Meghan S. Miller

Present-Day Motion of the Sierra Nevada Block and some Tectonic Implications for the Basin and Range Province, North American Cordillera

Global Positioning System (GPS) data from five sites on the stable interior of the Sierra Nevada block are inverted to describe its angular velocity relative to stable North America. The velocity data for the five sites fit the rigid block model with rms misfits of 0.3 mm/yr (north) and 0.8 mm/yr (east), smaller than independently estimated data uncertainty, indicating that the rigid block model is appropriate. The new Euler vector, 17.0°N, 137.3°W, rotation rate 0.28 degrees per million years, predicts that the block is translating to the northwest, nearly parallel to the plate motion direction, at 13–14 mm/yr, faster than previous estimates. Using the predicted Sierra Nevada block velocity as a kinematic boundary condition and GPS, VLBI and other data from the interior and margins of the Basin and Range, we estimate the velocities of some major boundary zone faults. For a transect approximately perpendicular to plate motion through northern Owens Valley, the eastern California shear zone (western boundary of the Basin and Range province) accommodates 11±1 mm/yr of right-lateral shear primarily on two faults, the Owens Valley-White Mountain (3±2 mm/yr) and Fish Lake Valley (8±2 mm/yr) fault zones, based on a viscoelastic coupling model that accounts for the effects of the 1872 Owens Valley earthquake and the rheology of the lower crust. Together these two faults, separated by less than 50 km on this transect, define a region of high surface velocity gradient on the eastern boundary of the Sierra Nevada block. The Wasatch Fault zone accommodates less than 3±1 mm/yr of east-west extension on the eastern boundary of the Basin and Range province. Remaining deformation within the Basin and Range interior is also probably less than 3 mm/yr.

Continuing Colorado plateau uplift by delamination-style convective lithospheric downwelling

The Colorado plateau is a large, tectonically intact, physiographic province in the southwestern North American Cordillera that stands at ∼1,800–2,000 m elevation and has long been thought to be in isostatic equilibrium. The origin of these high elevations is unclear because unlike the surrounding provinces, which have undergone significant Cretaceous–Palaeogene compressional deformation followed by Neogene extensional deformation, the Colorado plateau is largely internally undeformed. Here we combine new seismic tomography and receiver function images to resolve a vertical high-seismic-velocity anomaly beneath the west-central plateau that extends more than 200 km in depth. The upper surface of this anomaly is seismically defined by a dipping interface extending from the lower crust to depths of 70–90 km. The base of the continental crust above the anomaly has a similar shape, with an elevated Moho. We interpret these seismic structures as a continuing regional, delamination-style foundering of lower crust and continental lithosphere. This implies that Pliocene (2.6–5.3 Myr ago) uplift of the plateau and the magmatism on its margins are intimately tied to continuing deep lithospheric processes. Petrologic and geochemical observations indicate that late Cretaceous–Palaeogene (∼90–40 Myr ago) low-angle subduction hydrated and probably weakened much of the Proterozoic tectospheric mantle beneath the Colorado plateau. We suggest that mid-Cenozoic (∼35–25 Myr ago) to Recent magmatic infiltration subsequently imparted negative compositional buoyancy to the base and sides of the Colorado plateau upper mantle, triggering downwelling. The patterns of magmatic activity suggest that previous such events have progressively removed the Colorado plateau lithosphere inward from its margins, and have driven uplift. Using Grand Canyon incision rates and Pliocene basaltic volcanism patterns, we suggest that this particular event has been active over the past ∼6 Myr.

Dynamics of continental accretion

Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth’s history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions

[1] Delineating mantle interfaces can provide important clues for understanding the formation of continents. We use S-wave receiver functions to investigate lithospheric structure along a transect extending from Vancouver Island to Baffin Island. Observed Sp converted waves allow for interpretation of boundaries in the depth range expected for tectonic plates. Receiver functions show a distinct negative amplitude feature, interpreted as the lithosphere-astbenosphere boundary, at shallow depths beneath British Columbia (∼85km), deepening abruptly at the eastern edge of the Cordillera to ∼260km beneath the Canadian Shield. Dipping mid-lithospheric discontinuities extend beneath several giant ca. 1.8 Ga epicontinental magmatic arcs, consistent with formation of cratonic lithosphere by arc accretion. This model provides a plausible explanation for global mid-lithospheric discontinuities within cratons and aids in understanding their formation.

Three‐dimensional visualization of a near‐vertical slab tear beneath the southern Mariana arc

The use of a three-dimensional ray-tracing inversion algorithm has greatly enhanced the resolution of gradients and strong variations in wave speeds to create improved P wave tomographic images of the Mariana arc region. The images obtained from the Mariana arc region show relatively low amplitudes of heterogeneity due to the limited number of seismic stations in the area. Despite these limitations, detailed interpretations of the three-dimensional geometry and morphology of the Pacific Plate subducting beneath the Philippine Sea Plate have provided a three-dimensional model of the steep dip of the Pacific plate and the curvature of the slab beneath the Mariana arc in unprecedented detail. The new P wave tomography and seismicity depict a previously unidentified E-W trending near-vertical tear in the subducting plate at the southern end of the Mariana arc that divides the arc into two distinct segments: a steeply dipping curved slab penetrating the lower mantle and a short (∼250 km depth) slab along the Challenger Deep segment of the arc. The slab tear is likely to be the result of the need to accommodate the reduced volume the slab must occupy as it is subducted beneath the Philippine Sea plate along a curved arc.

Upper mantle structure beneath the Caribbean‐South American plate boundary from surface wave tomography

[1] We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean-South American boundary region by analysis of fundamental mode Rayleigh waves in the 20- to 100-s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike-slip fault system along the northern coast of Venezuela, illustrating the differences between the South American continental lithosphere, the Venezuelan archipelago, and the Caribbean oceanic lithosphere. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block, there is evidence of underthrusting of the Caribbean plate, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. In eastern Venezuela, linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the Guayana shield lithosphere. The subducting oceanic part of the South American plate is imaged beneath the Antilles arc. The surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental South American plate offshore of northeastern Venezuela. The continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere. We interpret a nearly vertical low-velocity ‘‘column’’ west of the tear centered beneath the Cariaco Basin, with three-dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere, with the asthenosphere escaping from beneath continental South America and rising into the plate boundary zone. The complex plate boundary structure is best examined in three dimensions. We discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity, teleseismic shear wave splits, and interpretations from active source profiling.

Seismic imaging of the Cocos plate subduction zone system in central Mexico

Broadband data from the Meso-America Subduction Experiment (MASE) line in central Mexico were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental North America. The converted and reverberated phase image shows complete horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continental lithosphere, with a clear image of a very thin low-velocity oceanic crust (7–8 km) which dips at 15–20 degrees at Acapulco then flattens approximately 300 km from the Middle America Trench. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans-Mexican Volcanic Belt (TMVB). Where the slab bends underneath the TMVB, the migrated image depicts the transition from subducted oceanic Moho to continental Moho at ∼230 km from the coast, neither of which were clearly resolved in previous seismic images. The deeper seismic structure beneath the TMVB shows a prominent negative discontinuity (fast-to-slow) at ∼65–75 km within the upper mantle. This feature, which spans horizontally beneath the arc (∼100 km), may delineate the top of a layer of ponded partial melt.

Imaging crustal and upper mantle structure beneath the Colorado Plateau using finite frequency Rayleigh wave tomography

A new 3-D shear velocity model of the crust and upper mantle beneath the Colorado Plateau and surrounding regions of the southwestern United States was made with finite frequency Rayleigh wave tomography using EarthScope/USArray data. The goal of our study is to examine the Colorado Plateau lithospheric modification that has resulted from Cenozoic tectonism and magmatism. We have inverted for the isotropic Vs model from a grid of Rayleigh wave dispersion curves obtained by a modified two-plane wave method for periods from 20 to 167 s. We map the lithosphere-asthenosphere boundary under the Colorado Plateau by identifying the middle of the shallowest upper mantle negative Vs gradient. The depths of the lithosphere-asthenosphere boundary inferred here agree well with receiver function estimates made independently. The strong lateral heterogeneity of shear velocity can be mainly attributed to 200–400 K variations in temperature together with ∼1% partial melt fraction in the shallow upper mantle. The resulting Vs structures clearly image the upper mantle low-velocity zones under the Colorado Plateau margins that are associated with magmatic encroachment. These upper mantle low-velocity zones resulted from the convective removal of the Colorado Plateau lithosphere that had been rehydrated by subduction-released water, refertilizing and destabilizing it. This convective erosion by the asthenosphere at the low-viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended Basin and Range since the Mid-Cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast, south, and northwest. Moreover, the Rayleigh wave tomography model images parts of a high-velocity drip in the western Colorado Plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography. The widespread edge convective erosion, which the regional delamination-style downwelling processes are a 3-D manifestation of, could provide additional buoyancy sources to support the excess uplift at the margins of the plateau.

Reactivated lithospheric-scale discontinuities localize dynamic uplift of the Moroccan Atlas Mountains

The Atlas Mountains of Morocco, an example of an intracontinental mountain belt, display only modest tectonic shortening, yet have unusually high topography. We present new evidence from receiver functions and shear-wave splitting for localized, nearly vertical offset deformation of both crust-mantle and lithosphere-asthenosphere interfaces at the fl anks of the High Atlas. These offsets coincide with the locations of Jurassic-aged normal fault reactivation that led to tectonic inversion of the region during the Cenozoic. This suggests that a lithospheric-scale discontinuity is involved in orogeny. Another signifi cant step in lithospheric thickness is inferred within the Middle Atlas. Its location corresponds to the source of regional Quaternary alkali volcanism, where the infl ux of melt induced by the shallow asthenosphere appears to be restricted to the lithospheric-scale fault on the northern side of the range. Inferred stretching axes from shear-wave splitting are aligned with the highest topography, suggesting along-strike asthenospheric shearing in mantle fl ow guided by lithospheric topography. Isostatic modeling based on these improved crustal thickness and offset estimates indicates that lithospheric thinning alone does not explain the anomalous Atlas topography. Instead, an upwelling component induced by a hot mantle anomaly is also required to support the Atlas, suggesting that the timing of uplift is contemporaneous with the recent volcanism in the Middle Atlas. These observations provide a refi ned understanding of intracontinental orogeny and localized volcanism.

Craton formation: Internal structure inherited from closing of the early oceans

The closure of ancient oceans created a dynamic setting suitable for craton formation via the thickening of continental material over a mantle downwelling. This process subjected the thickening lithosphere to extensive deformation, forming internal structure that can be preserved over the lifetime of the craton. Recent seismic imaging of cratonic lithosphere has led to observations of anomalous features colloquially known as midlithospheric discontinuities. These discontinuities are attributed to a range of sources, including the lithosphere-asthenosphere boundary, melt accumulation, and phase transitions. However, the internal structure imaged within these cratons might be reflective of their formation. In particular, the orientation and nature of the variable depths of the midlithospheric discontinuities suggest a more complicated origin such as that which could be introduced during the formation and thickening phase of cratonic lithosphere. Here, we present geodynamic models demonstrating the internal structures produced during the formation of cratonic lithosphere as well as new seismological observations of midlithospheric discontinuities in the West African craton, together with reassessment of midlithospheric discontinuities observed in the North American, South African, Fennoscandia, and Australian cratons. We suggest that the midlithospheric discontinuities observed in these cratons could be remnants of deformation structures produced during the formation of the cratons after ancient oceans closed.