R-3 Repository :: Browsing by Author "Levander, Alan"

Browsing by Author "Levander, Alan"

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3D Shear Velocity Structure of the Caribbean—Northwestern South America Subduction Zone From Ambient Noise and Ballistic Rayleigh Wave Tomography
(Wiley, 2024) Miao, Wenpei; Cornthwaite, John; Levander, Alan; Niu, Fenglin; Schmitz, Michael; Li, Guoliang; Dionicio, Viviana; Prieto, German
The Caribbean-South America subduction zone is a flat subduction zone, with Laramide-style thick-skinned uplifts occurring in the Merida Andes, Sierra de Perija Range, and Santa Marta Massif. Geodetic measurements and historical seismicity show this region is storing strain energy and is capable of a mega-thrust earthquake (M ≥ 8.0). Previous seismic investigations of the lithosphere and upper mantle in this area are either very large scale, very local, or only peripheral to this area; therefore, details of the Caribbean plate subduction geometry beneath the Maracaibo block remain unclear. In this study, we used a new data set acquired by the Caribbean-Merida Andes seismic experiment (CARMA), which comprised 65 temporary broadband stations and 44 permanent stations from the Colombian and Venezuelan national seismic networks. We jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8–30 s band and ballistic Rayleigh wave phase velocities in 30–80 s band to construct a 3-D S-wave velocity model in the area between 75°–65°W and 5°–12°N. The 3-D model reveals a general increase in crust thickness from the trench to the southeast. An anomalous area is the Lake Maracaibo, which is underlaid by the thinnest crystalline crust in the region. This observation may indicate that the Maracaibo block is experiencing a contortion deformation within the crust. We also identified a high velocity anomaly above the subducting Caribbean slab, likely representing a detached piece of eclogitized Caribbean large igneous province from the base of the Maracaibo block. Additionally, our Vs model clearly indicates a slab tear within the subducted Caribbean slab, approximately beneath the Oca-Ancon Fault.
Applying high-resolution filtering based on non-convex regularization of Radon Transform on seismic imaging
(2018-06-28) Sun, Yen; Levander, Alan
In this study we propose a novel high-resolution filtering method based on non-convex regularization of the Radon Transform and demonstrate several possible applications. The Radon transfer sums signals exhibiting linear, parabolic or hyperbolic moveout in the original domain (t-Δ domain) to a single event in the new domain (Radon domain). Consequently, in the Radon domain, the target signals can be isolated easily from some types of noise. A high-resolution Radon Transform can be achieved by applying least-square inversion in the frequency domain. To further enhance the spatial resolution in the Radon domain and decrease the computational cost, a pre-processing method was developed in our group (Aharchaou and Levander 2016) based on the linear Radon transform in the frequency domain implemented with compressive sensing theory. The compressive sensing approach helps recover the sparsest solutions in the Radon domain for underdetermined inverse problems. Instead of least square minimization, a non-convex minimization (Lp regularization with 0
Autocorrelation Analysis of the Seismic Data Recorded on Mars
(2022-12-02) Deng, Sizhuang; Levander, Alan
There are eight planets in our solar system, which can be divided into two categories, terrestrial planets and Jovian planets. Mars, the last terrestrial planet away from the sun, is the target we want to investigate in this thesis. Mars has a very thin atmosphere and ice caps in its polar areas. Knowledge of the Martian interior informs theories for the formation and dynamic evolution of another terrestrial planet, hence providing information on the history of the solar system. On Earth, subsurface structure is discovered by analysis of seismic signals recorded by large seismograph arrays deployed worldwide. The InSight (Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport) lander carried one seismic station to Mars at the end of 2018, providing the opportunity to investigate the internal structure of Mars. As only one station is deployed on Mars to record seismic vibrations, some tomographic and imaging methods based on seismic array analysis are not suitable to investigate Mars data. In this thesis, we applied the autocorrelation method to retrieve two types of seismic phases, body-wave reflection signals and Mars orbiting surface wave signals. The body-wave reflection signal originated from subsurface interfaces of Mars can inform the depth range of these seismic boundaries, including Moho, olivine-wadsleyite transition and core-mantle boundary. As for Mars orbiting surface waves, it can be used to improve the velocity models of Martian upper mantle.
Autocorrelation R2 on Mars
(Wiley, 2022) Deng, Sizhuang; Levander, Alan
A purpose of the Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) mission is to reveal the Martian interior structure with seismic data. In this work, ambient noise autocorrelation of the continuously recorded vertical-component seismic signals has extracted the Rayleigh waves that propagate around Mars for one cycle, R2. The Mars orbiting surface waves are observed at a lag time of ∼6,000 s in the stacked autocorrelation series filtered between 0.005 and 0.01 Hz. Synthetic seismograms from a set of radially concentric velocity models were computed to find the best-fitting one as the starting model for a Monte Carlo inversion. The starting model was randomly perturbed iteratively to increase the correlation coefficients and reduce the absolute time shifts between the synthetic and observed R2. An S-wave low-velocity layer in the inverted velocity model extends to ∼400 km depth, consistent with Marsquake observations, geophysical inversion, and high-pressure experiments.
Autocorrelation Reflectivity of Mars
(Wiley, 2020) Deng, Sizhuang; Levander, Alan
The seismic structure of the Martian interior can shed light on the formation and dynamic evolution of the planet and our solar system. The deployment of the seismograph carried by the InSight mission provides a means to study Martian internal structure. We used ambient noise autocorrelation to analyze the available vertical component seismic data to recover the reflectivity beneath the Insight lander. We identify the noise that is approximately periodic with the Martian sol as daily lander operations and the diurnal variation in Martian weather and tides. To investigate the seismic discontinuities at different depths, the autocorrelograms are filtered and stacked into different frequency bands. We observe prominent reflection signals probably corresponding to the Martian Moho, the olivine-wadsleyite transition in the mantle, and the core-mantle boundary in the stacked autocorrelograms. We estimate the depths of these boundaries as ~35, 1,110–1,170, and 1,520–1,600 km, consistent with other estimates.
Crustal structure beneath the Rif Cordillera, North Morocco, from the RIFSIS wide-angle reflection seismic experiment
(American Geophysical Union, 2014) Gil, Alba; Gallart, Josep; Diaz, Jordi; Carbonell, Ramon; Torne, Montserrat; Levander, Alan; Harnafi, Mimoun
The different geodynamic models proposed since the late 1990s to account for the complex evolution of the Gibraltar Arc System lack definite constraints on the crustal structure of the Rif orogen. Here we present the first well-resolved P-wave velocity crustal models of the Rif Cordillera and its southern continuation toward the Atlas made using controlled-source seismic data. Two 300+ km-long wide-angle reflection profiles crossed the Rif along NS and EW trends. The profiles recorded simultaneously five land explosions of 1Tn each using ~850 high frequency seismometers. The crustal structure revealed from 2-D forward modeling delineates a complex, laterally varying crustal structure below the Rif domains. The most surprising feature, seen on both profiles, is a ∼50 km deep crustal root localized beneath the External Rif. To the east, the crust thins rapidly by 20 km across the Nekkor fault, indicating that the fault is a crustal scale feature. On the NS profile the crust thins more gradually to 40 km thickness beneath Middle Atlas and 42 km beneath the Betics. These new seismic results are in overall agreement with regional trends of Bouguer gravity and are consistent with recent receiver function estimates of crustal thickness. The complex crustal structure of the Rif orogen in the Gibraltar Arc is a consequence of the Miocene collision between the Iberian and African plates. Both the abrupt change in crustal thickness at the Nekkor fault and the unexpectedly deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African passive margin at late Oligocene-early Miocene times.
Fluid Controls on the Heterogeneous Seismic Characteristics of the Cascadia Margin
(Wiley, 2018) Delph, Jonathan R.; Levander, Alan; Niu, Fenglin
The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate‐depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.
From mantle shear-zones to crustal detachment surfaces: Geophysical investigations of rifting at continental margins
(2022-12-02) Nguyen, Luan Chan; Morgan, Julia; Levander, Alan
This dissertation contributes to the understanding of the tectonic evolution of the lithosphere as it undergoes extensional deformation during continental rifting. The study consists of two main projects carried out over two rifted margins using distinct geophysical methods to probe the subsurface at different scales. Over the Gulf of Mexico and its bounding margins, Rayleigh surface-wave tomography from the cross-correlation of the seismic ambient noise field was used to construct a 3D velocity model for the top 150 km. The model unveils a significant feature in the region beneath the extended terrain of the northwestern Gulf of Mexico. This anomalous pinch-and-swell structure in the mantle lithosphere is interpreted as geologic boudinage that reflects the deformational history of the continental lithosphere as it was stretched and deformed. Our analysis, in combination with previous findings, demonstrate that geologic boudins play an important role in enhancing localized deformation leading to the well-documented asymmetric geometry of conjugate rifted margins worldwide. We infer that boudinage development is influenced by inherited thickness of the lithosphere and by the process of mantle refertilization triggered by melt infiltration from the ascending asthenosphere during rifting. Lithospheric boudinage proves to be a lasting feature that persists long after the breakup of continents such that it provides a means to investigate other rifted margins around the world. In the second part of this project, we show that crustal variation along the US Gulf Coast margin as well as the structural asymmetry observed across its conjugate Yucatan margin are consistent with the scenarios in which rifting was accompanied by mantle shear-zones. The direction of plate motion is determined to have changed significantly during the transition from rift to drift which is attributed to the development of deep shear-zones in the western part of the rift. Our estimated thickness of the lithosphere indicates that the extended continental lithosphere over the Gulf of Mexico margins has mostly regained its thickness since the time of breakup. The topic of shear-zones, but in the brittle regime, is revisited in the context of rifting at the Galicia margin at depth of less than 12 km below sea level , where 3D active-source reflection seismic imaging clearly shows the morphology of the S reflector, a major detachment surface that facilitated the motion of the overlying crustal blocks during continental extension. The nature of the rock layer above this surface contains a record of block displacements and can yield insight rifting kinematics. Defining the physical properties of this rock layer has been a challenge in previous studies. Here, we applied the analysis of amplitude-variation-with-offset to the S reflector to determine the elastic properties of the rock layer immediately overlying S. We identified a wide distribution of areas of low elastic properties above the S reflector, consistent with the presence of a fault gouge. Our derived rock densities and Vp/Vs ratios indicate that the gouge’s composition is highly heterogeneous throughout the study area, but with systematic distributions that reflect the evolution of the fault system. Our results suggest an increasing level of mantle serpentinization within the fault gouge toward the eastern part of the Galicia margin, an indication of a longer history of mantle hydration attributed to water ingress along early-formed crustal faults. In addition, we infer from the correlation between gouge composition and thickness that heterogenuous fault strength resulting from differential serpentinization of the mantle detachment surface may have affected block displacement and overall geometry of the rifted margin.
Imaging the Great Plains of the Central U.S. using Finite-Frequency Rayleigh Wave Tomography and Implications for Asthenosphere-Driven Uplift
(2014-08-26) Margolis, Rachel Ellen; Levander, Alan; Zelt, Colin; Lenardic, Adrian
Here we present a 3D shear velocity model for the lower crust and upper mantle beneath the Great Plains in the central United States using finite frequency Rayleigh wave tomography. We use USArray Transportable Array recordings of teleseismic Rayleigh waves and first invert for phase velocity using the two-plane wave method with finite frequency kernels, then invert the resulting dispersion curves for shear velocity structure. We characterize the lithospheric structure in this tectonically transitional regime to illuminate the differences between the actively deforming west and stable continental interior. The west is defined by slow velocities and thin lithosphere, whereas the east has fast velocities and thick lithosphere, with the thickest lithosphere under the Superior craton. From our tomography and heat flow data, we infer warm temperatures in the west and suggest that the asthenospheric mantle contributes to anomalously high elevation in the west with secondary contributions from crustal effects.
Insights on Formation of the Gulf of Mexico by Rayleigh Surface Wave Imaging
(Wiley, 2022) Nguyen, Luan C.; Levander, Alan; Niu, Fenglin; Morgan, Julia; Li, Guoliang
We used cross-correlation of ambient noise records from seismic stations in the US, Mexico, and Cuba to extract dispersion data of Rayleigh surface wave. Our derived 3D shear-wave velocity model of the greater Gulf of Mexico (GOM) region captures variations in the crustal and lithospheric structures across the continental margins of the US Gulf Coast and Yucatan, Mexico. The model shows a zone of reduced velocity in the mantle lithosphere underlying the extended continental margin of the northwestern GOM. We attributed this velocity reduction to extensional deformation and melt-induced weakening of the lithosphere during the Triassic continental rifting that preceded the seafloor spreading that formed the GOM. Melt extraction might have been hindered by the greater lithospheric thickness in the western region along the US Gulf Coast margin that resulted in the westward decrease of rift-related volcanism/magmatism reported from previous studies. The clear asymmetry between the US Gulf Coast and its conjugate Yucatan margin suggests extension along a shear-zone that focused more deformation on the North American plate prior to breakup. In contrast to the counterclockwise rotation of the Yucatan block during seafloor-spreading, our analyses using deformable plate models demonstrate that continental rifting occurred in a predominantly northwest-southeast direction. This change in plate motion is attributed to the development of mantle shear-zones in the western part of the rift. We estimated the depth of the lithosphere-asthenosphere boundary and determined that the extended continental and oceanic lithospheres have mostly regained their thickness since the time of breakup.
Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela
(Elsevier, 2015) Masy, Jeniffer; Niu, Fenglin; Levander, Alan; Schmitz, Michael
We have combined surface wave tomography with Ps and Sp receiver-function images based on common-conversion-point (CCP) stacking to study the upper mantle velocity structure, particularly the lithosphere–asthenosphere boundary (LAB), beneath eastern and central Venezuela. Rayleigh phase velocities in the frequency range of 0.01–0.05 Hz (20–100 s in period) were measured using the two-plane-wave method and finite-frequency kernels, and then inverted on a 0.5° × 0.5° grid. The phase velocity dispersion data at grid points were inverted for 1D shear velocity profiles using initial crust-mantle velocity models constructed from previous studies. The 3D velocity model and receiver-function images were interpreted jointly to determine the depth of the LAB and other upper mantle features. The tomographic images revealed two high velocity anomalies extending to more than ∼200 km depth. One corresponds to the top of the subducting Atlantic plate beneath the Serrania del Interior. The other anomaly is a highly localized feature beneath the Maturin Basin. The LAB depth varies significantly in the study region: It is located at ∼110 km depth beneath the Guayana Shield, and reaches ∼130 km at the northern edge of the Maturin Basin, which might be related to the downward flexural bending due to thrust loading of the Caribbean plate and pull from the subducting Atlantic plate. Immediately to the west, the lithosphere is thin (∼50–60 km) along the NE-SW trending Espino Graben from the Cariaco basin to the Orinoco River at the northern edge of the craton. The LAB in this region is the top of a pronounced low velocity zone. Westward, the lithosphere deepens to ∼80 km depth beneath the Barinas Apure Basin, and to ∼90 km beneath the Neogene Merida Andes and Maracaibo block. Both upper mantle velocity structure and lithosphere thickness correlate well with surface geology and are consistent with northern South American tectonics.
Localized waveform inversion applied to the D" region in teleseismic imaging
(2009) He, Yang; Levander, Alan
A localized waveform inversion technique based on hybrid modeling was developed to investigate the shear wave velocity structure of the lowermost mantle. Utilizing ray theory and the Kirchhoff integral, the source wavefield from the hypocenter and the receiver wavefield recorded at the Earth's surface can be extrapolated to the subsurface near the core-mantle boundary (CMB). Ray theory solutions for a smooth background Earth model are interfaced to a finite difference solution to the wave equation applied in a local heterogeneous region near the CMB. The velocity structure in the finite difference region is updated iteratively by zero-lag cross-correlation of the forward and backward propagated wavefields that represents the direction of minimizing the data misfit. As the finite difference method is only applied in a small region, the hybrid method requires much less computer memory when it is implemented to invert localized structures. This method is applied to study an event recorded by the RISTRA Array and the depth and amplitude of the D" discontinuity is recovered. This demonstrates that the hybrid waveform inversion is feasible in teleseismic imaging.
Ongoing lithospheric removal in the western Mediterranean: Evidence from Ps receiver functions and thermobarometry of Neogene basalts (PICASSO project)
(American Geophysical Union, 2014) Thurner, Sally; Palomeras, Imma; Levander, Alan; Carbonell, Ramon; Lee, Cin-Ty
The western Mediterranean tectonic system consists of the Betic Mountains in southern Spain and the Rif Mountains in northern Morocco curved around the back-arc extensional Alboran basin. Multiple tectonic models have been developed to explain the coeval compressional and extensional tectonic processes that have affected the western Mediterranean since the Oligocene. In order to provide constraints on these evolutionary models, we use Ps teleseismic receiver functions (RF), thermobarometric analyses of post-Oligocene basalts, and previous teleseismic tomography images to investigate the lithospheric structure of the region. Ps RFs were calculated using seismic data from 239 broadband seismic stations in southern Iberia and northern Morocco and thermobarometric analysis was performed on 19 volcanic samples distributed throughout the region. The RF images reveal a highly variable Moho depth (∼25 to ∼55 km), as well as a strong positive, sub-Moho horizon between ∼45 and ∼80 km depth beneath the central Betic and Rif Mountains, which we interpret to be the top of the previously imaged Alboran Sea slab. Thermobarometric constraints from magmas in the eastern Betics and Rif indicate mantle melting depths between 40 and 60 km, typical of melting depths beneath mid-oceanic ridges where little to no lithosphere exists. Together, the RF and thermobarometric data suggest ongoing and recent slab detachment resulting from delamination of the continental lithosphere.
Sedimentary and crustal structure of the US Gulf Coast revealed by Rayleigh wave and teleseismic P coda data with implications for continent rifting
(Elsevier, 2022) Miao, Wenpei; Niu, Fenglin; Li, Guoliang; Levander, Alan
We have developed an S-wave model of the south-central US focusing on the Gulf Coast sedimentary basin and its crust to understand continental rifting and regional tectonics. The model was derived by a joint inversion of Rayleigh wave phase velocities, Z/H ratios and P-coda data. The surface- and body-wave measurements were made, respectively, from ambient noise and teleseismic earthquakes recorded by 215 USArray stations in a rectangular area of 100°–87° west and 28°–37° north. We employed a cross-convolution function and H-κ analysis to better constrain sedimentary and Moho structure. We find that the southern edge of the Ouachita fold-and-thrust belt (OFTB) appears as a boundary in measured phase velocities, Z/H ratios, sediment basement depths, Moho depths, average crustal Vs and Vp/Vs ratios. The model shows southeastward thickening of the sedimentary basin, accompanied by thinning of the crystalline crust. The Moho gradient suggests that early rifting between North America and the Yucatan block commenced in a SE direction and involved most of the Pangea crust south of the OFTB boundary (i.e., Gondwana crust). We believe that a high velocity feature in the lowermost crust and upper mantle parallel to the southeast Texas coast was emplaced as a mafic body and is the source of the Houston magnetic anomaly. The seismic structures of the crust and uppermost mantle observed beneath the Mississippi Embayment and the Mississippi Valley Graben are consistent with plume induced Cretaceous uplift of the Mississippi Embayment as North America passed over the Bermuda hotspot.
Seismic Array Study of the Western Mediterranean and the United Stats Great Plains: Insight into the Modification and Evolution of Continental Lithosphere
(2014-11-17) Thurner, Sally; Levander, Alan; Niu, Fenglin; Lee, Cin-Ty; Pu, Han
Numerous tectonic processes are responsible for the modification and evolution of continental lithosphere. The continents, however, are generally resilient through geologic time and keep a record of Earth’s tectonic activity, both past and present. The focus of this work is to better understand the modification and evolution of continental lithosphere associated with continent-continent collisions. We do this by studying two orogenic systems: the Alpine Orogeny, associated with the ongoing collision between the African and Eurasian plates, and the Trans-Hudson Orogeny, associated with the initial formation of the North American craton during the Precambrian. This research focuses on the westernmost edge of the Alpine system in the western Mediterranean, where subduction and slab rollback have caused significant extension and Africa-Iberia convergence has caused simultaneous contraction. Here we calculate Pds receiver functions to constrain the discontinuity structure. Additionally, we jointly invert Pds receiver functions and Rayleigh wave phase velocity dispersion data to create a 3-D shear velocity model. These results show a deep Moho around the western portion of the Gibraltar Arc. Below this deep Moho we see the Alboran Slab extending down to ~250 km. In the eastern Gibraltar Arc, there is a very shallow Moho where the slab has detached from the surface and removed continental lithosphere. In the Trans-Hudson Orogen we use receiver functions and gravity data to determine the discontinuity and density structure of the shallow lithosphere. This analysis reveals crustal-scale thrusting associated with the Wyoming-Superior suture zone. We also find a relatively low Moho density contrast throughout the Trans-Hudson and northern Yavapai Province. This low Moho density contrast is associated with a deep Moho (>50 km) and is interpreted to be evidence of a dense lower crustal layer resulting from mafic underplating. Finally, we investigate the contribution that this dense thick crust may have played in the isostatic stabilization of the North American craton as well as other cratons around the world. We find that the lithospheric mantle must provide a negative component to cratonic lithospheric buoyancy in order to account for the low elevations observed along with thick crust in the cratons.
Seismic evidence for lithospheric boudinage and its implications for continental rifting
(The Geological Society of America, 2022) Nguyen, Luan C.; Levander, Alan; Niu, Fenglin; Morgan, Julia; Li, Guoliang
The continental rifting that precedes the breakup of a continent and the formation of a new ocean basin is one of the key processes of plate tectonics. Although often viewed as a two-dimensional process, rifted margins exhibit significant variations along strike. We document along-strike variations developed during the ca. 200–160 Ma continental rifting that formed the margins of the Gulf of Mexico ocean basin. Rayleigh-wave ambient noise tomography reveals a zone of high and low seismic velocity resembling large scale geologic boudins in the mantle lithosphere of the northwestern Gulf of Mexico margin. These features become progressively less prominent eastward following the transition from a magma-poor to a magma-rich passive margin. We infer that mantle refertilization and thickness of the pre-rift lithosphere control deformation style and the along-strike variations in continental rifting. Our results also suggest that deformation during rifting produces long-lived features that persist long after breakup and, therefore, can be used to study rifted margins globally.
Tectonic Evolution of the South Caribbean-Northwest South America Subduction Zone
(2022-12-02) Cornthwaite, John Paul; Levander, Alan
In this work I image the northwest South American-southern Caribbean subduction zone beneath the Maracaibo Block from the crust to the mantle transition zone. I tie together the different structures I model to propose the tectonic evolution that explains the physiography observed at the surface. Finite-frequency teleseismic P-wave tomography illuminated the large structures of the subducting Caribbean plate and provided the first-order explanation of how a flat-subducting Caribbean plate contributed to the uplift of three mountain ranges far from the trench. P- and S-wave local earthquake tomography showed the finer structure of the upper mantle and crust and supported the idea that the Caribbean plate interacted with the South American plate deep into the interiors of Colombia and Venezuela. Ps receiver functions revealed critical boundaries in the upper mantle and crust and corroborated substantial crustal deformation in the crust in response to subduction. My first project focused on the large-scale structures of the Caribbean-northwest South American subduction zone. I modeled the subduction structures using finite frequency teleseismic P-wave tomography and connected those structures to the Laramide-style deformation on the overriding South American plate. I identified three segments of subducting Caribbean plate with one segment completely detached from the surface. The timing of the detachment aligns with other regional events, including the uplift of the Mérida Andes, about 10 Ma. Slab buoyancy post-detachment likely resulted in recoupling with the overriding plate, reactivation of Jurassic-aged rift structures and subsequent uplift of the Mérida Andes. Mantle counterflow over the broken segment induced by rollback of the attached slab likely contributed to the uplift of the Mérida Andes. I concluded that the northern limit of subduction lies south of the Oca-Ancón fault, though the fault itself may be the surface expression of the boundary. In my second study I use P- and S- wave local earthquake tomography to model shallow structures within the subduction zone that connect subduction processes to deformation in the overriding plate. I identified several large fast anomalies (dlnVp ~ +4%) extending from the base of the Maracaibo block to near the top of the currently subducting plate under the whole length of the Maracaibo block. I hypothesized this anomaly is buoyant Caribbean lithosphere that has remained attached to the base of the Maracaibo block lithosphere during slab fragmentation. Upwelling mantle that replaced the descending slab is observed beneath the Serrania de Perijá and the Mérida Andes and possibly contributes to dynamic uplift of those mountains. A fast anomaly observed beneath the Moho under Lake Maracaibo is likely similarly negatively buoyant eclogitized slab crust that has not yet detached and contributes to subsidence in the overriding plate. My third study provides a regional view of crustal and upper mantle structure using Ps receiver functions. I revealed variable crustal thickness that thickens due continental underthrusting under the Serrania de Perijá and the Mérida Andes. South American crust appears to underthrust Maracaibo block crust under the Mérida Andes. I found that the crust within the Maracaibo Block is highly heterogeneous and blocky. Poisson ratios within the Maracaibo block appear to trend higher than outside of the block. In the mantle I identified the transition from subduction to underthrusting of the Caribbean plate south of the Oca-Ancón fault, in agreement with my first study. Finally, I speculated that mantle upwelling could be occurring throughout the Maracaibo Block.
Three-dimensional seismic structure of the Leeward Antilles arc from seismic refraction and refraction and reflection tomography
(2006) Arogunmati, Adeyemi T.; Zelt, Colin A.; Levander, Alan
We have developed new velocity and Moho interface models for the Dutch Antilles region of the Leeward Antilles arc from independent and simultaneous 3-D inversion of first arrival and PmP traveltimes. The first arrival and PmP traveltimes were picked from wide-angle offshore-onshore seismic data acquired in the Leeward Antilles arc region using 70 offshore and onshore instruments as part of the Broadband Onshore-Offshore Lithosphere Investigation of Venezuela and the Antilles are region (BOLIVAR) experiment. The resulting velocity models show evidence for fault bounded basins and features associated with the under-thrusting of the Caribbean plate beneath South America. Velocities beneath the arc are higher than velocities elsewhere in the study region at all depths and they suggest that the arc may be made of intermediate granulites. Moho interface models vary in depth from 19 km to 37 km in the study area. Beneath the arc, shallow basement topography lies above deep Moho topography and vice versa. In other words, the Moho structure mirrors the basement topography. This observation indicates the possibility of displacement partitioning during the formation of the arc. We compare an average 1-D velocity model from the Leeward Antilles arc to models from other island and continental arcs and find that the Leeward Antilles profile has velocities lower than mafic oceanic island arcs such as the Aleutians, Tonga and the Bonin arc, but has higher velocities than continental arcs such as the Sierra Nevada. Its velocities are however, similar to those of the Honshu island arc. The range of velocities in the Leeward Antilles arc profile is similar to that in the average continental crustal profile. Key words. Leeward Antilles arc, 3D crustal structure, seismic velocity, seismic tomography.

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