Maria Beatrice Magnani

The Yavapai-Mazatzal boundary: A long-lived tectonic element in the lithosphere of southwestern North America

A seismic reflection profile crossing the Jemez lineament in north-central New Mexico images oppositely dipping zones of reflections that converge in the deep crust. We interpret these data as a Paleoproterozoic bivergent orogen, centered on the Jemez lineament, that formed during original Proterozoic crustal assembly by collision of Mazatzal island arcs with Yavapai proto–North American continent at ca. 1.68–1.65 Ga. The two major sets of reflections within the Yavapai-Mazatzal transition boundary dip at 15°–20°, and we interpret them as part of a south-dipping thrust system and as a north-dipping crustal-scale duplex that formed synchronously with the thrust system. The upper crust shows structures recording a succession of tectonic and magmatic events from the Paleoproterozoic to the Holocene. Notable among these structures is a system of nappes that formed during development of the bivergent orogen. Elements of the nappe system are exposed in Rocky Mountain uplifts and have been dated as having formed at 1.68 Ga, at depths of 10 km and at temperatures of >500 °C. We also see continuous bright reflections in the upper part of the middle crust that we associate with basaltic sills that postdate accretion. The data show that the Yavapai-Mazatzal suture is low angle (∼20°), an observation that explains why the boundary between the provinces has previously been so hard to define in the surface geology. The Jemez lineament overlies the root of this bivergent orogen that we also suggest is a Paleoproterozoic zone of weakness that has subsequently acted as a conduit for magmas and a locus of tectonism.

Characterizing the Caribbean–South American plate boundary at 64°W using wide‐angle seismic data

[1]xa0We present wide-angle velocity modeling results from profile 64°W of the Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region (BOLIVAR) project. Line 64W is a 460-km-long, approximately north-south, onshore-offshore reflection/refraction transect located approximately at 64°W longitude. The profile extends across the transform plate boundary between the southeastern Caribbean (CAR) and South American (SA) plates. East of the profile the plate boundary bends to the north, and SA subducts beneath CAR. We utilize first-arrival tomography to resolve a smooth velocity field for the sediments and upper/middle crust and then use a layered approach to resolve a sharp velocity contrast for the Moho, simultaneously inverting refracted Pn and reflected PmP arrivals. We image crustal and upper mantle structure across the plate boundary zone. We interpret that the strike-slip system that accommodates relative motion between CAR and SA extends near vertically through the entire crust and offsets the Moho. We see no evidence supporting a major component of convergence, and rather than a wide boundary zone of overlapping lithospheric plates, we interpret the plate boundary to be confined to the 33-km-wide, near-vertical strike-slip system. Previously interpreted thrust faults flanking the strike-slip system appear to be confined to the upper/middle crust and may be related to the detachment of subducting South American lithosphere at the southern terminus of the Lesser Antilles subduction zone east of 64°W.

Evolution of the Southern Caribbean Plate Boundary

It is generally accepted that the cores of the continents, called cratons, formed by the accretion of island arcs into proto-continents and then by proto-continental agglomeration to form the large continental masses. Mantle-wedge processes, combined with higher melting temperatures during the Archean (2.5–3.8 billion years ago) and possibly thrust stacking of highly depleted Archean oceanic lithosphere, produced a strong, buoyant, upper mantle chemical boundary layer. This stabilizing mantle layer, known as the tectosphere, has shielded the Archean cratons from most subsequent tectonic disruption and is highly depleted in iron, providing the positive buoyancy that is required to ‘float’ the continents more than four kilometers above the surrounding ocean basins.

Identification and tectonic implications of a tear in the South American plate at the southern end of the Lesser Antilles

[1]xa0In the southeast corner of the Caribbean, westward subduction of (Atlantic) oceanic South America beneath the Lesser Antilles transitions to east-west transform motion between continental South America and the Caribbean plate. This geometry requires negatively buoyant, subducting, oceanic South American lithosphere to progressively detach from positively buoyant, continental South American lithosphere. The most widely accepted model is slab break-off, with oblique arc-continent collision and northwest dipping, continental subduction precipitating narrow rifting in the subducting slab. In contrast, the subduction-transform edge propagator (STEP) model conceptualizes progressive detachment along a vertical, dip-slip tear through the lithosphere, with stress focused at the edge of the propagating transform boundary. We present four types of seismic data to resolve the ongoing lithospheric detachment: local seismicity, receiver functions, wide-angle seismic velocity inversion, and a regional, balanced cross section constrained by petroleum industry data. These four data sets image a near-vertical tear extending through the entire lithosphere, revealing a key mechanism for the structural evolution of Venezuela.

Crustal structure of the South American–Caribbean plate boundary at 67°W from controlled source seismic data

[1]xa0We present the results of new seismic reflection and wide-angle data across the SE Caribbean plate boundary. The 550 km long N–S profile crosses the structures involved in the active 55 Ma long continent-arc oblique collision between the Caribbean (CAR) and the South American (SA) plate. From the north to the south these structures include the accretionary prism, the extinct volcanic arc (Leeward Antilles arc), the Tertiary Bonaire basin, the continental-size dextral strike-slip fault system (San Sebastian–El Pilar fault), the allochthonous exhumed terranes, and the authocthonous fold and thrust belt (Caribbean Mountain system) and foreland basin. The wide-angle data show that these elements are characterized by different velocity structures and that they are separated by sharp lateral velocity variations. The Leeward Antilles arc exhibits a velocity structure similar to that of the Lesser Antilles active volcanic arc, indicating that the extinct arc has not been modified by the collision with the SA plate. The data show a ∼20 km change in crustal thickness across the San Sebastian fault, suggesting that the dextral strike-slip fault is a crustal feature that likely continues in the mantle as a primary strand of the plate boundary between the South American and the Caribbean plates. South of the strike-slip fault and beneath the exhumed eclogitic terranes, the data image a north dipping, high-velocity (>6.5 km/s) anomaly in the upper crust (3–11 km), indicating that high-pressure/low-temperature rocks are the likely lithologies responsible for the high seismic velocities and suggesting that exhumation of these assemblages is enabled by the strike-slip fault.

Crust and Upper Mantle Velocity Structure of the Southern Rocky Mountains from the Jemez Lineament to the Cheyenne Belt

We have interpreted the refraction/wide-angle reflection seismic profile acquired as part of the Continental Dynamics of the Rocky Mountains project. The profile extends ∼955km from northern New Mexico to southern Wyoming, crossing the Tertiary-Quaternary volcanics of the Jemez lineament, the Paleoproterozoic Mazatzal and Yavapai terranes, the Cheyenne belt, and the southern Archean Wyoming Province. We inverted the travel-time data from the 10 shot profile with both a layer based inversion method and a tomographic method. The two techniques yield comparable upper and middle crustal velocity structures. Lower crustal velocities are well constrained in the layer based model but are not in the tomographic model. From the layer based model, velocities in the crystalline crust and the upper mantle are lower than typical for continents and for modern orogens. Lower crustal velocities rarely exceed 7.00 km/s, likely due to the regionally high heat flow. We infer that the low upper mantle velocities beneath the Jemez lineament (7.70-7.76 km/s) are indicative of upper mantle partial melt. Crustal thickness increases from south to north, with thinner crust under the Jemez lineament (40-42 km), and thicker crust under northern Colorado, the Cheyenne belt, and southern Wyoming (51-53 km). Although the Cheyenne belt outcrops as a narrow zone separating Paleoproterozoic and Archean terranes, the seismic model shows broad lateral variation in crustal velocity near the suture, and a thick crust in the northern half of the profile. Part or all of the crustal thickening is likely to have occurred subsequent to continental accretion.

Negligible convergence and lithospheric tearing along the Caribbean–South American plate boundary at 64°W

[1]xa0Prior studies of the Caribbean–South American plate boundary have suffered from poor constraint on the structure of the crust and uppermost mantle. We use a recent wide-angle velocity model from the Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region project to constrain new seismic reflection data and previously published line drawing interpretations of the Caribbean–South American plate boundary at 64°W. Though commonly characterized as obliquely convergent, we determine that convergence is negligible in our study area. Previous estimates of Miocene to present north-south shortening onshore eastern Venezuela have commonly been 115 km or higher, but we constrain shortening to ∼35 km onshore, with an additional ∼30 km offshore. With such minor convergence, we conclude that uplift and basin subsidence in eastern Venezuela does not derive from typical collisional orogeny. Instead, the largely vertical tectonics likely result from mantle dynamics associated with an eastward propagating, near-vertical tear in the lithosphere along the former passive margin.

The Caribbean–South American plate boundary at 65°W: Results from wide‐angle seismic data

[1]xa0We present the results of the analysis of new wide-angle seismic data across the Caribbean–South American plate boundary in eastern Venezuela at about 65°W. The ∼500 km long profile crosses the boundary in one of the few regions dominated by extensional structures, as most of the southeastern Caribbean margin is characterized by the presence of fold and thrust belts. A combination of first-arrival traveltime inversion and simultaneous inversion of PmP and Pn arrivals was used to develop a P wave velocity model of the crust and the uppermost mantle. At the main strike-slip fault system, we image the Cariaco Trough, a major pull-apart basin along the plate boundary. The crust under the Southern Caribbean Deformed Belt exhibits a thickness of ∼15 km, suggesting that the Caribbean Large Igneous Province extends to this part of the Caribbean plate. The velocity structures of basement highs and offshore sedimentary basins imaged by the profile are comparable to those of features found in other parts of the margin, suggesting similarities in their tectonic history. We do not image an abrupt change in Moho depth or velocity structure across the main strike-slip system, as has been observed elsewhere along the margin. It is possible that a terrane of Caribbean island arc origin was accreted to South America at this site and was subsequently bisected by the strike-slip fault system. The crust under the continental portion of the profile is thinner than observed elsewhere along the margin, possibly as a result of thinning during Jurassic rifting.

Quaternary deformation and fault structure in the Northern Mississippi Embayment as imaged by near‐surface seismic reflection data

Seismicity in the New Madrid seismic zone (NMSZ) in the central United States constrains the location of present deformation at depth along four main distinct arms, while the surface expression of the ongoing deformation is still unclear. To better constrain the surface deformation in the NMSZ, we integrate existing seismic reflection data with a new ~300u2009km-long high-resolution seismic reflection profile acquired along the Mississippi River from Cape Girardeau, MO, to Caruthersville, MO. Based on the data, we interpret the Reelfoot Thrust and the New Markham Fault as upward splays of a blind master fault defined by the seismicity and extending at depth farther north. To the south, two faults, the Axial Fault and the Cottonwood Grove Fault, are imaged above the southern arm of the NMSZ. Both fault display deformation of the Paleozoic through the Tertiary sediments, and a relief of ~20–25u2009m at the base of the Quaternary alluvium, which we interpret as the result of strike-slip motion along a complex fault plane geometry. We propose two alternative interpretations for the relationship between the shallow faults and the seismicity in this area: (1) the faults merge at depth and are presently both active and (2) the faults are distinct at depth and were active during the Quaternary and only the Axial Fault is presently deforming. Geological structures mapped at the surface as part of this study show that Quaternary deformation is accommodated along a fault network that is more complex than the simple four-arm system illuminated by the seismicity, a behavior predicted by analog and computer models.

Long‐lived deformation in the southern Mississippi Embayment revealed by high‐resolution seismic reflection and sub‐bottom profiler data

Three high-resolution seismic reflection profiles and two sub-bottom profiler sections acquired along the Mississippi River in southern-Central U.S. image deformation in post-Paleozoic sediments. The northernmost profile images two faults offsetting Cretaceous through at least Eocene Cane River reflectors, interpreted to strike northwest and to be part of the Arkansas River fault zone. The central profile shows a down-to-the-north fault, displacing Cretaceous and Paleocene Midway Group reflectors by ~210 m and ~160 m, respectively. The fault is interpreted as the northern edge fault of the Monroe Uplift, a Late Cretaceous uplift associated with igneous intrusions. The southernmost profile displays a down-to-the-south fault, offsetting Cretaceous and Paleocene-Eocene Wilcox Group reflectors by ~125 m and ~32 m, respectively. Tilted reflectors in the first 80 m indicate Eocene-Oligocene activity of the fault, although Quaternary activity cannot be ruled out. Quaternary tectonic activity is proposed for a series of faults that offset shallow (<40 m depth) Eocene sequences and the base of the Quaternary alluvium as imaged on two sub-bottom profiler sections. These shallow faults are imaged in the vicinity of Holocene earthquake-induced liquefaction fields, corroborating the evidence for recent tectonic activity in the area. The spatial coincidence of the imaged faults with the inferred location of the Alabama-Oklahoma transform strongly argues toward a long-lived influence of this Precambrian continental margin in focusing tectonic activity in the southern U.S. by controlling the reactivation of Triassic-Jurassic syn-rift basement structures and guiding the emplacement of Late Cretaceous igneous intrusions and the location of Cenozoic deformation.