 km below Tian Shan and Junggar region and to ~280 km below the Altai Range. Lateral variations of the seismic anomalies are compatible with variations in the lithospheric mantle composition retrieved from global petrological data. We also model a preexisting profile in the eastern India-Eurasia collision zone and discuss the along-strike variations of the lithospheric structure. We confirm the presence of a noticeable lithospheric mantle thinning below the Eastern Tibetan Plateau, with the LAB located at 140 km depth, and of mantle compositional differences between the Tibetan Plateau and the northern domains of Qilian Shan, Qaidam Basin, and North China.

The crustal structure of the Arizona Transition Zone and southern Colorado Plateau from multiobservable probabilistic inversion

The Arizona Transition Zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western US, namely the southern Colorado Plateau and the southern Basin and Range provinces. As such, the internal crustal structure and physical state of this transitional zone hold clues for understanding (i) the amalgamation of these provinces, (ii) the partitioning of deformation due to both past and present-day stress fields, and (iii) the role of thermal versus compositional effects in controlling surface observables. Here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode Rayleigh phase velocities, receiver functions, surface heat flow, geoid height, and absolute elevation to obtain an internally consistent 3-D model of the temperature, density, Vs, and Vp of the Arizona Transition Zone and the southern portions of the Colorado Plateau and Basin and Range. Our results confirm a significant crustal thickening from ∼28 km in the SW of the Arizona Transition Zone and southern Basin and Range to ∼48 km beneath the southern Colorado Plateau. Inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere-asthenosphere boundary is not deeper than ∼70 km in most of the region. We find that major pre-Cambrian surface structures and/or shear zones separate crustal domains with distinct bulk properties, suggesting that the juxtaposed crustal blocks still retain, at least in part, their original characteristics. However, widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths, locally overprinting their original compositions and creating highly heterogeneous crustal sections. A dominant and large-scale internal crustal pattern of SW dipping planes/structures is evident in our models, coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms. While we cannot categorically corroborate the presence of melt or aqueous fluids within the crust, our results are compatible with these scenarios beneath some parts of the Basin and Range, the Mogollon-Datil, and Springerville volcanic fields.

An efficient and general approach for implementing thermodynamic phase-equilibria information in geophysical and geodynamic studies

We present a flexible, general, and efficient approach for implementing thermodynamic phase equilibria information (in the form of sets of physical parameters) into geophysical and geodynamic studies. The approach is based on Tensor Rank Decomposition methods, which transform the original multidimensional discrete information into a separated representation that contains significantly fewer terms, thus drastically reducing the amount of information to be stored in memory during a numerical simulation or geophysical inversion. Accordingly, the amount and resolution of the thermodynamic information that can be used in a simulation or inversion increases substantially. In addition, the method is independent of the actual software used to obtain the primary thermodynamic information, and therefore, it can be used in conjunction with any thermodynamic modeling program and/or database. Also, the errors associated with the decomposition procedure are readily controlled by the user, depending on her/his actual needs (e.g., preliminary runs versus full resolution runs). We illustrate the benefits, generality, and applicability of our approach with several examples of practical interest for both geodynamic modeling and geophysical inversion/modeling. Our results demonstrate that the proposed method is a competitive and attractive candidate for implementing thermodynamic constraints into a broad range of geophysical and geodynamic studies. MATLAB implementations of the method and examples are provided as supporting information and can be downloaded from the journals website.