Dale E. Bird

Gulf of Mexico tectonic history: Hotspot tracks, crustal boundaries, and early salt distribution

A Late Jurassic mantle plume may have generated hotspot tracks on the North American plate and the Yucatan Penninsula tectonic block as the Gulf of Mexico opened (ca. 150 Ma). The tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data. They are associated with high-amplitude, distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20° clockwise rotation of the Yucatan block or almost one-half the total rotation required to open the Gulf of Mexico Basin. The duration of track generation is estimated to have been about 8–10 m.y. or almost one-half the total time required to open the Gulf of Mexico Basin. Prior to this rotation, extension of continental crust over a 10–12-m.y. interval was the result of approximately 22° of counterclockwise rotation and crustal thinning. Autochthonous salt appears to be confined to the continental flanks of the hotspot tracks, confirming that salt was deposited during continental extension and not after ocean floor had begun to form. A prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge, which was created along the ocean-continent transform boundary as the basin opened. The eastern flank of this marginal ridge and the northernmost, easternmost, and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic-continental crustal boundary in the basin. Dale E. Bird consults in the oil- and gas-exploration industry as a potential fields specialist. Since 1981, his experience in the industry includes acquisition, processing, interpreting, and marketing geophysical data, including positions with Aerodat, World Geoscience, Marathon Oil Company, Digicon, and Aero Service. He earned a Ph.D. from the University of Houston in 2004, and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data. He is a member of several local and international geological and geophysical societies.Kevin Burke has been a professor of geology in the University of Houston since 1983, having previously lived and worked in several parts of the world. His main research is in tectonics: The large-scale evolution of planetary lithospheres. His current interests include African and Asian geology and the derivation of large igneous provinces from the core-mantle boundary. Stuart A. Hall is a professor of geosciences specializing in the study of gravity and magnetic fields. He received a B.Sc.(honors) degree in physics from the University of Birmingham, United Kingdom (1968) and a Ph.D. in geophysics from the University of Newcastle-upon-Tyne, United Kingdom (1976). His current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts. He is a member of the Society of Exploration Geophysicists, American Geophysical Union, Royal Astronomical Society, and Sigma Xi Research Society. John F. Casey is a professor of geology and chair of the Department of Geosciences at the University of Houston specializing in structure, tectonics, and geochemisty. He received a B.A. degree in geology from the University of Pennsylvania in 1975 and a Ph.D. in geology with honors from the State University of New York, Albany, in 1980. His research has focused on the tectonics and geochemistry of midocean ridges, transform faults, and ophiolite-bearing orogenic belts.

Shear margins: Continent-ocean transform and fracture zone boundaries

As exploration extends into deeper water, it has become more important to understand the nature of the earths crust beneath offshore sediments. That is, heat flow and related source rock potential usually require that sediments be deposited over continental crust. Hence, delineating the boundary between oceanic and continental crust becomes important. The material in this article was originally presented at the workshop “The Crust and its Structure” at SEGs recent Annual Meeting in Calgary. These results, part of a larger dissertation research project, also have been presented to several oil and gas exploration companies in Houston.

Early Central Atlantic Ocean seafloor spreading history

Twenty-three Mesozoic “Chrons” (specific time intervals) from M0 to M40, including several in the Jurassic Magnetic Quiet Zone (“Jurassic Quiet Zone”), as well as Cenozoic Chron C34, are identified and mapped between the Atlantis and Fifteen-Twenty fracture zones on the North American plate, and between the Atlantis and Kane fracture zones on the African plate. Asymmetric seafloor spreading is indicated by the distances spanned over Chron intervals for the western and eastern flanks of the Central Atlantic ocean basin: C34 to the Mid-Atlantic Ridge (84 Ma to 0 Ma), M0 to C34 (120.6 Ma to 84 Ma), and M25 to M0 (154 Ma to 120.6 Ma). Chron M40 (167.5 Ma) is mapped ∼65 km outboard of the S1 magnetic anomaly over the African flank, and its conjugate, the Blake Spur Magnetic Anomaly (“Blake Spur Anomaly”) over the North American flank. Another pair of conjugate anomalies, the S3 magnetic anomaly over the African flank, and the East Coast Magnetic Anomaly (“East Coast Anomaly”) over the North American flank, are respectively located ∼30 km and 180 km inboard of the S1-Blake Spur Magnetic Anomaly pair. Therefore, the ridge jump to the east between “Blake Spur” and “East Coast” anomalies at ∼170 Ma theorized by [Vogt and others in 1971][1] is supported by this study. Between the Atlantis and Kane fracture zones, the width of the African Jurassic Magnetic Quiet Zone is ∼70 km greater (22%) than the North American Jurassic Magnetic Quiet Zone. Correlatable anomalies exist over the African plate, suggesting a second ridge jump, to the west. Modeling results indicate that this jump occurred between 164 Ma and 159 Ma (Chrons M38 and M32). The ridge jumps can be related to plate interactions as North America separated from Gondwana. However, we note that the second ridge jump occurred approximately at the time suggested for the onset of seafloor spreading in the Gulf of Mexico. [1]: #ref-40

Interpretation of magnetic anomalies over the Grenada Basin

The Grenada Basin is a back arc basin located near the eastern border of the Caribbean Plate. The basin is bounded on the west by the north-south trending Aves Ridge (a remnant island arc) and on the east by the active Lesser Antilles island arc. Although this physiography suggests that east-west extension formed the basin, magnetic anomalies over the basin exhibit predominantly east-west trends. If the observed magnetic anomalies over the basin are produced by seafloor spreading, then the orientation of extension is complex. Extension in back arc basins is roughly normal to the trench, although some basins exhibit oblique extension. Present models for the formation of the Grenada Basin vary from north-south extension through northeast-southwest extension to east-west extension. An interpretation of magnetic anomalies over the Grenada Basin supports basin development by nearly east-west extension. Low amplitude magnetic anomaly trends subparallel to the island arc magnetic anomaly trends over the southern part of the basin and the results of forward three-dimensional (3-D) magnetic modeling are consistent with this conclusion. Late Cenozoic tectonic movements may have been responsible for disrupting the magnetic signature over the northern part of the basin. On the basis of our 3-D analysis, we attribute the prominent east-west trending anomalies of the Grenada Basin to fracture zones formed during seafloor spreading at low latitude. This east-west trend is not interpreted as indicating north-south extension of the basin.

Chapter 15 Tectonic evolution of the grenada basin

Abstract Detailed analyses of gravity, seismic reflection and refraction data are integrated with an earlier interpretation of magnetic data to produce a coherent model for the tectonic evolution of the Grenada basin that suggests that the basin formed by near east-west extension. Although the seafloor of the Grenada basin changes from smooth and undisturbed in the south to rugged with relatively high relief in the north, Bouguer anomalies and two-dimensional and three-dimensional gravity models, based upon seismic refraction and reflection data, reveal that the crust gradually thins in an east-west sense towards the center of the basin. Typical back-arc crust is observed in the southern part of the basin, but refraction data are not sufficiently reliable in the northern part to adequately determine the nature of the crust. Several curvilinear discontinuities in magnetic, gravity and bathymetric trends are observed. These discontinuities, when integrated with two-dimensional and three-dimensional modeling and analyses of Bouguer gravity anomalies, are interpreted to be due to late Tertiary compressional forces in the northern part of the region. These compressional forces have resulted in the bifurcation of the Lesser Antilles island arc north of 15∘N, the westward displacement of part of the Aves Ridge (a remnant island arc), and the crustal deformation observed in the northern Grenada basin. The compressional forces also appear to have sufficiently disrupted the crust in the northern Grenada basin such that earlier magnetic anomaly patterns have been modified to yield the observed magnetic signature.

Reconstruction of the East Africa and Antarctica continental margins

The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana and Pangea. Previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean-boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate Antarctic margin near the Riiser-Larsen Sea. Satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Crustal thicknesses together with fracture zone terminations reveal COBs that are significantly closer to the African and Antarctic coasts than previously recognized. Correlation of fracture zone azimuths and identified COBs suggests Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. An areal-balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins. Both margins reveal a trend of increasing extension from east to west. Our results suggest Africa underwent extension of 60–120 km, while Antarctic crust was stretched by 105–180 km. Various models tested to determine the direction of extension during rifting suggest that Antarctica moved away from Africa in a WNW-ESE direction during the period between 184 and 171 Ma prior to the onset of seafloor spreading.

Pangea Breakup: Mexico, Gulf of Mexico, And Central Atlantic Ocean

Late Triassic breakup of the super-continent of Pangea (ca. 230 Ma) preceded the final assembly of Mexico, the birth of the Gulf of Mexico, and the formation of the Central Atlantic Ocean. Extensional rifting in passive margins essentially stops once new oceanic lithosphere is created. Therefore closing ocean basins along geomagnetic isochrons is an objective method for analyzing reconstructed continental margins. New finite-difference rotation poles define relative motions between North America and Residual Gondwana (Afro-Arabia and South America) for geomagnetic isochrons M0 (124.6 Ma or Early Aptian), M25 (154.1 Ma or Kimmeridgian), and particularly M40 (165.1 Ma or Late Bathonian) (Gradstein et al., 2004), which lies within the Jurassic Magnetic Quiet Zone (JMQZ) (Figure 1).

Aeromagnetic interpretation of basement structure using over 3,000 control wells, Central Alberta: A case history

The basement slopes down to the southwest from about 750 m to over 2,000 m bsl. Dominant basement structures are series of north-south trending inter-connected grabens and half grabens. These structures are interpreted to act as control for deposition of Granite Wash sediments as well as influencing structural and facies development on overlying sediments, particularly economically important Devonian and Carboniferous rocks.

Integrated geophysical interpretation of the Grenada Basin

The Grenada Basin formed via back arc extension in Early Tertiary time The north-south orientation of major physiographic elements along the eastern margin of the Caribbean Plate suggests that simple east-west sea floor spreading resulted in the formation of the Basin. However gridded magnetic anomalies over the Basin exhibit prominent east-west trends, suggesting north-south sea floor spreading. Present models for the formation of the Grenada Basin vary from extension oriented north-south, to extension oriented northeastsouthwest, to extension oriented east-west. An integrated interpretation of magnetics, gravity. seismic (both reflection and refraction), geologic, and well data for the Grenada Basin support near east-west extension for the formation of the Basin. Subtle, magnetic anomaly trends, subparallel to the Lesser Antilles island arc, over the southern portion of the Basin are consistent with this conclusion. Late Tertiary tectonic movements may be responsible for disrupting the magnetic signature north of 15’N.

Integrated seismic illumination and gravity modeling in refinement of a subsalt interpretation

Summary Top salt geometry exerts a strong effect on the illumination of both the base salt and on underlying subsalt reflectors. A 3D seismic illumination study can be useful for the interpretation of these events. It may also distinguish subsalt amplitude anomalies due to rock property changes (hydrocarbon saturation for example) from amplitude anomalies due to illumination differences. A 3D illumination study was carried out to assess the significance of an apparent bright spot on a key subsalt mapping horizon. The study indicated that the anomaly in question was probably not due to focusing effects. The illumination results also provided evidence for the existence of a salt root coinciding with the sudden disappearance of the base salt reflection event. The presence of the salt root is further supported by the results of a gravity modeling study.