The Jurassic Quiet Zone (JQZ) is a region of low amplitude, difficult-to-correlate magnetic anomalies located over Jurassic oceanic crust. We collected 1200 km of new deep-tow magnetic anomaly profiles over the Pacific JQZ that complement 2 deep-tow profiles reported in Sager et al. (1998). Our primary goals were to extend the correlation of deep-tow magnetic anomalies farther back in time, to evaluate the correlatability and repeatability of anomalies, and to refine the Jurassic geomagnetic polarity reversal time scale (GPTS). Correlations of anomalies were excellent over M34 and over supposedly older seafloor to the south of ODP Site 801. In contrast, the correlation in the region between M34 and Site 801 was difficult. Using anomaly correlation models, we made magnetic polarity block models to establish a revised Jurassic GPTS extending until 169.4 Ma. Age calibration was accomplished with radiometric dates from two ODP holes. Systematic changes in anomaly amplitudes occur along the survey lines with the amplitudes decreasing backward in time and then increasing again in the oldest part of survey area. The zone of the most difficult to correlate anomalies corresponds to a period of ~4 m.y. that appears to have an abrupt end. This low amplitude zone suggests unusual magnetic behavior during the Jurassic. It has been said that many of the larger anomalies are likely caused by changes in polarity, whereas smaller anomalies may be intensity fluctuations. Although it is impossible to identify which anomalies are caused by reversals and which are not, magnetization structures observed in ODP Hole 801C suggest that many of the smallest anomalies, particularly around Hole 801C indicate polarity reversals. We concluded that (1) the new data demonstrates repeatability and correlatability of the JQZ magnetic anomalies implying that they are seafloor spreading lineations and (2) good correlations made new GPTS models extending back to 169.4 Ma; and (3) the origin of the JQZ may be a combination of rapid polarity reversals in the Jurassic low magnetic dipole field and closely spaced, tilted magnetization structure in the oceanic crust.

The Jurassic Quiet Zone (JQZ) is a region of low amplitude, difficult-to-correlate magnetic anomalies located over Jurassic oceanic crust. We collected 1200 km of new deep-tow magnetic anomaly profiles over the Pacific JQZ that complement 2 deep-tow profiles reported in Sager et al. (1998). Our primary goals were to extend the correlation of deep-tow magnetic anomalies farther back in time, to evaluate the correlatability and repeatability of anomalies, and to refine the Jurassic geomagnetic polarity reversal time scale (GPTS). Correlations of anomalies were excellent over M34 and over supposedly older seafloor to the south of ODP Site 801. In contrast, the correlation in the region between M34 and Site 801 was difficult. Using anomaly correlation models, we made magnetic polarity block models to establish a revised Jurassic GPTS extending until 169.4 Ma. Age calibration was accomplished with radiometric dates from two ODP holes. Systematic changes in anomaly amplitudes occur along the survey lines with the amplitudes decreasing backward in time and then increasing again in the oldest part of survey area. The zone of the most difficult to correlate anomalies corresponds to a period of ~4 m.y. that appears to have an abrupt end. This low amplitude zone suggests unusual magnetic behavior during the Jurassic. It has been said that many of the larger anomalies are likely caused by changes in polarity, whereas smaller anomalies may be intensity fluctuations. Although it is impossible to identify which anomalies are caused by reversals and which are not, magnetization structures observed in ODP Hole 801C suggest that many of the smallest anomalies, particularly around Hole 801C indicate polarity reversals. We concluded that (1) the new data demonstrates repeatability and correlatability of the JQZ magnetic anomalies implying that they are seafloor spreading lineations and (2) good correlations made new GPTS models extending back to 169.4 Ma; and (3) the origin of the JQZ may be a combination of rapid polarity reversals in the Jurassic low magnetic dipole field and closely spaced, tilted magnetization structure in the oceanic crust.

The East African Rift System diverges in the Lake Natron Basin of Northern Tanzania and is a major zone of continental extension and crustal thinning with resulting in active tectonics and volcanism. The discovery of Acheulean technology in Olduvai Gorge and Peninj as well as the presence of significant volcanic centers, has made in the region subject to studies in various disciplines. However, lack of precise radiometric age constraints due to the complex geology of the region is a major drawback. The basin is bordered on the western side by an escarpment that contains thick sequences of volcanic (nephelinites, basanites, hawaiites, alkali basalts), volcaniclastic and lacustrine strata that predates 1.2 Ma. This thesis is based on 41 rock samples that were collected from two geological sections, the Endukai Kete (EK) and Waterfall (WF) sections and aims to establish a preliminary geomagnetic polarity time scale (GPTS) for the Natron Escarpment, together with establishing possible flow directions of the volcanic lavas within these sections. Nephelinites of EK section have an inferred NW-SE direction of flow, based on study of anisotropy of magnetic susceptibility. They record a normal polarity that most likely correspond to the Cobb Mountain Event (CMT; 1.187-1.208 Ma), although there is an 80-ka discrepancy between the CMT event and the dated lavas. The most probable source is the Mosonik that erupted nephelinitic lavas 1.28 Ma ago. The palagonitic tuff layer below the nephelinites displays reverse polarity and a NE-SW direction of flow. Due to the absence of approximately 200 m strata within the basanite series of the section, regional lithological correlation is used to constrain the GPTS pattern. Hajaro Beds of the Peninj Group to the north of the escarpment, postdates the Olduvai Event (1.71 to 1.86 Ma) and lacustrine strata of the escarpment for EK and WF sections are deposited over the same unconformity and share depositional similarities. Therefore, the lacustrine strata are correlative to Hajaro beds and the normal event observed within the basanite series of both sections is attributed to the Réunion Event (2.116 – 2.137 Ma). The establishment of a preliminary magnetostratigraphic sequence presented in this thesis demonstrate that the rift escarpment in northern Tanzania is suitable for paleomagnetic dating. Future studies should be conducted to establish a more detailed and constrained magnetostratigraphic section, which will be of great use in this part of the African Rift where radiometric dating has been challenging.

My dissertation research consists of two themes: (a) the analysis of Middle Jurassic - Early Cretaceous marine magnetic anomalies (M-anomalies) in order to construct a comprehensive geomagnetic polarity timescale and (b) the investigation of the upper oceanic crustal architecture using downhole geophysical logs. These themes were chosen to better understand how remotely-sensed geophysical signals elucidate the formation and evolution of oceanic crust. This revised Pacific-wide MGPTS model shows significant improvement in its reliability, exhibits global applicability, and highlights changes in the paleo-Pacific spreading regime. By integrating Atlantic Manomaly analyses with the new MGPTS model and reviewing previous seismic studies, we shed new light on the causes of a ubiquitously distributed ?Atlantic anomaly smooth zone? where little coherency among M5-M15 anomaly sequence is observed. For the second theme, I analyzed the architecture of 15 m.y. old superfast spreading East Pacific Rise crust drilled at Ocean Drilling Program Hole 1256D in the eastern Pacific. An intact upper oceanic crustal section was penetrated at this site to a depth of 1507 mbsf. In situ crustal architecture was mapped from resistivity imagery (electrofacies by Formation MicroScanner) combined with recovered cores and other logs. Highlights of this research are: (1) most of the extrusive section consists of massive flows and fragmented formations including breccias, which has important implications for the magnetic source layer and pathways of hydrothermal alteration; (2) the dike complex is composed of sheeted-dikes dipping away from the paleo-spreading axis consistent with submersible observations at other sites in the eastern Pacific; (3) the crustal construction processess from ridge axis to abyssal plain during 0-50 kyr time are consistent with previous seismic reflection studies based on the integration of our stratigraphy model with lava flow observations from the southern East Pacific Rise.