Abstract We present a reduced-to-pole, total magnetic intensity map derived from merged aeromagnetic surveys in and around the Gulf of Mexico. Most of the deep central Gulf crust has a magnetic pattern of orthogonally intersecting features similar to, and interpreted as, fracture zones and ridge segments of oceanic crust formed by seafloor spreading. This spreading or drift phase occurred after the primary synrift phase of continental stretching across the greater Gulf of Mexico region, and thus the ocean crust rests within a broader zone of stretched continental crust with Yucatán, western Florida, the southern USA, and eastern Mexico forming the surrounding continental margins. We identify three regional magnetic anomaly trends that can be used to constrain the Gulf of Mexico’s Late Jurassic through earliest Cretaceous spreading history. A central magnetic anomaly trend is interpreted to accord with the later increments and cessation of seafloor spreading, for which a stage pole of rotation is estimated. Two flanking magnetic anomaly trends to the north and south of the central one, respectively, occur just basin-ward from the inferred depositional limits of autochthonous Callovian-Early Oxfordian salt. These anomalies appear to define the landward limits of oceanic crust in the northern and southern Gulf, and probably lie in crust that is medial or Late Oxfordian in age. They have similar mapped patterns that can be reconstructed onto one another and hence are probably genetically related but separated by spreading. These landward anomalies are best fit around a different stage pole than the central anomaly; thus the rotation pole appears to have jumped during spreading in the Gulf. Seismic reflection data show that the two outer anomalies occur at the basement “step ups” to the oceanic crust or the basinward shoulders of the “outer marginal troughs.” Until specific magnetic source modelling is done on the outer anomaly pair, we favor an edge-effect interpretation caused by the intrusive interface between Oxfordian oceanic crust and serpentinized and exhumed subcontinental mantle, the latter of which we infer forms the step ups to the oceanic crust. In addition, the aeromagnetic map shows a north-south trending “Campeche Magnetic Anomaly” downslope from the western shelf edge of Yucatán that we argue helps to constrain the reconstruction of Yucatán along Texas at the start of the synrift stage. Thus, the aeromagnetic map provides vital insights into the kinematics of all three stages of the basin's development, namely the synrift, the drift, and the interpreted intervening transitional phase of crustal hyperextension/mantle exhumation along the Gulf’s magma-poor continent-ocean transitions.

The structure and the tectonic development of the southern Panama plate boundary have been derived from an interpretation of marine geophysical data, including GLORIA (Geological LOng Range Inclined Asdic) long-range side-scan sonar and seismic reflection profiles, most of which were acquired in 1989 from a cruise of the RRS Charles Darwin. The northern boundary of the Nazca plate runs within the continental margin of southern Panama. South of the Gulf of Panama this boundary, the Southern Panama fault zone, is predominantly left-lateral strike-slip and occupies an elongate sedimentary basin. South and southwest of the Azuero Peninsula the boundary becomes one of oblique subduction, with active formation of an accretionary complex. The eastern part of this accretionary complex slips around the bend in the overriding Panama block to the purely strike-slip portion of the plate boundary, where it ceases to accrete sediment. South of the Gulf of Panama, the fault zone is flanked on its southern side by a bathymetrie ridge, containing rocks of a high density, that was once a part of the Panamanian continental margin. The ridge has been displaced 140 km eastward by the motion between the Panama block and the Nazca plate. The eastern end of this ridge is being subducted beneath South America, and at the ridge crest, the deformation front of the Colombian accretionary complex meets the Southern Panama fault zone. The inactive trench, filled with sediment, that lies at the foot of the Southern Panama continental margin owes its existence, in the west, to the downward flexure of the Nazca plate beneath the overriding Panama block at the oblique subduction boundary, and in the east, where the lithosphere of the Nazca plate is “broken” along its transform boundary, to the flexural load of the displaced basement ridge. A continuation of the Southern Panama fault zone runs southeastward behind the Colombian accretionary complex, separating it from forearc basin sediments deposited on the “continental” basement of northernmost Colombia. Deformed mud diapirs indicate a component of left-lateral strike-slip motion on the fault zone. This pattern of tectonics around this northernmost corner of the Nazca plate has probably been active since the collision of Panama and South America about 3.5 Ma.

Seismic reflection profiles and SeaMARC II imagery from the southwest Panama margin demonstrate that oblique convergence is presently occurring along what had previously been thought of as a transform margin. Our seismic profiles image landward-dipping thrust faults and seaward-verging folds at the toe of the slope. The frontal deformation zone as imaged on the SeaMARC II mosaic is 12 to 15 km wide with individual east-west-trending folds and thrusts that are laterally continuous for 5 to 10 km. Much of the terrigenous trench sediment is offscraped and accreted, forming an accretionary prism (South Panama deformed belt). Three linear ridges (part of the Panama Fracture Zone complex) are being obliquely subducted along the southwest Panama margin. The oblique convergence causes the ridges to sweep eastward along the trench. The SeaMARC II mosaic shows that the regional structure of the South Panama deformed belt is dominated by east-west-trending trench segments that are separated by the north-south fracture zone ridges. The trench shallows where the ridges intersect the trench, and the deformation front is warped around the ridges. On the east side of each ridge the accretionary complex bends to a northwest-southeast trend, suggesting that the ridges are deforming the accretionary complex. As the accretionary prism rides up over each ridge, it thickens markedly. By the time the prism reaches the top of the ridge, its surface slope has been greatly oversteepened and large portions of accreted material slump into the trench. After passage of the ridge, the system returns to its “normal” state, and accretion resumes, adding the slumped material back into the accretionary prism. The accretionary prism is thus only temporarily disrupted by the subduction of the Panama Fracture Zone system ridges.

Gilbert (1877) proposed that the level of emplacement of laccoliths is controlled by the density contrast between rising magma and the weighted mean density of the overburden. For felsic laccoliths, his hypothesis is strongly supported by gravity surveys of a number of laccolith groups. Epizonal felsic laccoliths are consistently found to have zero density contrast with the host rocks. Constraining the emplacement level provides a basis for analysis of the growth of laccoliths. Mechanical analysis suggests that the diverse shapes of laccolithic intrusions observed in the field can be represented by a continuous series of intrusion modes between two distinct end members. The simplest end member is an epizonal intrusion formed by a single sill that acts mechanically as a vertical punch. Punched laccoliths are characterized by flat tops, peripheral faults, and steep or vertical sides. The other end member results from the intrusion of multiple sills stacked vertically in a fashion suggestive of a Christmas tree. The multiple-level loading results in plastic deformation of the country rock. Christmas-tree laccoliths lack peripheral faults and have a characteristic rounded dome appearance on the surface. The floor of these laccoliths may, or may not, sag. Gilbert’s (1877) ideal laccolith falls between these two end members. The end members of the laccolith growth series are treated as boundary value problems in continuum mechanics. Geometrically and materially nonlinear finite element analysis is used to solve the boundary value problems. Field observation, a physical model, and the theoretical models provide convergent answers to the mechanical analysis of the growth of laccoliths. As a check on the theoretical models, a gazetteer of the dimensions and locations of approximately 900 laccoliths is included. Of these, approximately 600 are located in the United States. If North America represents a statistically valid sample, then there must be between 5,000 and 10,000 laccoliths around the world.