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You are browsing the archive for tectonics Archives - AGU Blogosphere.

16 April 2021

Listening to avalanches half a state away

High on the broken pyramid of Iliamna Volcano, rotten rock held in place by volcano-warmed ice sometimes loses its grip. Several times over the years, rock-and-ice avalanches have blasted down Iliamna at 150 miles per hour. Left behind on the mountain’s face is a dirty, five-mile scar, in the same place as the last one.

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11 January 2021

The Ray Sponaugle well: A 13,000-ft lesson in Appalachian Valley and Ridge structure

“To the surprise of the drillers and geologists involved with the project, the well bore never got anywhere close to the Cambrian quartzite. At 10,000 ft (3,010 m) below the surface, the well passed through a thrust fault and entered a tight, nearly recumbent syncline cored by the same Ordovician shale unit into which drilling began.”

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4 December 2020

Lidar hillshade imagery hints at the location of a future coal spoil landslide

A coal spoil landslide in southeastern Wise County, Virginia, appears traceable to a faint scarp visible in the spoil pile in a 2017 lidar dataset. The slide pre-dates October 2019 Google Earth imagery and post-dates the 2017 lidar data acquisition.

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15 October 2020

Normal fault, reverse fault, or both?

The new model, whose color scheme is admittedly quite shocking (think Pepto-Bismol bottle), is shown… The interesting fault is at the center of the image. The fault is traced in black in the lower image, with arrows indicating movement sense.

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14 September 2020

The interesting geologic setting of Thomas Cole’s “The Oxbow”

New from The Geo Models blog: “Earlier this year, I became aware of the longer, geographically-specific title and learned that the painting does portray a real location with a particularly interesting geologic context…. Cole’s vantage point on Mt. Holyoke is east-northeast.

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28 August 2020

The Geo Models: Landslides associated with historic iron mining in the Virginia Valley and Ridge

The sharpness of these landslide features suggests they may still be slowly moving, but very little disruption to vegetation is visible in satellite imagery, so movement is probably very slow. Since their maximum age is known (the time of mining; late 1800s-1920s), they offer interesting comparison to older, natural landslides in the area, which tend to have softened, rounded features due to weathering and erosion.

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28 July 2020

Fault-propagation folds in a sandbox model

New from The Geo Models: “These anticlines are recognizable as fault-propagation folds because the fault that offsets the deepest blue layer does not cut upward through the entire section. Displacement along the fault at depth is accommodated by folding of the overlying, un-faulted layers.”

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24 July 2020

Earthquake adds missing piece to puzzle

Late in the evening of July 21, 2020, State Seismologist Michael West heard a text alarm. His phone informed him of a large earthquake beneath the ocean, just south of the Alaska Peninsula, about 60 miles southeast of the village of Sand Point. His first thought was that this — the biggest earthquake on the planet so far in 2020 — would cause a devastating tsunami. His second thought was that a longstanding earthquake mystery may have just been solved.

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17 July 2020

Anticlines and synclines in exhumed model rift basins

Several Mesozoic rift basins have been exhumed along the Atlantic margin of North America, creating interesting patterns in sedimentary rock layers and igneous intrusions that originated during the breakup of Pangaea.

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15 May 2020

A simple rift basin sandbox model with normal faults

A simple model of a continental rift basin that develops some characteristics of the real thing can be made by constructing a layered sand cake on top of two overlapping sheets of paper, one of which is anchored to the underlying board, etc. This model setup will produce an asymmetric half-graben style of basin, which has a single, high displacement breakaway fault on one side and several smaller faults on the other.

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22 April 2020

Listric normal faults with glass microbeads

Latest from from The Geo Models: Deformation associated with listric (downward flattening) normal faults produces very interesting patterns.

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7 April 2020

Interesting reverse faults in a simple extensional sandbox model

The different mechanical properties of the layers are apparent in the dip angles of the normal faults in the model. The master fault on the left side of the model (black line) is less steep in the weak microbeads, an expression of how their failure behavior differs from the stronger layers above and below.

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25 March 2020

Cracked mountaintops Part 2: Sinking summits?

Some upper Devonian sandstone mountains in the Virginia Valley and Ridge show evidence of deep-seated landsliding, resulting in the formation of a downthrown block (graben) along the summit ridge.

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20 March 2020

Cracked mountaintops of the Virginia Valley and Ridge

A newly-released LiDAR data set reveals impressive ridge-top cracks associated with large rock slides in the Virginia Valley and Ridge. While the cracks are easily visible with LiDAR hillshade imagery, they appear to be covered by normal forest vegetation and would probably look like elongated depressions in the forest.

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10 March 2020

A sandbox model without fault(s)?

Fold-thrust belts (both real and model, like this one) develop fault and fold patterns that reflect the properties of the rock (or sand-like materials) being deformed. The model section shown (shown here) is interesting because it results from shortening a granular layer sequence by 50% and does not show any major thrust fault structures that cut through all of the layers…

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9 March 2020

‘Remarkable groove’ slides gold into Alaska

In the 160 miles between the towns of Eagle and Circle, a half-dozen gold-mining settlements — most of them ghosted out — were on the south bank of the Yukon River. Not one was on the north side. That seemed like more than a coincidence.

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17 February 2020

Outcrop patterns in a fold-thrust belt analog model, round 1

The model shown here did not work out as planned because I shortened it too much, but the overall appearance is still cool and reflects local variations in the layer pack. In real fold-thrust belts, the local or regional variations in folding and faulting style also reflect the details of the layer sequence being folded and faulted, among many other conditions.

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23 January 2020

Discount inverted basin models: You get what you pay for (mostly)

A basic Google search of “inversion geology” will produce a tremendous number of results, including conceptual illustrations, analog model results, and actual cross sections generated from subsurface imaging and drilling exploration.

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13 January 2020

An abandoned river valley and the “true depth” of Appalachia’s deepest river gorge

This post centers around an oddly meandering, dry valley 800 ft above the northwest wall of the gorge. It’s not much to look at in Google Earth imagery alone, but with enhanced maps and some understanding of how river gorges spread through topography, it gives great context for different ways of thinking about gorge development.

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6 January 2020

Surface uplift and deep fault structure in sandbox models

A particularly interesting method of attempting to understanding deep fault geometry is using patterns of surface landscape evolution to identify the moving zone of uplift above a deep fault ramp. A useful analogy for this concept is to visualize sliding a spatula underneath a cooking egg.

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