Kevin L. Mickus

Geophysical project in Ethiopia studies continental breakup

As continental rift zones evolve to sea floor spreading, they do so through progressive episodes of lithospheric stretching, heating, and magmatism, yet the actual process of continental breakup is poorly understood. The East African Rift system in northeastern Ethiopia is central to our understanding of this process, as it lies at the transition between continental and oceanic rifting [Ebinger and Casey, 2001]. We are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts, a region providing an ideal laboratory to examine the process of breakup as it is occurring. EAGLE is a multidisciplinary study centered around the most advanced seismic project yet undertaken in Africa (Figure l). Our study follows the Kenya Rift International Seismic Project [e.g., KRISP Working Group, 1995],and capitalizes on the IRIS/PASSCAL broadband seismic array [Nyblade and Langston, 2002], providing a telescoping view of the East African Rift within this suspected plume province.

The complete gravity gradient tensor derived from the vertical component of gravity: a Fourier transform technique

Abstract A technique has been to developed to determine the complete gravity gradient tensor from pre-existing vertical gravity data using the fast Fourier Transform (FFT). Since direct measurement of the entire gravity gradient tensor is generally unavailable, our technique provides an alternative determination of the gravity gradient tensor components. Traditionally, derivatives of vertical gravity ( g z,x , g z,y , and g z,z ) have been the only gravity gradient tensor components that have been computed directly. Gravity gradient tensor components are computed for four different, three-dimensional (3-D), idealized horst-and-graben models, with varying depths to the horst. Comparing the FFT results with calculated gradient components from the 3-D models shows that the RMS error for each component, between the two results, is at most ∼3.3 Eotvos Units. In addition, measured gravity gradient components from an airborne survey over the Wichita Uplift and Anadarko Basin region of southwest Oklahoma compare favorably with the FFT-derived results using available vertical gravity data. No error analysis was attempted between the two results due to a low signal-to-noise ratio in the measured data. Our technique offers a novel way to transform and visualize the available data, and it also offers an inexpensive and previously unavailable subsurface mapping capability.

Lithospheric structure of the south-central United States

Recent seismic data in the Ouachita Mountains area and the Gulf of Mexico make it possible to construct a lithospheric-scale cross section (transect) from the midcontinent region to the gulf. The authors constructed a transect in the form of a gravity model, but it incorporates all available seismic, drill hole, and geologic data as constraints. The thrust sheets of the Ouachita orogenic belt appear as a thin veneer covering the southern part of the Arkoma basin and the preserved Paleozoic continental margin. Mesozoic rifting is evident in three areas: (1) southern Arkansas and northern Louisiana where extension was minor, (2) the vicinity of the Texas-Louisiana coastline where modification of the lithosphere and subsidence were considerable, and (3) the deep Gulf of Mexico where rifting was successful. A significant variation in the average density of the mantle, which could delineate the North American craton as a lithospheric feature, was detected near the Paleozoic continental margin.

Regional gravity analysis of the crustal structure of Tunisia

Abstract Gravity data were integrated with seismic refraction/reflection data, well data and geological investigations to determine a general crustal structure of Tunisia. The gravity data analysis included the construction of a complete Bouguer gravity anomaly map, residual gravity anomaly maps, horizontal gravity gradient maps and a 2.5-D gravity model. Residual gravity anomaly maps illustrate crustal anomalies associated with various structural domains within Tunisia including the Sahel Block, Saharian Flexure, Erg Oriental Basin, Algerian Anticlinorium, Gafsa Trough, Tunisian Trough, Kasserine Platform and the Tell Mountains. Gravity anomalies associated with these features are interpreted to be caused either by thickening or thinning of Palaeozoic and younger sediments or by crustal thinning. Analysis of the residual gravity anomaly and horizontal gravity gradient maps also determined a number of anomalies that may be associated with previously unknown structures. A north-south trending gravity model in general indicated similar subsurface bodies as a coincident seismic model. However, thinner Mesozoic sediments within the Tunisian Trough, thinner Palaeozoic sediments in the Gafsa Trough, and a greater offset on the Saharian Flexure were required by the gravity data. Additionally, basement uplifts under the Kasserine Platform and Gafsa Trough, not imaged by seismic data, were required by the gravity data. The gravity model revealed two previously unknown basins north and south of the Algerian Anticlinorium (5 km), while the Erg Oriental Basin is composed of at least two sub-basins, each with a depth of 5 km.

Potential field evidence for a volcanic rifted margin along the Texas Gulf Coast

Potential fi eld data along the Texas portion of the Gulf of Mexico indicate a large-amplitude coast-parallel magnetic maximum and a smaller Bouguer gravity high. Models constrained by seismic-refraction data indicate that these maxima manifest a deeply buried volcanic rifted passive margin or other magnetic high in the outer transitional crust. Buried 12‐15 km, the source is 220 km wide, similar to the Voring Plateau in Norway and the U.S. East Coast. This margin, which formed during the opening of the Gulf of Mexico, differs in origin from the transform boundary of the northeast Mexico margin (Tehuantepec transform), and we infer a Jurassic triple junction related to the Borderland rift system, which is traceable as far as southeast California.

Magnetic stripes of a transitional continental rift in Afar

Magnetic stripes parallel to mid-ocean ridges are one of the most signifi cant consequences of seafl oor spreading, and have played an essential role in the establishment of the plate tectonics theory and the determination of seafl oor spreading rates. Similar magnetic anomaly patterns have not been well documented subaerially in continental rifts transitioning into seafl oor spreading centers. Here, using high-resolution magnetic data that were collected across the Tendaho Graben in the Afar Depression, Ethiopia, we document one of the fi rst examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafl oor spreading centers. The ~50-km-wide graben is the southernmost structural and geomorphological expression of the on-land continuation of the Red Sea propagator, which is taken to represent the Arabian-Nubian plate boundary within Afar. The graben is bounded by northwest-trending border faults, with the footwalls dominated by ca. 1.7 Ma basalts and the downthrown blocks constituting progressively younger basalts toward the center of the graben, reaching ca. 35 ka. The Tendaho magnetic fi eld is characterized by an ~10-km-wide linear negative magnetic anomaly that corresponds to a normal-polarity zone that is fl anked by two parallel, ~20-km-wide linear positive magnetic anomalies of reversed polarity. This work shows that magnetic stripes can be developed in transitional continental rifts before the development of oceanic spreading centers. The common assumption that magnetic stripes can be used to date the onset of seafl oor spreading may need to be re-evaluated in light of the evidence provided here.

Crustal Structure and Evolution Beneath the Colorado Plateau and the Southern Basin and Range Province: Results from Receiver Function and Gravity Studies

Over the past several decades, contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long-term stability of the Colorado Plateau (CP) and crustal thinning beneath the Basin and Range Province (BRP) in the southwestern United States. Here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations on the southwestern CP and the southern BRP. Our results reveal that the BRP is characterized by a thin crust (28.2 ± 0.5 km), a mean Vp/Vs of 1.761 ± 0.014 and a mean amplitude (R) of P-to-S converted wave (relative to that of the direct P wave) of 0.181 ± 0.014 that are similar to a typical continental crust, consistent with the model that the thin crust was the consequence of lithospheric stretching during the Cenozoic. The CP is characterized by the thickest crust (42.3 ± 0.8 km), largest Vp/Vs (1.825 ± 0.009) and smallest R (0.105 ± 0.007) values in the study area. In addition, many stations on the CP exhibit a clear arrival before the P-to-S converted phase from the Moho, corresponding to a lower crustal layer of about 12 km thick with a mafic composition. We hypothesize that the lower crustal layer, which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines, contributed to the long-term stability and preuplift low elevation of the Colorado Plateau.

Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8–15 km) and high heat flow (60–90 mW m−2) beneath a ~60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16–30 km) and the heat flow estimates are lower (30–50 mW m−2), corresponding to thicker crust (~40–50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement.

Gravity and aeromagnetic constraints on the extent of Cenozoic volcanic rocks within the Nefza–Tabarka region, northwestern Tunisia

Bouguer gravity and aeromagnetic data are analyzed to determine the extent of Miocene magmatism in the Nefza and Tabarka regions of northwestern Tunisia. Construction of magnetic intensity and enhanced analytic signal (EAS) maps indicated the existence of at least two regions containing probable subsurface igneous bodies that correlate to the small scattered igneous outcrops in the Nefza and Tabarka regions. Because of the lack of lateral resolution of the EAS techniques, 3-D magnetic and 2.5-D gravity models were constructed over the anomalies at Nefza and Tabarka. The final models indicate that the maximum depths of the igneous bodies are between 2.5 and 2.7 km with maximum widths between 15 and 22 km. The final models also indicate that the bodies are tabular with a combination of laccolithic and lopolithic shapes and were probably emplaced in the shallow levels of the crust (at least 3 km). These widths greatly expand the region of known Miocene magmatism in northwestern Tunisia. Combined with geochemical and petrological data of the surface volcanic rocks, the gravity and magnetic models imply a wider range of Miocene volcanic activity in northern Tunisia, probably related to a subduction zone.

Abandoned PbZn mining wastes and their mobility as proxy to toxicity: A review.

Lead and zinc (PbZn) mines are a common occurrence worldwide; and while approximately 240 mines are active, the vast majority have been abandoned for decades. Abandoned mining wastes represent a serious environmental hazard, as Pb, Zn and associated metals are continuously released into the environment, threatening the health of humans and affecting ecosystems. Iron sulfide minerals, when present, can form acid mine drainage and increase the toxicity by mobilizing the metals into more bioavailable forms. Remediation of the metal waste is costly and, in the case of abandoned wastes, the responsible party(ies) for the cleanup can be difficult to determine, which makes remediation a complex and lengthy process. In this review, we provide a common ground from a wide variety of investigations about concentrations, chemical associations, and potential mobility of Pb, Zn and cadmium (Cd) near abandoned PbZn mines. Comparing mobility results is a challenging task, as instead of one standard methodology, there are 4-5 different methods reported. Results show that, as a general consensus, the metal content of soils and sediments vary roughly around 1000mg/kg for Zn, 100 for Pb and 10 for Cd, and mobilities of Cd>Zn>Pb. Also, mobility is a function of pH, particle size, and formation of secondary minerals. New and novel remediation techniques continue to be developed in laboratories but have seldom been applied to the field. Remediation at most of the sites has consisted of neutralization (e.g. lime,) for acid mine discharge, and leveling followed by phytostabilization. In the latter, amendments (e.g. biochar, fertilizers) are added to boost the efficiency of the treatment. Any remediation method has to be tested before being implemented as the best treatment is site-specific. Potential treatments are described and compared.