One of the geologic phenomena that shows up strikingly in space imagery is that of the structural deformation of crustal rocks which produces folds and faults. Warping 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 typical examples of space-observed folding from above.
Space imagery is well-suited to recognizing and interpreting 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 achieved in Landsat/SPOT-type imagery, 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). A group of folds, shown here in color, is at about the same scale as the above outcrop. 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. The folds shown above occur in supracrustal rocks, those above what is commonly called the "basement" rocks - those 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). 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 kilometers/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 Highway synclinal fold shown first on page 2-1 and reproduced here is such a case since it has one inclined limb (its western [left] segment) tilted down to the right (east), then an inflection point (rocks horizontal), and a companion limb inclined to the west. Such folds have limb pairs inclined in opposing directions.
We saw examples of large folds - both anticlines and synclines - in the Section 1 Exam you may have completed. There, the Folded Appalachians were highlighted. These folds tend to be elongated but do end, and are thus referred to as closed (or pitching) when viewed from above.
An obvious example are these next folds found in the Ouachita Moountains of Oklahoma-Arkansas (the full scene is shown near the bottom of 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 elongate folds plunging in two directions (causing a curvature at each end; thus closing) to areas where the folded units remain parallel for a considerable distance.
A similar style of folding is displayed in the Sierra Madre Orientale (fold belt) near Monterrey, Mexico, as shown in these two Landsat 7 ETM+ image. Some structural geologists consider this belt to be an extension of Appalachian folding, but inclined rocks of younger age (early Mesozoic) are present here. 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.
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). 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 anticline seen here eroded to create a cross-sectional view. 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 (as the writer excitedly witnessed when flying over that part of Iran in 1975 (the good ol' Shah days). Here is a more detailed look using a Landsat-7 ETM+ image: 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: 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 or diapirs (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. Sometimes anticlinal folds form ridges or linear mountains that have widely separated interfold segments (synclines may not be well-formed). An example is this next style of folding that makes up a decollement - a French term that describes the detachment of a sheet of upper crustal rock in which the folds are likened to creases in a slip rug that has been wrinkled. The area shown in this Landsat-1 image is in the Sichuan Basin of central China: 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. Even where carefully mapped, the size of such belts is so great that their overall characteristics are often hard to appreciate. This next image (Landsat-2, Band 6) illustrates again the value of large-scale or large region coverage: The Tapa Shan mountain belt in west-central China seems to have split, with the two western branches swerving south and north; such tectonic behaviour is unusual:
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 western edges 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.
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 of these high ranges (much like the coastal mountains, Sierra Nevada, and Cascades do in the United States; rain on the west, rain shadow [dry] on the east). 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.
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. The High Atlas has peaks reaching above 4500 m (13000 ft). Here it is seen from space in a Landsat mosaic: 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: It may come as a surprise to read this claim that the Anti-Atlas and Atlas Mountains of northwestern Africa are geologically tied to the Appalachian Mountains of North America. When Africa split from North and South America following the closing of the Iapetus Ocean, part of the rumpled collision area resulting from closure was detached onto the African plate. Here is a map of the Appalachians before the Pangaea breakup: 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. On the next page, we look at 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.
