The Central Indian Ocean region has heat flow higher than expected for its lithospheric age. This heat flow anomaly is thought to be associated with deformation of sediment and crust and high seismicity. To better constrain the nature of this deformation, we examine the spatial variation of the heat flow. Previous work suggested high heat flux also in the Wharton Basin to the east, which shows less seismicity and deformation. Using new values for lithospheric age from reinterpretation of the magnetic anomalies, we have reexamined the heat flow and found it no higher than expected, in contrast to the Central Indian Basin. This spatial distribution of heat flow highs and expected values is consistent with the pattern of seismicity and deformation and the predictions of the recent diffuse plate boundary model [Wiens et al., 1985] for the region.
An estimated total of over 20,000 feet of Palaeozoic sediments accumulated in the Bonaparte Gulf Basin. The thickest known continuous section is that in Bonaparte No. 1 Well, abandoned at 10,530 feet in Upper Devonian sandstone and shale. Rocks of the Basin margins are mainly sandstones and limestones (in part reef), whereas a thick shale section has been discovered in the deeper parts. Data from recent seismic surveys indicate that the seaward extension of the Basin is considerable and that a thick pile of sediments is preserved there.The Bonaparte Gulf Basin formed as a result of subsidence of the north-eastern part of the Kimberley Block along fault lines associated with the Halls Creek Mobile Zone. This zone borders the south-eastern margin of the Basin and trends north-east. One basement block, represented by the presentday Pincombe Range, remained relatively high. The Bonaparte Gulf Basin can be divided into two subsidiary basins, the Carlton Basin to the west and north-west and the Burt Range Basin in the east and south-east. The Pincombe Range separates the two.Marine sediments were deposited in the Carlton Basin during the Middle and Upper Cambrian, Lower Ordovician, Upper Devonian and Lower Carboniferous epochs. Angular unconformities have been mapped between the Lower Ordovician and Upper Devonian rocks, and between Upper Devonian and Lower Carboniferous rocks. In the Burt Range Basin, deposition began in the Upper Devonian and continued with minor breaks through the Lower Carboniferous. Faults along the south-eastern margin were active through this period and affected the character of the sediments.Permian sediments are widely distributed and lie with unconformity on older units.
We review data and models for the recent tectonics of the region in the northern Indian Ocean from the Central Indian Ridge to the Sumatra Trench, long considered an anomalous tectonic region.Seismicity is much greater than expected for an area not traditionally considered to be a plate boundary, and the crust and sediments are deformed by both folding and faulting.Recently these phenomena, previously considered examples of deformation within a nonrigid Indo-Australian Plate, have been explained in terms of rigid plate tectonics.Analysis of relative plate motion data along the Carlsberg and Central Indian Ridges indicates the presence of distinct Indian and Australian plates.The seismicity and deformation thus reflect a diffuse boundary between these two rigid plates.This plate motion model eliminates the nonclosure of the Indian Ocean triple junction.The kinematic models, which describe the present-day plate motions, do not directly address the mechanics of the formation of the diffuse plate boundary.Further insight can be obtained from mechanical models of the stresses in the Central Indian Basin, including ones based on plate driving forces.The predicted stresses are consistent with the diffuse boundary model and distribution of deformation shown by seismicity, and by marine geophysical and satellite gravity data.Furthermore, they provide an explanation for the widespread pattern of compressional folding east, west, and on the Ninetyeast Ridge.These data and the kinematic and mechanical models provide insight into three enigmatic tectonic features: the complex blocky structure of the Ninetyeast Ridge north of 10°S, the trend of the southernmost 85°E Ridge, and the rapid recent subsidence of the southern Chagos-Laccadive Ridge.
We present shear-wave splitting analyses of SKS and SKKS waves recorded at sixteen Superior Province Rifting Earthscope Experiment (SPREE) seismic stations on the north shore of Lake Superior, as well as fifteen selected Earthscope Transportable Array instruments south of the lake. These instruments bracket the Mid-Continent Rift (MCR) and sample the Superior, Penokean, Yavapai and Mazatzal tectonic provinces. The data set can be explained by a single layer of anisotropic fabric, which we interpret to be dominated by a lithospheric contribution. The fast S polarization directions are consistently ENE-WSW, but the split time varies greatly across the study area, showing strong anisotropy (up to 1.48 s) in the western Superior, moderate anisotropy in the eastern Superior, and moderate to low anisotropy in the terranes south of Lake Superior. We locate two localized zones of very low split time (less than 0.6 s) adjacent to the MCR: one in the Nipigon Embayment, an MCR-related magmatic feature immediately north of Lake Superior, and the other adjacent to the eastern end of the lake, at the southern end of the Kapuskasing Structural Zone (KSZ). Both low-splitting zones are adjacent to sharp bends in the MCR axis. We interpret these two zones, along with a low-velocity linear feature imaged by a previous tomographic study beneath Minnesota and the Dakotas, as failed lithospheric branches of the MCR. Given that all three of these branches failed to propagate into the Superior Province lithosphere, we propose that the sharp bend of the MCR through Lake Superior is a consequence of the high mechanical strength of the Superior lithosphere ca. 1.1 Ga.
The Midcontinent Rift has characteristics of a large igneous province, causing geologists to rethink some long-standing assumptions about how this giant feature formed.
The magnitude of inferred depth and heatflow "anomalies" at hotspot swells relative to "normal" seafloor plays a major role in constraining the causes of these swells. Hotspot heatflow anomalies were first believed to be large and consistent with the uplift expected from thermal thinning of the lower lithosphere. However, these anomalies were overestimated because reference thermal models predicted greater depths and lower heatflow than was typical of lithosphere older than 70 Ma. In contrast, models GDH1 and GDH2, derived by joint fitting of heatflow and bathymetry (and geoid slope for the latter) yield a hotter and thinner lithosphere than previous...
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