Juan M. Lorenzo

Thermal uplift and erosion across the continent-ocean transform boundary of the southern Exmouth Plateau

Thermal evolution of the continental lithosphere at a continent-ocean transform margin is examined using a two-dimensional heat conduction model. All heating is assumed to result from the emplacement of new oceanic ridge against the continent. The assumed initial continental temperature gradient is probably best applied to regions having experienced finite-duration periods of rifting lasting tens of millions of years. A deep seismic reflection profile across the southern paleo-transform margin of the Exmouth Plateau (northwest Australia) tests the predictions of the model. Using this reflection profile, it is estimated that up to 3.5 km of sediments have been eroded from the continental rim, diminishing to almost no erosion at 60 km from the continent-ocean transform boundary. This trend and these values for erosion can be matched approximately using the model under the condition of local isostasy. A finite-difference scheme is employed, where the surface elevation is the result of the competing (1) thermal uplift (up), (2) surficial erosion (down), and (3) local isostatic rebound (up) in response to the erosion. The model predicts that most of the erosion ceases by 40 Ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length.

Sheared continent–ocean margins: an overview

Abstract Continent–ocean fracture zones are the fossil transform offsets located along passive rifted continental margins. Kinematic models identify at least two principal stages in their evolution. During the first stage as rifting proceeds, continent–continent shearing dominates a narrow region in which the transform fault will eventually rupture. High-standing continental marginal ridges 50–100 km wide and bounding deep sedimentary basins, are derived in such settings. In stage two as sea-floor spreading proceeds, the younger oceanic block slides along the active transform, heating the older continental block, and possibly induces thermal uplift and accompanying denudation. Magnetic injection into the continental block at depth may also induce an isostatic uplift. After ridge–transform intersection time, mechanical coupling between the continental and oceanic blocks may influence the stratigraphy and structure of these margins.

Transform tectonics and thermal rejuvenation on the Côte d'Ivoire-Ghana margin, west Africa

Formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break-up, including transpressional crustal thickening, and thermal rejuvenation and igneous underplating during passage of a spreading ridge. ODP drill holes on the Côte d’Ivoire-Ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin. Apatite fission-track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra-continental wrench zone that predated margin formation. Rapid cooling of >120°C at 120–115 Ma corresponds to erosion of 3.5–5 km along the conjugate Brazilian margin, reflecting c. 1 km of tectonically driven uplift, subaerial erosion, and isostatic uplift due to unloading. Following rift initiation at 120 Ma (Aptian), an oceanic spreading axis passed adjacent to this part of the margin at 90 Ma (Cenomanian). Maximum uplift during the ridge-transform intersection was 390 m, considerably less than the 2000+ m predicted by heat conduction models in local isostatic equilibrium. The modern ridge is partially the product of thicker crust (22 km) underlying the ridge than the adjacent Deep Ivorian Basin (19 km), and partially related to flexural unloading of the transform ridge between the end of intra-continental wrenching and ridge-transform intersection. Flexural coupling between the continental and oceanic plates since ridge-transform intersection has caused a progressive depression of the offshore margin, estimated at about 650 m in the study area.

Timing of rifting in the Alboran Sea basin — correlation of borehole (ODP Leg 161 and Andalucia A-1) to seismic reflection data: implications for basin formation

Abstract To constrain the timing of rifting in the western, eastern, and northern parts of the Alboran Sea basin, seismic reflectors, corresponding to biostratigraphic boundaries and unconformities from the Miocene to Recent, are correlated using synthetic seismograms. Regions of adjacent coeval compression and extension are found in the Alboran Sea basin. Normal faulting continues in parts of the eastern Alboran Sea basin later than in the western Alboran Sea basin. Seismic reflection data in the vicinity of ODP Site 976 (western Alboran Sea basin) suggest that rifting ended in the Miocene. Near Site 976 (western Alboran Sea basin), no compressional features affect Miocene and younger sediments. In the eastern Alboran Sea basin north of Al-Mansour Seamount (ODP Site 977), normal faults are observed in Miocene and Early Pliocene sediments. Later folds affect Pleistocene-Recent sediments in the eastern Alboran Sea basin (Site 977). After the folding event, there is evidence for normal faulting in Pleistocene-Recent sediments close to Site 977. South of Al-Mansour Seamount (ODP Site 978), compressional features in the eastern Alboran Sea basin from Miocene to Recent are evidenced by reverse faulting followed by folding. In the northern Alboran Sea basin (Andalucia A-1 well), there is evidence for strike-slip faulting in the Late Miocene that can be related to the Serrata fault system. We envision the development of the Alboran Sea basin through a southeasterly migration of the delaminating continental lithosphere to explain younger extension in the eastern Alboran Sea basin. The rate of the migration of the delamination front is of the range of millimeters-centimeters/year. We see evidence for the migration of the delamination front from Miocene to Recent as tectonic inversion occurs near Site 977. In contradiction to an extensional collapse hypothesis for the formation of the Alboran Sea, rifting did not end in the Late Miocene in the entire Alboran Sea basin. We exclude retreating subduction in a westward direction or a westward continental delamination as a model for Alboran Sea basin development because it would predict younger extension in the western part of the basin.

An Alternative Interpretation of Microseismic Events during Hydraulic Fracturing

It is common practice in industry to monitor hydraulic fracturing jobs by picking major, micro-earthquake events in seismograms whose source locations form a spatial pattern used for interpreting induced fractures. Surprisingly, controversy still surrounds the interpretation of these scattered, discrete events. Many authors conclude that hydraulic fractures are generated by shear failure events rather than tensile failures. This interpretation contradicts our common understanding of fracture mechanics, which describes the hydraulic fracture process as taking place predominantly in mode-I failure (pure tension). We propose that band-width limited instrumentation during seismic field recording may be partly to blame. Lowfrequency (<5 Hz) tensile-source events which are expected to occur continuously between shear events are largely ignored. Herein, we try to compare the total energy of detected shear events with the total energy of expected low-frequency tensile events in the background in order to justify the main (tensile) mechanism of fracturing. Major, shear-mechanism events may not describe accurately the temporal and spatial pattern of induced fractures in the subsurface. Shear events may only represent the locations where hydraulic fractures intersect pre-existing discontinuities. Therefore, by only considering shear events, we may not be able to make a correct estimation of the orientation and extension of the hydraulic fractures. We suggest that only by recording silent (low frequency) events, we will truly be able to describe induced subsurface fracture geometries.

Benchmark hydrogeophysical data from a physical seismic model

Theoretical fluid flow models are used regularly to predict and analyze porous media flow but require verification against natural systems. Seismic monitoring in a controlled laboratory setting at a nominal scale of 1:1000 in the acoustic frequency range can help improve fluid flow models as well as elasto-granular models for uncompacted saturated-unsaturated soils. A mid-scale sand tank allows for many highly repeatable, yet flexible, experimental configurations with different material compositions and pump rates while still capturing phenomena such as patchy saturation, flow fingering, or layering. The tank (~6x9x0.44m) contains a heterogeneous sand pack (1.52-1.7phi). In a set of eight benchmark experiments the water table is raised inside the sand body at increments of ~0.05m. Seismic events (vertical component) are recorded by a pseudowalkaway 64-channel accelerometer array (20Hz-20kHz), at 78kS/s, in 100- scan stacks so as to optimize signal-to-noise ratio. Three screened well sites monitor water depth (+/-3mm) inside the sand body. Seismic data sets in SEG Y format are publicly downloadable from the internet (http://github.com/cageo/Lorenzo-2012), in order to allow comparisons of different seismic and fluid flow analyses. The capillary fringe does not appear to completely saturate, as expected, because the interpreted compressional-wave velocity values remain so low (<210m/s). Even at the highest water levels there is no large seismic impedance contrast across the top of the water table to generate a clear reflector. Preliminary results indicate an immediate need for several additional experiments whose data sets will be added to the online database. Future benchmark data sets will grow with a control data set to show conditions in the sand body before water levels rise, and a surface 3D data set. In later experiments, buried sensors will help reduce seismic attenuation effects and in-situ saturation sensors will provide calibration values.

RESISTIVITY AND SHEAR WAVE VELOCITY AS A PREDICTIVE TOOL OF SEDIMENT TYPE IN COASTAL LEVEE FOUNDATION SOILS

Levee foundation soils in New Orleans, USA, are composed of unconsolidated Holocene deltaic sedim ents. Traditionally, geotechnical tests at point locations can identify the more unstable zones, but cannot predict accurately the laterally heterogeneous facies of the Mississippi delta. Together, electrical resistivity and seismic shear wave studies can aid in the interpretation of different soil types between geotechnical sites. In such highly conductive, coastal soils, resistivity measurements are limited to shall ow depths, but remain useful for describing variations in saturation and the presence of clays. Similar studies conducted in Japanese fluvial and Australian calcrete environments do not consider the influe nce of brackish water in coastal settings. The London Avenue Canal levee flank of New Orleans, which failed in the aftermath of Hurricane Katrina, 2005, presents a suitable site in which to pioneer these geophysical relationships in a coastal setting. Shear wave velocity and resistivity are related to soil properties through Hertz-Mindlin Theory and Archie’s Law. Preliminary cross-plots show electrically resistive, high-shear-wave velocity are as interpreted as low-permeability, resistive silt. In brackish coastal environments, low-resistivity and low- shear-wave-velocity areas may indicate both saturated, unconsolidated sands and low-rigidity clays. Publi shed polynomial approximations to similar cross-plots must be modified for use in the near-surface sediments of the Mississippi River Delta. We present new relationships between soil type, resistivity, and shear wave velocity to distinguish the three main sediment groups found in deltaic environments: sand, silt, and clays.

The competing effects of stress and water saturation on in situ Q for shallow (< 1 m), unconsolidated sand, evaluated with a modified spectral ratio method

A publicly available seismic dataset from a lab experiment shows the simultaneous dependence of quality factor (Q) on water saturation and stress in unconsolidated sand. Large Q gradients (e.g., > 10 m−1) necessitate a spectral ratio method modified to assume that Q changes with each ray path, thereby eliminating false Q values (e.g., < 0). Interval Q values (Qint) increase the most with depth (dQ/dz = 43 m−1) and stress (dQ/dσ = 0.0025/Pa) in dry sand and the least in partially saturated sand (dQ/dz = 10 m−1 and dQ/dσ = 0.0013/Pa) where attenuation created by local fluid flow reaches a maximum. Expected Qint values can be extrapolated from dQ/dσ and are bounded by Qint of the dry (Qdry) and partially saturated (Qwet) media (e.g., Qdry ≥ Qint ≥ Qwet). Qint deviations outside this range may be explained by changes in effective stress, attenuation mechanism, or sediment composition. Field values of seismic attenuation in natural settings may be helped by these constraints, although attenuation remains subject to careful consideration of other factors, e.g., grain size, sorting, and shape.

Soil-type Estimation beneath a Coastal Protection Levee, Using Resistivity and Shear Wave Velocity

Unconsolidated Holocene deltaic sediments comprise levee foundation soils in New Orleans, USA. Whereas geotechnical tests at point locations are indispensable for evaluating soil stability, the highly variable sedimentary facies of the Mississippi delta create difficulties to predict soil conditions between test locations. Combined electrical resistivity and seismic shear wave studies, calibrated to geotechnical data, may provide an efficient methodology to predict soil types between geotechnical sites at shallow depths (0- 10 m). The London Avenue Canal levee flank of New Orleans, which failed in the aftermath of Hurricane Katrina, 2005, presents a suitable site in which to pioneer these geophysical relationships. Preliminary cross-plots show electrically resistive, high-shear-wave velocity areas interpreted as low-permeability, resistive silt. In brackish coastal environments, low-resistivity and low-shear-wave-velocity areas may indicate both saturated, unconsolidated sands and low-rigidity clays.

Seismic Velocity Prediction in Shallow (<30 m) Partially Saturated, Unconsolidated Sediments Using Effective Medium Theory

ABSTRACT Seismic velocity models of the near-surface (<30 m) better explain seismic velocities when all elements of total effective stress are considered, especially in materials with large cohesive and soil suction stress such as clays. Traditional constitutive elastic models that predict velocities in granular materials simplify the effect of total effective stress by equating it to net overburden stress, while excluding interparticle stresses and soil suction stress. A new proposed methodology calculates elastic moduli of granular matrices in near-surface environments by incorporating an updated definition of total effective stress into Hertz-Mindlin theory and calculates the elastic moduli of granular materials by extending Biot-Gassmann theory to include pressure effects induced by water saturation changes and cohesion. At shallow depths, when water saturation increases, theoretically calculated seismic velocities decrease in clay and increase in sand because interparticle stresses suppress the Biot-...