Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Bibliography: leaves 422-441. / by Raymon Lee Brown, Jr. / Ph.D.

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Plate tectonic theory provides logical explanations for the major tectonic events in the eastern US during Paleozoic time. The details of these tectonic events are becoming more apparent with the accumulation of new data, especially radiometric age dates. When plate tectonic theory is applied to specific tectonic events for which there is no substantial evidence, such as intracontinental hotspot rifting environments and Precambrian subduction zones, the proposed models may become very speculative. A misconception concerning the geology of the central US is that this region is structurally stable. However, geologists are currently paying considerable attention to the interlocking network of faults that in a general way follow the 38th parallel of latitude from west-central Virginia into Central Missouri (and may extend farther to the east and west). Most of the displacement along this zone occurred during the Precambrian, but different parts have moved during several periods of post-Precambrian time. In the basement the lineament may be a wide fracture zone that extends deep into the crust and is thus responsible for the magmatic iron deposits of the Southeast Missouri and may be either directly or indirectly responsible for the localization of the Mississippi Valley type deposits that occur sporadically along its length. Whether or not plate-tectonic processes operated during the Precambrian is open to speculation and the lineament may or may not be related to plate tectonic activity, but it is obvious that throughout time inherent zones of weakness are important in the localization of ore deposits. The occurrence of several major mineral districts at the intersections of the 38th parallel lineament with other major structural features, particularly in some uplifted areas and fault zone intersections, suggests that other similar structural uplifts and fault-zone intersections should be investigated for undiscovered new districts or extensions of known districts. Small uneconomic mineral occurrences along fault zones intersecting the lineament may merit further examination as they may be indications of undiscovered deposits at depth. The overall tectonic environment in the Appalachian region was an important control on the localization of massive sulfide, gold, titanium, and tungsten deposits. The deposits occur in clusters, either in Late Precambrian spreading centers and associated rift systems related to the breakup of proto-Pangea, or in Eocambrian and Devonian low-potassium tholeiitic volcanic and plutonic rocks associated with the volcanic island arc systems which developed during the closing of the Iapetus Ocean. Feiss and Hauck (1980) are confident that moderate sized (1-10 million ton) massive sulfide deposits are yet to be found at depth in these regions of the southern Appalachians, but large (greater than 20 million ton) massive sulfide deposits are unlikely to exist. The Mississippi Valley carbonate-hosted deposits of lead-zinc-baritefluorite, that occur to some extent throughout the Paleozoic section, and the Silurian "Clinton" iron ores owe their origin and distribution to normal sedimentary and diagenetic processes resulting from the transgressions of the epeiric seas. Others, such as the residual deposits of managnese iron, and aluminum, owe their existence to the afore mentioned processes, but must also have had subsequent exposure to the concentrating mechanism of weathering in a stable environment. The Mississippi Valley type occur primarily around paleo-basement highs and paleoshorelines; therefore, the formation of domes and arches within the continental interior during bathygenic episodes was a major factor controlling the localization of these deposits. These broad upwarps were preferential sites for reefal development and facies changes, and, during epeirogenic periods, these positive features have resulted in erosion and karsting of the the carbonate rocks by meteoric waters and have thus been prepared for mineralization. Deposits of this type are most common below a pre-Middle Ordovician unconformity and should be sought along major domes and arches, and along major lineaments. The association of Applachian type deposits with arches is indeterminate because a structure as subtle as an arch would be difficult to detect following overprinting by the deformation of the Alleghany orogeny; however, there is no reason to suspect that this type of positive feature did not play a role in their location. In conclusion, plate movements were a major control on the Paleozoic tectonic history of the eastern US and were also the primary control on the localization of the base metal, gold, tungsten, chromite, and titanium deposits of the southern Appalachians. However, important sedimentary and diagenetic deposits were localized primarily by arch, dome, and basin development during bathygenic episodes. Whether these submergent episodes are the result of plate motion or whether plate motion is indirectly related to submergent episodes, as suggested by Sloss and Speed (1974), remains a problem that needs to be investigated and debated further. / KMBT_363 / Adobe Acrobat 9.53 Paper Capture Plug-in

Studies of the evolution of hotspot swells, mantle plumes, and Hawaiian basalts are presented in three parts in this dissertation. In part 1, the evolution of mid-plate hotspot swells are simulated numerically as an oceanic plate rides over a hot, upwelling mantle plume. The transient heat transfer equations, with time- and space-dependent boundary conditions, are solved in cylindrical coordinates. Geophysical data are used to constrain the models. Formation of the Hawaiian swell requires a mechanism of convective thinning of the lithosphere. The models constrain the Hawaiian heat source to have a maximum anomalous temperature of 250-300°C, and a perturbing heat flux 5-6 times the background value. On the other hand, the Bermuda swell is likely produced by heat conduction due to weakness of the heat source. In part 2, an analytic model of axisymmetric mantle plumes is presented. Plume parameters beneath the lithosphere, which are constrained from the swell models, are used to infer the plume source regions. The Hawaiian plume likely originates near the core-mantle boundary, but other hotspots may have shallower sources. Chemical plumes are much narrower than thermal plumes because of low chemical diffusivity in the mantle. For mantle plumes driven by combined thermal-chemical diffusion, the chemical signature of the source regions may only be observed near plume centers. Finally, melt generation and extraction along the Hawaiian volcanic chain are discussed in part 3. As a part of the plate moves over the heat source, melting largely takes place in the region where the lithospheric material is engulfed and swept away by the flow of the heat source. At least three mantle components must be involved in the melt generation: the plume material, the asthenosphere, and the engulfed lithospheric material. Significant amount of melts may also come from direct melting of the upwelling plume at depths below the initial plate-plume boundary. Melt extracts continuously from an active partial melting zone of 10-20 km thick, which moves outward as heating and compaction proceed. The models explain quantitatively the general characteristics of Hawaiian volcanism as the result of plume-plate interaction.

The Guerrero terrane of western Mexico is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of arc affinity. The arc assemblage rests unconformably on partially metamorphosed rocks of possible Triassic-Jurassic age. These "basement units," the Arteaga and Placeres Complexes and the Zacatecas Formation, are composed of deformed turbidites, basalts, volcanic-derived graywackes, and blocks of chert and limestone. Sandstones from the basement units are mostly quartzitic and have a recycled orogen-subduction complex provenance. They have negative ᵋNdi (-5 to -7), model Nd ages of 1.3 Ga., and enrichment in light REE, indicating that they were supplied from an evolved continental crust. The volcanic graywackes are derived from juvenile sources (depleted in LREE and ᵋNd = +6), though they represent a small volume of sediments. Primary sources for these turbidites might be the Grenville belt or NW South America. Basement rocks in western North America are not suitable sources because they are more isotopically evolved. Igneous rocks from the basement units are of MORB affinity (depleted LREE and ᵋNdi = +10 to +6). The Jurassic(?)-Cretaceous arc volcanic rocks have ᵋNdi (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. Sandstones related to the arc assemblage are predominantly volcaniclastic. These sediments have positive ᵋNdi values (+3 to +6) and REE with IAV-affinity. The Guerrero terrane seems to be characterized by two major tectonic assemblages. The Triassic-Middle Jurassic "basement assemblage" that corresponds to an ocean-floor assemblage with sediments derived from continental sources, and the Late Jurassic-Cretaceous arc assemblage formed in an oceanic island arc setting. During the Laramide orogeny the arc was placed against nuclear Mexico. Then, the polarity of the sedimentation changed from westward to eastward, and sediments derived from the arc-assemblage flooded nuclear Mexico. This process marks the "continentalization" of the Guerrero terrane, which on average represents a large addition of juvenile crust to the western North American Cordillera during Mesozoic time.

This study describes the tectono-stratigraphic and climatic record of the NE Arabian Sea during the Cenozoic.  Compilation of regional knowledge and subsurface observations has in this thesis provided new interpretations and insights into the records present along this passive margin.  The first is the rifting period (80-65 Ma) and the identification of a syn-rift volcanic sequence, comparable to other volcanic rifted margins.  This is followed by the record of a drift sequence (~65 Ma to present day), composed of extensive carbonate platforms and an infill sequence of siliciclastic deposits.  The analysed drift sequence (sink) is partly the result of the erosion of the hinterland (source) characterised by the India-Eurasia continent-continent collision.  Influence of regional climate and/or tectonic forces on the accumulation rate in the sink was tested, but not conclusive as the study area (Upper Indus Fan) covers only a limited part of the sedimentary record of the Indus Fan.  The thermal regime of the western margin of India is sparsely sampled, but once analysed, allows the definition of first order constraints on multiple rifting events.  The post-rift subsidence of the margin is slow and anomalous for >28 m.y. after break-up, potentially in relation with vigorous asthenospheric convection and a sharp ocean-continent boundary.  Past and present fluid flow is recorded in the sedimentary sequence of the Upper Indus Fan.  The first is related to gas hydrate occurrence and is the result of the migration of fluids by a plumbing system to the shallow subsurface, expressed by bottom-simulating reflections crosscutting stratal reflections.  A longer term fluid migration is recorded in this basin by the longest lived (~22 m.y.) mud volcano field recorded to date.

In northern British Columbia, the Sylvester Allochthon of the Slide Mountain terrane is the most inboard of Cordilleran suspect terranes, resting as a vast klippe upon miogeoclinal strata of the Cassiar Platform. The Sylvester is oceanic; it comprises gabbro, pillowed and massive basalt, banded chert, carbonate, argillite, ultramafics and minor arenite, which range in age from Late Devonian to Late Triassic. Internal structure in the Sylvester Allochthon is characterized as a stack of innumerable interleaved tectonic slices, bounded by subhorizontal, layer-parallel faults. These lithotectonic units are an order of magnitude smaller than the terrane itself and may consist of only a single or a few repeated rock types. The internal structure of the Sylvester is complex but not chaotic; small numbers of slices occur together in larger second-order packages which are also fault-bounded and lensoidal. However, tectonic juxtaposition of unrelated lithologies and older-over-younger faults are common. The "stratigraphy" of the Sylvester assemblage is thus tectonic. Sliver-bounding faulting within the Sylvester is known to have, at least in part, predated its post-Triassic, pre-mid Cretaceous emplacement. The Sylvester was emplaced onto North America as the roof thrust to a foreland-style duplex within underlying North American strata. vii viii The Sylvester Allochthon is the most inboard of accreted terranes, however it does not represent a simple marginal basin. New microfossil dating demonstrates that most rock types occur through the complete range of Sylvester ages. Coeval but depositionally incompatable lithologies must have accumulated in separate ocean floor paleoenvironments. Lithologies of the allochthon derive almost exclusively from layer 1, only the surface of oceanic crust. Thus, Sylvester slices are telescoped remnants detached from a vast area of ocean crust which ranged in age and width through the upper Paleozoic but which is now otherwise entirely consumed. Similarities of rock type, internal structure, age range, and regional tectonic setting have identified the Sylvester Allochthon as broadly correlative with a discontinuous series of terranes extending the length of the Cordillera. Together, these terranes may represent the remnants of what was once the late Paleozoic proto-Pacific ocean floor.

In this thesis, I present results that broadly fall into two themes. The first involves understanding active tectonic and magmatic processes at mid-ocean ridges. The second involves using small stress changes due to the tides to probe earthquake processes at mid-ocean ridges. The four main results of my thesis are as follow: (1) The spatiotemporal evolution of an eruption at a fast-spreading mid-ocean ridge, the East Pacific Rise, is now characterized and understood to be mainly controlled by the buildup of tectonic stress to a critical level rather than magma overpressure. (2) Microearthquakes at the East Pacific Rise are found to be strongly modulated by tides in the years before an eruption but not immediately after the eruption, suggesting the potential utility of tidal triggering strength for eruption forecasting. (3) Earthquake size-frequency distribution, often quantified using the b value, is shown to vary systematically with tidal stresses which lends support to the use of earthquake b value as an in-situ stressmeter. (4) The 2015 Axial Seamount eruption is revealed to be preceded by variable rates of melt influx into the shallow reservoir, highlighting the short-timescale variability of magmatic systems as they are primed for an eruption.

Graduation date: 1995 / Best scan available for figures.

Anomaly timescales for the last 90 million years, derived from marine magnetic profiles and published prior to mid-1979, are summarized, illustrated for comparison, and critically reviewed. A revised timescale is constructed using calibration points which fix the ages of anomalies 2.3', 5.5, 24, and 29. An equation is presented for converting K-Ar dates that is consistent with the recent adoption of new decay and abundance constants. The calibration points used in the revised timescale, named NLC-80, are so converted, as are the boundary ages of geologic epochs within the range of the timescale. NLC-80 is then used, along with recently acquired and rigorously navigated underway geophysical data from the region of the mouth of the Gulf of California, to prepare detailed bathymetric, gravimetric, and seismo-tectonic maps of the area. The basement ages at DSDP Leg 63 drilling sites 471, 472, and 473 are estimated from magnetic anomalies fit to timescale NLC-80. The estimates agree with biostratigraphically determined basement ages and support the proposal that an aborted ridge of about 14 MY age has left a small fragment of the Farallon Plate beneath the Magdalena Fan. Several large inactive faults are identified on the deep-sea floor west of the tip of the peninsula of Baja California. Additional magnetic anomaly profiles and bathymetric profiles across the Rivera Ridge are interpreted. These contradict the existence of a 3.5 MY old aborted spreading center on the Maria Magdalena Rise. Instead, it is proposed that an episode of subduction of the Pacific Plate beneath the southeastern tip of Baja California, concomitant with strike-slip faulting west of the peninsula, occurred and that this subduction may be responsible for the uncentered location of the Rivera Ridge within the mouth of the Gulf of California. A single magnetic anomaly profile obtained northeast of the Tamayo Fracture Zone is used to determine that the rate of Pacific/North American plate motion, for the last 3 MY is 68 km/MY at this location. This result, if correct, indicates that the peninsula of Baja California is separating from mainland Mexico faster than the Rivera Ridge is generating oceanic crust in the wake of opening in the gulf. This, in turn, requires that either slow diffuse extension is occurring presently across the Maria Magdalena Rise, or across the Cabo Corrientes-Colima region, or that the portion of North America south of the trans-Mexican volcanic belt is moving right-slip with respect to the North American Plate at a rate of 10-20 km/MY. Large horsts and many smaller continental fragments are found within the southern gulf. Several of them have active seismic boundaries, while others have apparently foundered. The gulf began to open approximately 14-15 MY ago with slow, diffuse block-faulting and the deposition of the Maria Magdalena Fan at the mouth of the gulf. Oceanic crust was exposed in the gulf by about 9-10 MY, at the same time that the Rivera Ridge began reorienting by clockwise rotation. Strike-slip motion along the Tosco-Abreojos Fault took up some of the Pacific/North American motion with the remainder occurring within the gulf itself. During this period the Pacific Plate forming within the gulf was slowly subducting beneath Baja California. By 4-5 MY subduction ceased and all of the Pacific/North American plate motion was shifted to the Gulf of California fault system. The gulf and peninsula of California are still in the process of adjusting to the change from Pacific/Farallon to Pacific/North American motion. / Graduation date: 1982