Horizontal mantle flow controls subduction dynamics

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age –the main control on buoyancy– exhibits little correlation with most of the present-day subduction velocities and slab dips. “West”-directed subduction zones are on average steeper (~65°) than “East”-directed (~27°). Also, a “westerly”-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an “easterly”-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth

The effects of short-lived radionuclides and porosity on the early thermo-mechanical evolution of planetesimals

The thermal history and internal structure of chondritic planetesimals, assembled before the giant impact phase of chaotic growth, potentially yield important implications for the final composition and evolution of terrestrial planets. These parameters critically depend on the internal balance of heating versus cooling, which is mostly determined by the presence of short-lived radionuclides (SLRs), such as aluminum-26 and iron-60, as well as the heat conductivity of the material. The heating by SLRs depends on their initial abundances, the formation time of the planetesimal and its size. It has been argued that the cooling history is determined by the porosity of the granular material, which undergoes dramatic changes via compaction processes and tends to decrease with time. In this study we assess the influence of these parameters on the thermo-mechanical evolution of young planetesimals with both 2D and 3D simulations. Using the code family I2ELVIS/I3ELVIS we have run numerous 2D and 3D numerical finite-difference fluid dynamic models with varying planetesimal radius, formation time and initial porosity. Our results indicate that powdery materials lowered the threshold for melting and convection in planetesimals, depending on the amount of SLRs present. A subset of planetesimals retained a powdery surface layer which lowered the thermal conductivity and hindered cooling. The effect of initial porosity was small, however, compared to those of planetesimal size and formation time, which dominated the thermo-mechanical evolution and were the primary factors for the onset of melting and differentiation. We comment on the implications of this work concerning the structure and evolution of these planetesimals, as well as their behavior as possible building blocks of terrestrial planets.Comment: 19 pages, 11 figures, 5 tables; accepted for publication in Icarus; for associated video files, see http://timlichtenberg.net/2015_porosity.html or http://dx.doi.org/10.1016/j.icarus.2016.03.00

Coupling SPH and thermochemical models of planets: Methodology and example of a Mars-sized body

Giant impacts have been suggested to explain various characteristics of terrestrial planets and their moons. However, so far in most models only the immediate effects of the collisions have been considered, while the long-term interior evolution of the impacted planets was not studied. Here we present a new approach, combining 3-D shock physics collision calculations with 3-D thermochemical interior evolution models. We apply the combined methods to a demonstration example of a giant impact on a Mars-sized body, using typical collisional parameters from previous studies. While the material parameters (equation of state, rheology model) used in the impact simulations can have some effect on the long-term evolution, we find that the impact angle is the most crucial parameter for the resulting spatial distribution of the newly formed crust. The results indicate that a dichotomous crustal pattern can form after a head-on collision, while this is not the case when considering a more likely grazing collision. Our results underline that end-to-end 3-D calculations of the entire process are required to study in the future the effects of large-scale impacts on the evolution of planetary interiors.Comment: 29 pages, 10 figures, accepted for publication in Icaru

Backarc Lithospheric Thickness and Serpentine Stability Control Slab-Mantle Coupling Depths in Subduction Zones

A key feature of subduction zone geodynamics and thermal structure is the point at which the slab and mantle mechanically couple. This point defines the depth at which traction between slab and mantle begins to drive mantle wedge circulation and also corresponds with a rapid increase in temperature along the slab-mantle interface. Here, we consider the effects of the backarc thermal structure and slab thermal parameter on coupling depth using two-dimensional thermomechanical models of oceanic-continental convergent margins. Coupling depth is strongly correlated with backarc lithospheric thickness, and weakly correlated with slab thermal parameter. Slab-mantle coupling becomes significant where weak, hydrous antigorite reacts to form strong, anhydrous olivine and pyroxene along the slab-mantle interface. Highly efficient (predominantly advective) heat transfer in the asthenospheric mantle wedge and inefficient (predominantly conductive) heat transfer in the lithospheric mantle wedge results in competing feedbacks that stabilize the antigorite-out reaction at depths determined primarily by the mechanical thickness of the backarc lithosphere. For subduction zone segments where backarc lithospheric thickness can be inverted from surface heat flow, our results provide a regression model that can be applied with slab thermal parameter to predict coupling depth. Consistently high backarc heat flow in circum-Pacific subduction zones suggests uniformly thin overriding plates likely regulated by lithospheric erosion caused by hydration and melting processes under volcanic arcs. This may also explain a common depth of slab-mantle coupling globally

Impact splash chondrule formation during planetesimal recycling

Chondrules are the dominant bulk silicate constituent of chondritic meteorites and originate from highly energetic, local processes during the first million years after the birth of the Sun. So far, an astrophysically consistent chondrule formation scenario, explaining major chemical, isotopic and textural features, remains elusive. Here, we examine the prospect of forming chondrules from planetesimal collisions. We show that intensely melted bodies with interior magma oceans became rapidly chemically equilibrated and physically differentiated. Therefore, collisional interactions among such bodies would have resulted in chondrule-like but basaltic spherules, which are not observed in the meteoritic record. This inconsistency with the expected dynamical interactions hints at an incomplete understanding of the planetary growth regime during the protoplanetary disk phase. To resolve this conundrum, we examine how the observed chemical and isotopic features of chondrules constrain the dynamical environment of accreting chondrite parent bodies by interpreting the meteoritic record as an impact-generated proxy of planetesimals that underwent repeated collision and reaccretion cycles. Using a coupled evolution-collision model we demonstrate that the vast majority of collisional debris feeding the asteroid main belt must be derived from planetesimals which were partially molten at maximum. Therefore, the precursors of chondrite parent bodies either formed primarily small, from sub-canonical aluminum-26 reservoirs, or collisional destruction mechanisms were efficient enough to shatter planetesimals before they reached the magma ocean phase. Finally, we outline the window in parameter space for which chondrule formation from planetesimal collisions can be reconciled with the meteoritic record and how our results can be used to further constrain early solar system dynamics.Comment: 20 pages, 11 figures, 2 tables; accepted for publication in Icarus; associated blog article at goo.gl/5bDqG

Thermo-mechanical controls of flat subduction: insights from numerical modeling

This study was supported by National Basic Research Program of China (2014CB440901), the Strategic Priority Research Program (B) of CAS (XDB18020104), National Science Foundation of China (41190073, 41372198 and 41304071), and NERC grant NE/J021822/1.Numerical experiments are used to investigate the thermo- mechanical controls for inducing flat subduction and why flat subduction is rare relative to normal/steep subduction. Our modeling results demonstrate that flat subduction is an end-member of a steady state subduction geometry and is characterized by a curved slab with a nearly-horizontal slab section. Intermediate cases between normal/steep and flat subduction appear to be transient in origin and evolve toward one of the stable end-members. Physical parameters inducing flat subduction can be classified into four categories: buoyancy of the subducting oceanic lithosphere (e.g., slab age, oceanic crustal thickness), viscous coupling between the overriding and downgoing plates (e.g., initial subduction angle), external kinematic conditions, and rheological properties of the subduction zone. On the basis of parameter sensitivity tests and the main characteristics of present-day flat subduction zones, positive buoyancy from either the young slab or the thickened oceanic crust are considered the primary controlling parameter. Our results show that the possibility of flat subduction is directly proportional to oceanic crustal thickness and inversely proportional to the slab age. Furthermore, oceanic crust must be thicker than 8 km to induce flat subduction, when the slab is older than 30 Ma with an initial subduction angle of ≥ 20°, and without absolute trenchward motion of the overriding plate. The lower the initial subduction angle or the thicker the overriding continental lithosphere, the more likelihood for flat subduction. The initial subduction angle is more influential for the development of flat subduction than the overriding lithospheric thickness, and a thick overriding lithosphere induces flat subduction only under the condition of an initial subduction angle of ≤ 25°, with a slab age of ≥ 30 Ma and without absolute trenchward motion of the overriding plate. However, when the initial subduction angle is increased to > 25°, no flat subduction is predicted. All the parameters are evaluated within the constraints of a mechanical framework in which the slab geometry is regarded as a result of a balance between the gravitational and hydrodynamic torque. Any factor that can sufficiently reduce gravitational torque or increase hydrodynamic torque will exert a strong effect on flat subduction development. Our results are consistent with the observations of modern flat subduction zones on Earth.PostprintPeer reviewe

A water budget dichotomy of rocky protoplanets from 26^{26}Al-heating

In contrast to the water-poor inner solar system planets, stochasticity during planetary formation and order of magnitude deviations in exoplanet volatile contents suggest that rocky worlds engulfed in thick volatile ice layers are the dominant family of terrestrial analogues among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained, and it is not clear whether the solar system is a statistical outlier or can be explained by more general planetary formation processes. Here we employ numerical models of planet formation, evolution, and interior structure, to show that a planet's bulk water fraction and radius are anti-correlated with initial $^{26}$Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals prior to accretion onto larger protoplanets and yields a system-wide correlation of planet bulk abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models suggest two main scenarios of planetary systems' formation: high-$^{26}$Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of $^{26}$Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%.Comment: Preprint version; free-to-read journal version at https://rdcu.be/bmdlw; blog article at https://t.co/p6SValG1i

Subduction of young oceanic plates : a numerical study with application to aborted thermal‐chemical plumes

We investigated numerical models of initiation and subsequent evolution of subduction of young (10-30 Myr) oceanic lithosphere. Systematic numerical experiments were carried out by varying the age of the subducting plate (10, 12.5, 15, 17.5, 20, 25 and 30 Myr), the rate of induced convergence (2, 4 and 5 cm/yr) and the degree of hydration (0 and 2 wt% H(2)O) of the pre-existing weak oceanic fracture zone along which subduction is initiated. Despite the prescribed plate forcing, spontaneously retreating oceanic subduction with a pronounced magmatic arc and a backarc basin was obtained in a majority of the experiments. It was also found that the younger age of oceanic lithosphere results in more intense dehydration and partial melting of the slab during and after the induced subduction initiation due to the shallow dispositions of the isotherms. Partial melting of the subducted young crust may create thermal-chemical instabilities (cold plumes) that ascend along the slab-mantle interface until they either freeze at depth or detach from the slab and penetrate the upper plate lithosphere contributing to the nucleation and growth of a volcanic arc. Freezing of the plumes in the slab-mantle interface is favored by subduction of very young lithosphere (i.e., 10 Myr) at moderate rate (4 cm/yr) of convergence. Such aborted plumes may correspond to Cretaceous partially melted MORB-derived slab material and associated adakitic tonalitic-trondhjemitic rocks crystallized at ca. 50 km depth in the slab-mantle interface and exhumed in a subduction channel (serpentinite melanges) in eastern Cuba.We appreciated constructive comments by Erin Burkett and an anonymous reviewer and recommendations of Editor T. Becker. Blanco-Quintero would like to thank E. Sizova and Y. Mishin for provision of visualization routines, basic instructions and help during his stay in the ETH. We appreciate financial support from Spanish MCINN project CGL2009-12446 and support from Swiss National Science Foundation research grants 200020_129487, 200020_126832, 200021_113672/1, 200021_116381/1, 4D-Adamello, Swiss Federal Institute of Technology research grants ETH-06 09-2, ETH-08 07-2 and Crystal2Plate program to TVG. This is a contribution to IGCP-546 "Subduction zones of the Caribbean." Blanco-Quintero is supported by grant AP2005-5258 from the "Programa de Formacion del Profesorado Universitario" of the Spanish Ministry of Education

Arc-continent collision: The making of an orogen

Peer Reviewe

Isotopic and Petrologic Investigation, and a Thermomechanical Model of Genesis of Large-Volume Rhyolites in Arc Environments: Karymshina Volcanic Complex, Kamchatka, Russia

The Kamchatka Peninsula of eastern Russia is currently one of the most volcanically active areas on Earth where a combination of >8 cm/yr subduction convergence rate and thick continental crust generates large silicic magma chambers, reflected by abundant large calderas and caldera complexes. This study examines the largest center of silicic 4-0.5 Ma Karymshina Volcanic Complex, which includes the 25 × 15 km Karymshina caldera, the largest in Kamchatka. A series of rhyolitic tuff eruptions at 4 Ma were followed by the main eruption at 1.78 Ma and produced an estimated 800 km3 of rhyolitic ignimbrites followed by high-silica rhyolitic post-caldera extrusions. The postcaldera domes trace the 1.78 Ma right fracture and form a continuous compositional series with ignimbrites. We here present results of a geologic, petrologic, and isotopic study of the Karymshina eruptive complex, and present new Ar-Ar ages, and isotopic values of rocks for the oldest pre- 1.78 Ma caldera ignimbrites and intrusions, which include a diversity of compositions from basalts to rhyolites. Temporal trends in δ18O, 87Sr/86Sr, and 144Nd/143Nd indicate values comparable to neighboring volcanoes, increase in homogeneity, and temporal increase in mantle-derived Sr and Nd with increasing differentiation over the last 4 million years. Data are consistent with a batholithic scale magma chamber formed by primarily fractional crystallization of mantle derived composition and assimilation of Cretaceous and younger crust, driven by basaltic volcanism and mantle delaminations. All rocks have 35–45% quartz, plagioclase, biotite, and amphibole phenocrysts. Rhyolite-MELTS crystallization models favor shallow (2 kbar) differentiation conditions and varying quantities of assimilated amphibolite partial melt and hydrothermally-altered silicic rock. Thermomechanical modeling with a typical 0.001 km3/yr eruption rate of hydrous basalt into a 38 km Kamchatkan arc crust produces two magma bodies, one near the Moho and the other engulfing the entire section of upper crust. Rising basalts are trapped in the lower portion of an upper crustal magma body, which exists in a partially molten to solid state. Differentiation products of basalt periodically mix with the resident magma diluting its crustal isotopic signatures. At the end of the magmatism crust is thickened by 8 km. Thermomechanical modeling show that the most likely way to generate large spikes of rhyolitic magmatism is through delamination of cumulates and mantle lithosphere after many millions of years of crustal thickening. The paper also presents a chemical dataset for Pacific ashes from ODDP 882 and 883 and compares them to Karymshina ignimbrites and two other Pleistocene calderas studied by us in earlier works