This asymmetry was used to support an interpretation that there were large non-dipolar components to the geomagnetic field at the time.
Recent data sets support the interpretation that this directional change was progressive and therefore a result of very rapid motion of Laurentia from high to low latitudes rather than a stepwise change across non-dipolar reversals.
We present high-precision U-Pb dates from Midcontinent Rift volcanics that result in an improved chronostratigraphic framework for rift volcanics and unconformities that improves correlations as well as constraints on rift development. We use these dates in volcanostratigraphic context to temporally constrain a new compilation of Midcontinent Rift paleomagnetic poles. The U-Pb dates constrain the rate of implied plate motion more precisely than has previously been possible.
We apply a novel Bayesian approach to assess the rate of implied plate motion through inverting for paleomagnetic Euler poles. The path is particularly well-explained by a model wherein there is continuous true polar wander in addition to rapid plate motion that changes direction and slows ca. We propose that upwelling of the Keweenawan mantle plume was associated with an avalanche of subducted slab material and associated downwelling that drove fast plate motion.
This fast plate motion was followed by the Grenvillian orogeny from ca. Prolonged collisional orogenesis could have been sustained due to this strong convective cell that therefore played an integral role in the assembly of the supercontinent Rodinia.
Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. Forgot your password? Get help. Password recovery. Geology Page. Home Latest News Video. Debris Flow Dynamics. Sampling Hot Molten Lava. The MCR is also a large igneous province, a region of extensive volcanism associated with upwelling and melting of deep mantle materials. Calculating the volume of volcanic rock causing this positive gravity anomaly reveals something interesting about the MCR. It is also a large igneous province LIP , a region of extensive volcanism associated with upwelling and melting of deep mantle materials.
MCR volcanics are significantly thicker than those at other LIPs because magma was deposited within a narrow rift rather than across a broad surface. Stein et al. Reflection seismic data Figure 2a across Lake Superior [ Green et al.
The MCR is a basin between dipping faults that contains volcanic rocks up to 20 kilometers thick, overlain by about 5—8 kilometers of sedimentary rocks. The upper volcanic layers and overlying sedimentary rocks dip from both sides and thicken toward the basin center, indicating that they were deposited after extension ended, as the basin continued to subside.
Radiometric dating shows that the volcanic rocks are about 1. Another seismic line to the east shows a similar sequence, but with extension on the southern fault. Working backward from the geometry of the volcanic and sedimentary rock layers seen today provides an evolutionary model of the feature Figure 2b. The MCR began as a half graben filled by flood basalts. After rifting extension stopped, the basin further subsided, accommodating more flood basalts.
After volcanism ended, subsidence continued, accompanied by sedimentation. The crust depressed and flexed under the load of the basalt and sedimentary rocks.
Long after subsidence ended, the area was compressed—a process called basin inversion—which reversed motion on the faults and activated new ones. The original crust thinned during rifting as the crust stretched, rethickened during the postrift phase because of the added volcanics and sediments, and thickened further because of the compression when the basin inverted.
If the model is correct, then crustal thickness along the MCR should vary with the amounts of extension and volcanism and the amount and direction of compression applied to that portion of the MCR. Seismic models covering much of the rift are being developed with these data [ Shen et al. These studies have already yielded surprising results. The basalt rift fill is presumably denser than the surrounding crust, but the velocities of shear waves traveling through it are similar or slightly lower than those of the surrounding rocks.
Another surprise is that the formation of the MCR left little signature in the upper mantle. Although vast quantities of melt were extracted to fill the MCR, the mantle shows no significant seismic velocity anomalies [ Shen et al. Unless the mantle beneath the MCR was replaced after the MCR formed, melt depletion seems to have had little effect on seismic velocities.
Moreover, mantle flows or oriented magma bodies usually make seismic wave speeds vary depending on the direction the waves travel, so waves tend to go faster along currently extending rifts than across them. However, Ola et al. Current and future studies combining petrology, geochemistry, geochronology, paleomagnetism, plate motions, and magnetotellurics will continue to explore what these surprises mean for the evolution of the rift.
The MCR is an extraordinary feature that arose from an unusual combination of a continental rift and a LIP, illustrating that over a billion years of Earth history, even unlikely events can happen. For the MCR, we suspect that the rifting continent by chance overrode a plume or a region of anomalously hot upper mantle, so both active and passive rifting may have been at play. Initial modeling [ Moucha et al.
Bowles-Martinez and Schultz [] find a highly conductive anomaly below western Lake Superior and northwestern Wisconsin, extending to depths below kilometers, that may have been formed by a mantle plume and somehow persisted. A question currently being discussed involves how the magma source operated over a long period of rapid plate motion [ Swanson-Hysell et al. The MCR was previously thought to have failed—stopped extending—because of regional compression associated with the Grenville orogeny [ Cannon , ].
However, new age dating shows that most of the compression recorded by reverse faulting occurred long after extension and volcanism ended. Instead, it stopped spreading much earlier, once seafloor spreading between Amazonia and Laurentia was fully established. By analogy, failed rifts similar to the MCR will have thick crust even if they have not been inverted, and inverted ones will have the thickest crust. Similarly, the gravity anomaly should change from a low to become progressively more positive as the rift fails and is later inverted.
The MCR has many features similar to those observed at volcanic passive continental margins. Volcanic margins arise where continental rifting is associated with large-scale melting that gives rise to thick igneous crust. Hence, the MCR shows what a passive margin looked like in its early stages.
Lake Superior and the surrounding spectacular scenery in national, state, and provincial parks are underlain by the MCR. Rift valleys differ from river valley s and glacial valley s in that they are created by tectonic activity and not the process of erosion. Tectonic plate s are huge, rocky slabs of Earth's lithosphere —its crust and upper mantle. Tectonic plates are constantly in motion—shifting against each other in fault zones, falling beneath one another in a process called subduction , crashing against one another at convergent plate boundaries, and tearing apart from each other at divergent plate boundaries.
Two arms of the triple junction can split to form an entire ocean. The Atlantic Ocean, for instance, is a result of a triple junction that started in what is now the Gulf of Guinea on the west coast of Africa. Rift valleys can also form at transform faults, where tectonic plates are grinding past each other. The San Andreas is a transform fault that marks the roughly northward movement of the Pacific plate and the roughly southern movement of the North American plate. As tectonic plates move away from one another at mid-ocean ridges, molten rock from the mantle may well up and harden as it contacts the frigid sea, forming new oceanic crust at the bottom of the rift valley.
In the northern Mid-Atlantic Ridge, the North American plate and the Eurasian plate are splitting apart at a rate of about 2. Over millions of years, the Mid-Atlantic Ridge has formed rift valleys as wide as 15 kilometers 9 miles. Like many underwater rift valleys, the East Pacific Rise is dotted with hydrothermal vents. Geologic activity beneath the underwater rift valley creates these vents, which spew super-heated water and vent fluid s into the ocean.
Continental Rift Valleys. Very few active rift valleys are found on continental lithosphere.
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