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One of the geologic phenomena that shows up strikingly in space imagery is that of the structural deformation of crustal rocks which produces folds. These warped or arching of units tend to stand out because of differential erosion which leaves the more resistant layers as ridges rising above the more easily eroded units. Several examples of folds as seen from the ground are compared with two typical examples of space-observed folding from above.


Geologic Folds from Space

Space imagery is well-suited for us to recognize and interpret types of distortions of layered strata that produce such geologic structures as folds, faults, and fracture sets (joints). Some of these structures are so small that we must identify them on the ground. As an example of what lies below the resolutions achievable from space, consider this photo of a small outcrop exposing crumpled shale layers, that form miniature anticlines and synclines (this same pattern occurs on grander scales such as the major folds of the Appalachians).

Anticlines are upfolds (arched upwards) whereas synclines are downfolds (shallow- to steep-sided U shape). In a series of folds anticlines are always next to synclines that in turn are next to anticlines (unless disrupted by faulting). This ground photo shows three anticlines and 2 and a half synclines:

Small folds (three distinct anticlines, with two synclines between them, and the left limb of a third syncline) of sedimentary rocks exposed in a road cut.

A similar group of folds, shown here in color, at about the same scale.

A crumpled (folded) assemblage of thin rock layers seen in an outcrop; scale is that of about 20 feet across the scene.

On a somewhat larger ground scale, look next at the folding shown in this outcrop photo of volcanic ash layers in Japan. There is an erosional discontinuity (unconformity) that separates earlier folding in the lower half from folding (above) after later ash flows were deposited.

Folds in volcanic ash in a Japanese deposit; at least two periods of successive folding are evident.

The folds shown above occur in supracrustal rocks, those above what is commonly called the "basement" rocks that make up the underlying (generally older) core of a continent (in the craton) and are composed of metamorphic rocks mixed with igneous rocks. At one time, before reaching the present surface, these deeply buried rocks were hot and "soft" (plastic-like). Under those conditions, layers in these crystalline rocks are deformed by squeezing and crumpling of heated units under high pressure as metamorphism proceeds. Here is an example of this type of folding (technically, called ptygmatic); note the hammer for scale. This folding is normally not detectable by space sensors (except those with very high spatial resolution).

Small folds and warps in metamorphic rocks exposed at the surface.

However, larger fold features at regional scales often show obvious patterns of geometric curvature or displacement that stand out in relation (context) to neighboring features, as best displayed in space images covering extensive areas. We used the Waterpocket Fold in the preceding pages of this Section to introduce this idea. Folds that extend over large areas (e.g., a single anticline may be one or more miles in width and nearly as high) are quite evident in space imagery. But when seen on the ground, typically only a small part of the arching or downfolding is visible at any local exposure of the folded strata, so that it is usually necessary to measure variations in inclinations (called "dip" by geologists) at separated locations in order to perceive the full nature (as a fold) of such large structures.

Thus the Maryland fold shown above is such a case since it has a single inclined limb (position 1). Such folds have limb pairs inclined in opposing directions. If in traveling along the Maryland Highway the next set of layers is inclined in the opposite direction (position 2), the structure would be an anticline. But if one continues further (from 2) in the same road direction and then encounters (at 3) layers inclined in the original direction (like 1), the structure is a syncline (between 2 and 3). We saw examples of large folds - both antinclines and synclines - in the Section 1 Exam you may have just completed. There, the Folded Appalachians were highlighted. These tend to be elongated but do end, and are thus referred to as closed.

An obvious example are these next folds found in the Ouachita Moountains of Oklahoma-Arkansas (the full scene is shown on page 6-3). The Ouachitas are an extension of the Southern Appalachians (the fold belt in between is buried under the Mississippi Embayment). The folds range here from those nearly circular to folds plunging in one direction (causing a curvature at one end) to areas where the folded units remain parallel.

The western end of the Ouachita Mountains, a fold belt.

A similar style of folding is displayed in the Sierra Madre Orientale (fold belt) near Monterrey, Mexico, as shown in this Landsat 7 TM image. Some structural geologists consider this to be an extension of Appalachian folding, but inclined rocks of younger age are present here.

Folds in the Sierra Madre Orientale, Coahuila State, Mexico; Landsat 7 TM+ image.

Next, we take a quick look at classic folds in Australia, Iran and northwest Africa. Elsewhere in this Tutorial (e.g., in Section 8 on Radar; Section 17, page 17-3), we describe other examples of folding depicted at regional scales.

For the moment we will preview the radar expression of folding with this look at a SIR-A image of closed structures composed of metasedimentary rocks in the Hamersley Range of northwest Australia.

SIR-A radar image of folds in the Hamersley Range of Australia.

The full Landsat scene below covers part of the Zagros Mountains along the southwest coast of Iran by the Persian Gulf. These mountains consist mainly of elongate folds which arch upwards as anticlines and downwards as synclines. The anticlines here make up distinct landforms as high hills with central ridges that taper at either end (a condition referred to as a closed fold).

Full Landsat MSS image of the Zagros Mountains in southwestern Iran; the blackish, roughly circular patterns mark the outpouring of salt - see below.

A simple analogy is to imagine cutting a watermelon in half through its longest dimension and laying the flat side on the floor. From above it resembles some of these Zagros anticlines. If we cut through it again across the long dimension at the mid-pointer, the exposed cross-section through green outer skin, white rind, and reddish center would appear similar to the folded strata within the anticlines.

An aerial oblique photo showing the Zagros Mountains; note the cross-sectional appearance of an anticline exposed by a stream cut.

These elliptical anticlinal folds in western Iran comprise a belt that is unsurpassed anywhere else in the world for their symmetry, extent and quality of exposure. Here is a more detailed look using a Landsat-7 ETM+ image:

Anticlines in western Iran; Landsat-7 ETM+ 10 m resolution image; note that several anticlines have been breached to expose shale (gray, much dissected) in their central cores.

2-11: Why don't you see the synclines in the Landsat images? ANSWER

An unusual phenomenon occurs in parts of the Zagros Mountains. Go up to the first image of this region; note patches of very dark gray material. These show up better in this perspective image made from ASTER and DEM data:

Salt glaciers (dark gray) emanating from anticlines in the Zagros Mountains.

These dark features are outspillings of salt that have been called "salt glaciers". Rock beds composed dominantly of salt (NaCl) can be produced, often in thick layers, in a marine environment in which salinity exceeds a certain value and direct precipitation removes the mineral halite and sometimes other mineral species that make up the class evaporites. As salt beds become more deeply buried, the overburden pressure or pressures associated with folding cause the salt material to flow like a very thick liquid under conditions that produce "plasticity". The salt may be pushed upwards, piercing overlying rocks, making salt domes (often excellent traps for gas and oil). The salt may reach the surface and "pour out", moving slowly to make the "salt glaciers" observed here.

Many folds do not stand out as individuals but are a larger part of continuous folding (and faulting) called orogenic belts or, more commonly, fold belts. There is a more or less continuous fold belt, of varying widths, running from Alaska to the tip of South America. The term "Cordillera" can be applied to this general mountain trend. The belt system is formed along the zone of convergence in which the Pacific tectonic plate is subducted near the edge of the North and South American plates, causing sedimentary rocks along the edge of these plates to crumple (fold) and be lifted up into mountain chains. In South America, the belt makes up the Andes Mountains. In this Landsat image, near Santiago, Chile, the Andes belt has narrowed to less than 120 km, with high plains on either side.

The narrowed Andes fold belt in northern Chile; note that the east side of the Andes is desert-like whereas the Altiplano (high plains) on the west side is heavily forested, with grasslands (but to its west at lower altitudes the rain shadow effect as moisture is precipitated out by orographic rise of air over the mountains; note also the band of vegetation at the lower edge of two alluvial fans, which results from water flowing through the alluvium and emerging to nourish local natural vegetation.

One of the best exposures of a complexly folded mountain belt anywhere occurs in the Atlas Mountain system of northwest Africa. This group is part of the great orogenic belt that includes the Alps, Appenines, the Betic Cordillera (southern Spain), and other chains that we can trace eastward through Turkey into the Zagros Mountains. These belts began to form about 70 million years ago, when the Tethys Ocean (precursor to today's Mediterranean) started to close as the African Plate moved northward against the Eurasian set of plates. The orogeny climaxed in the late Cenozoic period and is still active.

This perspective sketch map of northwest Africa shows the Atlas Mountains in context with the coast of Morocco and the extension into Algeria. Beyond the left end of the map lies the Anti-Atlas segment of this belt. The map includes the Middle Atlas and High Atlas segments, and the Rif mountains east of Tangiers. The mountain chain cuts out moisture coming in from the Atlantic such that this orogenic effect produces the Sahara Desert to the east these high ranges (much like the coastal mountains, Sierra Nevada, and Cascades do in the United States).

Map of northwestern Africa.

The Landsat scene below covers part of the Anti-Atlas mountains of southern Morocco. In the upper left is a deformed and metamorphosed core of Precambrian rocks against which the tight disharmonic folds of lower to mid-Paleozoic rocks (center) have been shoved northward along thrusts. The white sinuous band against a fold ridge is a dry stream or wadi.

 Part of a Landsat TM image of the Anti-Atlas Mountains of Morocco.

2-12: What does part of this Moroccan scene remind you of structurally in some other part of the world? ANSWER

The Atlas mountain belt was also involved in the major closing phase of the Tethys Sea about 80 million years ago, when the African Plate shoved northward, producing the Alps and other European mountains. A portion of the Anti-Atlas is imaged below using three SWIR bands on Terra's ASTER (see Section 9). Again, the folded structures stand out. The color composite shows various colors associated with rock units: yellow, orange, green, and dark blue denote limestone, sandstone, gypsum, and granite respectively. This is another strong confirmation that IR and thermal remote sensing can distinguish and identify major rock types (where vegetation cover is sparse) with considerable validity.

ASTER image of part of the Anti-Atlas Mountains of North Africa.

The High Atlas has peaks reaching above 4500 m (13000 ft). Here it is seen from space in a Landsat mosaic:

A Landsat TM mosaic of the High Atlas Mountains; prepared by Weldon Beauchamp of Cornell University.

These are the highest mountains in Africa and resemble parts of the Alps except that the vegetation is distinctly different. Here is a view taken from Marracech in the south interior of Morocco that typifies the terrain of the Anti-Atlas; below it is a view within the High Atlas segment:

The Anti-Atlas Mountains

View from a valley within the High Atlas Mountains.

Further south in Africa, in Namibia, is the Branberg Massif, a 21 by 31 km dome that reaches 2.5 km (1.6 miles) in height. A granite batholith intruded into the sedimentary rocks along its side.

MODIS image of the Branberg Massif in the Namib Desert.

A cluster of plutons composed of dark igneous rocks are exposed in the Air Mountains of Niger. The light-toned areas are mainly sand-covered desert.

Dark intrusions in the Air Mountains; Landsat Band 5 image.

On the next page, we look at three illustrations of how we can detect faulting from ground offsets and, more subtly, from discontinuities of landforms. Following that, we illustrate the advantages of space imagery in picking out lineaments (usually fractures in the outer crust that may be faults), together with an appraisal of how these can often be misidentified and misinterpreted. We then close this Section with a practical example of how fracture analysis leads to a successful search for a valuable natural commodity water.

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Primary Author: Nicholas M. Short, Sr. email: [email protected]