Wenrong Cao

Tracking paleodeformation fields in the Mesozoic central Sierra Nevada arc: Implications for intra-arc cyclic deformation and arc tempos

In this study, structures in plutons and host rocks are coupled with geochronology to track paleodeformation fields from the late Paleozoic to Late Cretaceous in the central Sierra Nevada. Regional NW-striking host-rock foliation, NE- or SW-vergent thrust faults, and associated folds developed from the early Mesozoic to Early Cretaceous. Dextral transpressional shear zones developed in the Late Cretaceous. Strikes of steep-dipping magmatic foliations in Mesozoic plutons temporally vary from approximately NW (Triassic–Jurassic) to WNW (Late Cretaceous), displaying a progressive counterclockwise rotation. Joint interpretation based on combining host-rock and magmatic structures suggests that intra-arc paleodeformation fields were dominated by coaxial and arc-perpendicular contraction from the early Mesozoic to Early Cretaceous, becoming increasingly dextral transpressive in the Late Cretaceous. The switch from contraction to transpression was likely caused by oblique convergence between the Farallon and North American plates. Based on observations in the study area and other host-rock pendants in the central Sierra Nevada, we propose that the intensity of intra-arc deformation is cyclic. To some extent, it mimics the episodic pattern of arc magmatism: Stronger deformation coincides with magmatic flare-ups. Magmatism promotes intra-arc deformation, which in turn causes crustal thickening during transfer of materials downward to the magma source regions, potentially fertilizing source regions with supracrustal materials and resulting in increased magma generation. Thus, models addressing continental arc tempos should include intra-arc processes. Evolution of continental arcs may be influenced by linked cyclic processes within the arcs accompanied by noncyclic processes driven by events external to the arcs.

Intrusion of granitic magma into the continental crust facilitated by magma pulsing and dike‐diapir interactions: Numerical simulations

We conducted a 2-D thermomechanical modeling study of intrusion of granitic magma into the continental crust to explore the roles of multiple pulsing and dike-diapir interactions in the presence of visco-elasto-plastic rheology. Multiple pulsing is simulated by replenishing source regions with new pulses of magma at a certain temporal frequency. Parameterized “pseudo-dike zones” above magma pulses are included. Simulation results show that both diking and pulsing are crucial factors facilitating the magma ascent and emplacement. Multiple pulses keep the magmatic system from freezing and facilitate the initiation of pseudo-dike zones, which in turn heat the host rock roof, lower its viscosity, and create pathways for later ascending pulses of magma. Without diking, magma cannot penetrate the highly viscous upper crust. Without multiple pulsing, a single magma body solidifies quickly and it cannot ascent over a long distance. Our results shed light on the incremental growth of magma chambers, recycling of continental crust, and evolution of a continental arc such as the Sierra Nevada arc in California.

Lithospheric foundering and underthrusting imaged beneath Tibet

Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.

Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion

ABSTRACT Elevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.

A mass balance and isostasy model: Exploring the interplay between magmatism, deformation and surface erosion in continental arcs using central Sierra Nevada as a case study

A one-dimensional mass balance and isostasy model is used to explore the feedbacks between magmatism, deformation and surface erosion and how they together affect crustal thickness, elevation, and exhumation in a continental arc. The model is applied to central Sierra Nevada in California by parameterizing magma volume and deformational strain. The simulations capture the first-order MesozoicCenozoic histories of crustal thickness, elevation and erosion including moderate Triassic crustal thickening and Jurassic crustal thinning followed by a strong Cretaceous crustal thickening, the latter resulting in a 60–70 km-thick crust plus a 20 km-thick arc eclogitic root, and a 5 km elevation in the Late Cretaceous. The contribution of contractional deformation to the crustal thickening is twice that of the magmatism. The contribution to elevation from magmatism is dampened by the formation of an eclogitic root. Erosion rate increases with the magnitude of crustal thickening (by magmatism and deformation) but its peak rate always lags behind the peak rate of thickening. We propose that thickened crust initially promotes magma generation by downward transport of materials to the magma source region, which may eventually jam the mantle wedge affecting the retro-arc underthrusting process and reducing arc magmatism.

Publisher Correction: Lithospheric foundering and underthrusting imaged beneath Tibet

The original version of the Supplementary Information associated with this Article contained an error in Supplementary Figure 4 in which the colours on the maps rendered incorrectly. The HTML has been updated to include a corrected version of the Supplementary Information

AN ISOSTATIC MASS BALANCE MODEL OF CONTINENTAL ARCS AND ITS APPLICATION TO PALEOZOIC-MESOZOIC ARGENTINEAN CORDILLERAN OROGENIC SYSTEMS

The Argentinean Cordilleran arc has been a hot spot to study arc evolution because of its well-preserved arc history. The proto-Andean arc system is on the Pacific side of the South American continent, and has at least 600 Myr of history (Franz et al., 2006). Previous studies have focus on a limited set of parameters. A newly proposed isostatic mass balance model, originally used to study the Sierra Nevada arc, is applied to the Argentinean Cordilleran arc. The one-dimensional model takes into account volumetric fraction of magma (β), tectonic shortening strain ( 3), erosion response time (τE), and mass-of-root to mass-of-melt ratio (γ). The model then reconstructs elevation and crustal thickness history of the arc.