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

Receiver Function and Gravity Constraints on Crustal Structure and Vertical Movements of the Upper Mississippi Embayment and Ozark Uplift

The Upper Mississippi Embayment (UME), where the seismically active New Madrid Seismic Zone resides, experienced two phases of subsidence commencing in the Late Precambrian and Cretaceous, respectively. To provide new constraints on models proposed for the mechanisms responsible for the subsidence, we computed and stacked P-to-S receiver functions recorded by 49 USArray and other seismic stations located in the UME and the adjacent Ozark Uplift and modeled Bouguer gravity anomaly data. The inferred thickness, density, and Vp/Vs of the upper and lower crustal layers suggest that the UME is characterized by a mafic and high-density upper crustal layer of ~30 km thickness, which is underlain by a higher-density lower crustal layer of up to ~15 km. Those measurements, in the background of previously published geological observations on the subsidence and uplift history of the UME, are in agreement with the model that the Cretaceous subsidence, which was suggested to be preceded by an approximately 2 km uplift, was the consequence of the passage of a previously proposed thermal plume. The thermoelastic effects of the plume would have induced wide-spread intrusion of mafic mantle material into the weak UME crust fractured by Precambrian rifting and increased its density, resulting in renewed subsidence after the thermal source was removed. In contrast, the Ozark Uplift has crustal density, thickness, and Vp/Vs measurements that are comparable to those observed on cratonic areas, suggesting an overall normal crust without significant modification by the proposed plume, probably owing to the relatively strong and thick lithosphere

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 ( \u3c 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

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 ( \u3c 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

Backus and Gilbert inversion of two and one-half-dimensional gravity and magnetic anomalies and crustal structure studies in western Arizona and the eastern Mojave Desert, California.

An inversion method that solves for a parameter as a continuous function (Backus-Gilbert) is used to determine the lower boundary of a two and one-half dimensional body which causes either a gravity or a magnetic anomaly and then assesses the vertical accuracy (variance) and the lateral resolution (spread). Also the gravity and magnetic data are simultaneously inverted and this procedure improves the resolution of the model over that obtained by inverting either data set individually. The usefulness of the Backus-Gilbert approach is evaluated by a study of the inversion of both synthetic gravity and magnetic data. The convergence characteristics, the amount of data and the distribution of data, the role of incorrect or correct parameters and the integration technique was considered. Gravity and magnetic data from the Sanford Basin, North Carolina are used to illustrate the inversion procedure. The second and third part of this study concerns the interpretation of regional geophysical data to obtain an idea of the crustal structure of the eastern Mojave Desert in California and western Arizona. The analysis of gravity and magnetic data in the Mojave Desert included the construction of Bouguer gravity, magnetic intensity, low and high band pass, and horizontal gradient maps. Based on the 100-250 km band pass gravity filtered map, four terranes were described: (1) Colorado River extensional corridor; (2) eastern Mojave gravity low; (3) Granite Mountain fault and (4) Colorado Plateau. Gravity modeling, geologic mapping and seismic refraction data indicate that the eastern Mojave gravity low is caused by a combination of low density Mesozoic intrusive rocks emplaced at depths of 8-9 km and a thickening of the crust. The Colorado River extensional corridor is underlain by a high density mid-crustal mylonitic zone based on seismic reflection and refraction data and gravity modeling. Analysis of gravity, magnetic and magnetotelluric data along a profile in western Arizona included the construction of Bouguer gravity and magnetic intensity maps, gravity and magnetotelluric crustal models, and the depth to the Curie isotherm. Low Bouguer gravity values, shallow Curie point depths and low resistivity values in the Castle Dome and Aquarius-Mohon Mountains indicate the potential of geothermal resources. The gravity cross-section also suggests the presence of a mid-crustal mylonitic zone underneath the Buckskin and Rawhide Mountains

Geology and Geophysics of the West Nubian Paleolake and the Northern Darfur Megalake (WNPL-NDML): Implication for Groundwater Resources in Darfur, Northwestern Sudan

The recent delineation of a vastly expanded Holocene paleo-lake (the Northern Darfur Megalake which was originally mapped as the West Nubian Paleolake and here will be referred to as WNPL-NDML) in Darfur in northwestern Sudan has renewed hopes for the presence of an appreciable groundwater resource in this hyper-arid region of Eastern Sahara. This paleolake which existed within a closed basin paleo-drainage system might have allowed for the collection of surface water which was subsequently infiltrated to recharge the Paleozoic-Mesozoic Nubian Aquifer. However, the presence of surface exposures of Precambrian crystalline rocks in the vicinity of the paleolake has been taken as indicating the absence of a thick Paleozoic-Mesozoic sedimentary section capable of holding any meaningful quantity of groundwater. This work integrates surface geology and gravity data to show that WNPL-NDML is underlain by NE-trending grabens forming potential local Paleozoic-Mesozoic aquifers that can hold as much as 1120 km3 of groundwater if the sedimentary rocks are completely saturated. Nevertheless, it is advised here that recharge of the Nubian aquifer under WNPL-NDML is insignificant and that much of the groundwater is fossil water which was accumulated during different geological times much wetter than today\u27s hyper-arid climate in Eastern Sahara. Excessive extraction will lead to quick depletion of this groundwater resource. This will result in lowering of the water table which in turn might lead to the drying out of the oases in the region which provide important habitats for humans, animals and plants in northern Darfur

Crustal Thickness, Poisson\u27s Ratio, and Moho Sharpness beneath Central Tien Shan: Constraints from Receiver Function Stacking

The Tien Shan is an approximately 2500-km long, 300-500-km wide chain of mountains located in central Asia. It is considered as one of the most tectonically active mountain ranges in the world and is a classic example of intraplate mountain building. Previous seismic and potential field studies were mostly concerned with lateral variations of crustal thickness, and systematic determinations of crustal Poisson\u27s ratio (which can be computed uniquely from Vp/Vs, the ratio of the P- and S-wave velocities) and Moho sharpness beneath Tien Shan have not be performed. Here we report preliminary results of such determinations using all the broadband seismic data archived at the IRIS Data Management Center, and use potential field data to constrain the interpretations of the seismic data. Results to date suggest that the average crustal thickness beneath Tien Shan from stacking of receiver functions is about 53 km which is consistent with previous geophysical determinations. The study area is characterized by a large Vp/Vs of 1.81±0.025 and a large overall stacking amplitude of the P-to-S converted phases beneath most stations, suggesting a mafic crust which is separated from the mantle by a sharp Moho. The thickest crust (about 68 km) is found beneath the northwestern corner of the Tarim basin. Distribution of earthquakes suggests that this area is among the seismically most active in Tien Shan. Stations in the vicinity of the Naryn Basin east of the Talas-Fergana strike-slip fault show low Vp/Vs ratios and anomalously thin crust of about 42 km, probably suggesting delamination of the lower crust. This interpretation is consistent with the positive regional Bouguer gravity anomalies which suggest a thinned crust beneath the area

Magnetic Stripes of a Transitional Continental Rift in Afar

Magnetic stripes parallel to mid-ocean ridges are one of the most significant consequences of seafloor spreading, and have played an essential role in the establishment of the plate tectonics theory and the determination of seafloor spreading rates. Similar magnetic anomaly patterns have not been well documented subaerially in continental rifts transitioning into seafloor 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 first examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafloor 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 field is characterized by an ~10-km-wide linear negative magnetic anomaly that corresponds to a normal-polarity zone that is flanked 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 seafloor spreading may need to be re-evaluated in light of the evidence provided here

Detailed Magnetic Study of the Red Sea Propagator within Afar, Ethiopia: Implications for the Transitioning from Continental Rifting to Sea Floor Spreading

A detailed magnetic study across the Tendaho Graben (the Red Sea propagator within the Afar Depression, Ethiopia) revealed features that can best be interpreted as a continental rift undergoing oceanization. This NW-trending extensional structure is ~50 km wide and it is confined within well-developed NW-trending boarder faults that deform the 2 km thick and ~ 2 Ma basalt flows of the Afar Stratoids. Faults within the graben are inward dipping and the deformed blocks show inward tilting. The age of the basaltic flows becomes progressively younger inward from the boarder faults until it reaches ~30,000 years close to the rift axis. The rift is filled with up to 2 km lacustrine sediments. A gravity analysis has indicated a gravity maximum over the center and thickest part of the graben that is caused by a thick zone of mafic dikes that extends into the basement rocks. Total-field magnetic measurements over the same region as the gravity study indicates that the central part of the Tendaho Graben has a 10 km wide magnetic minimum, which exhibits a narrow zone (~3 km) of a relatively higher magnetic values that coincides with the region of hydrothermal activities. This magnetic geometry is similar in dimension and magnitude to that observed from magnetic stripes of typical mid-ocean ridges. Forward modeling of the magnetic data (combined with geochronological data) shows that the basaltic rocks within the magnetic trough were crystallized after 0.78 Ma under normal magnetic polarity. Finally, the width of the magnetic trough (10 km) and the age of basaltic rocks ( \u3c 0.78 Ma) indicate a spreading rate of ~ 0.64 cm/year. However, to achieve the ~50 km width of the Tendaho Graben which started opening ~1.6 Ma, a 2.4 cm/year spreading rate is needed between 1.6 and 0.78 Ma. This suggests that the spreading rate with Tendaho Graben is slowing down and extension within Afar is taken somewhere else

Gravity Analysis of the Crustal Structure within the Tendaho Graben, Central Afar, Ethiopia

The Afar region of Ethiopia lies at a triple junction where northern section of the East African Rift System, the Red Sea rift and Aden rift meet. Within the Afar region, the Red Sea and Aden rifts are propagating from the ocean onto thinned continental crust. The Red Sea propagator is within the Tendaho Graben, occurs in the central Afar and ends near Aseyta. The Tendaho Graben which formed approximately 1.8 Ma ago, is approximately up to 60 km wide, is filled with lacustrine sediments and basalt flows, and bounded by basaltic and rhyolitic flows. Despite the numerous geological studies of the Tendaho graben, there have been few studies of the subsurface crustal structures. To aid in determining the region\u27s crustal structure, a detailed gravity, magnetic and broadband seismic study has been undertaken. Detailed gravity data were collected using differential GPS methods along all available roads at intervals between 0.5 and 1 km to supplement the available gravity data. Preliminary analysis of the gravity data including the construction of anomaly and 2-D models indicates that the youngest sections of the rift has the thickest lacustrine sediments (1.6 km) but is defined by a Bouguer gravity maximum caused by a series of dense mafic dikes extending into the basement. This regional gravity maximum which is thought to define the location of the Red Sea propagator extends to Aseyta where it ends. When combined with a detailed total-field magnetic measurements and analysis, and geochronology data, the gravity data are interpreted that the Tendaho graben represents a slowing down of the current rifting and that the current extension in the Afar is taken up further to the east. Future work will be to construct two and three-dimensional models and to collect additional data to further define the crustal structure of the region