Stage B - Hot Spot & Rifting

Hot Spot and Rifting

Underlying the stable Craton, from deep in the mantle a plume of hot mafic or ultramafic magma rises toward the surface and ponds at the base of the continent, creating a hot spot. Heat from the hot spot warms the continental crust, causing it to expand and swell into a dome 3-4 kilometers high and approximately a thousand kilometers in diameter. As the dome swells it thins and stretches like pulled taffy, until the brittle upper surface cracks along a series of three rift valleys radiating away from the center of the hot spot, forming a triple junction. Ideally the three rift valleys radiate from the center at 120o, but often the triple junction is not symmetrical and arms may diverge at odd angles. 
Mafic volcanism is normal and appears as intrusive sills, vent volcanoes and/or flood basalts from fissure volcanoes rising along feeder dikes. The volcanic rocks may be mostly volcaniclastic, or lava flows of vesicular and columnar-jointed basalt. Subaqueous pillow basalts are not unusual in later stages.
Mafic (hot spot) volcanoes are common and appear as vent volcanoes and/or flood basalts from fissure volcanos in the rift. Commonly the intense heat of the hot spot will fractionally melt the lower continental crust composed of granodiorites or plagiogranites. The results are alkali granitic magmas that rise to emplace as batholiths, frequently sending conduits to the surface to create large felsic volcanoes. The simultaneous formation of these two very different rock types (one from the bottom and one from the top of Bowen's Reaction Series) is called a bimodal distribution.
Active Rifting
Axial rifts are typically tens of kilometers across, and the elevation from the rift floor to the mountain crests on either side is as much as 4-5 km. Structurally, rift valleys are block-fault grabens bordered by horst mountains on either side. The edges of the major horsts bordering the axial graben are the continental terraces (also called hinge zones).
The major axial grabens contains numerous smaller horsts and grabens. The normal faults are listric in type. The fault surfaces are curved so that the graben blocks rotate as they subside, trapping small basins between the down-faulted block and the wall behind the fault. It is also typical for numerous, smaller lateral grabens to form for several hundred kilometers on either side of the axial graben. Initially the axial valley floor is subareal, that is above water (except for lakes), but in time the axial graben subsides and the sea invades, creating a narrow marine basin (making it subaqueous). 
Sedimentary rocks are deposited in the graben, mostly in short-system environments where facies changes are very rapid. The horst mountain highlands are composed of felsic and high-grade metamorphic continental basement, which erode rapidly to coarse, subareal arkosic breccias and conglomerates (fanglomerates) (red field on the QFL Diagram). All around the basin edges, at the base of the fault scarps, these accumulate in steep-faced alluvial fans. Away from the alluvial fans, toward the basin axis, the fans give way to braided rivers and then often lakes. See reconstructions for the Rifting of Pangaea.
The lakes are trapped depressions created when the graben floors drop and cause water to pond. Many of the lakes are very deep and, based on modern rift lakes, may be extremely alkaline with salt crusts floating on the surface. In the lake bottoms, black organic rich, anoxic clays accumulate because there is no circulation or oxygen in the deep water.
After the sea invades the rift, fan deltas develop. Here, alluvial fans still form next to the mountains, but now turbidity currents rather than braided rivers transport sediment toward the basin center. The basin center is still deep and anoxic, and thinly laminated black clays and silts are deposited. Thousands of meters of sediment may accumulate during this stage. 
In a geologically short time (~ 10 million years) the basin finishes filling. As the former great relief of the horst mountains and deep grabens smooth out, shelf and near-shore deposition takes over. Sands now dominate and abundant crossbeds and ripples indicate shallow water processes. By this time the Early Divergent Margin stage is beginning (Foundering of Rift Valley).

Contributed by Lynn Fichter 

Friday, August 24, 2018
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