Geologi struktur adalah bagian dari ilmu geologi yang mempelajari tentang bentuk (arsitektur) batuan sebagai hasil dari proses deformasi. Proses deformasi adalah perubahan bentuk dan ukuran pada batuan akibat dari gaya (force) yang terjadi di dalam bumi. Gaya tersebut pada dasarnya merupakan proses tektonik yang terjadi di dalam bumi. Di dalam pengertian umum, geologi struktur adalah ilmu yang mempelajari tentang bentuk batuan sebagai bagian dari kerak bumi serta menjelaskan proses pembentukannya.

Beberapa penulis menganggap bahwa geologi struktur lebih ditekankan pada studi mengenai unsur-unsur struktur geologi, misalnya perlipatan (fold), rekahan (fracture), sesar (fault), dan sebagainya, sebagai bagian dari satuan tektonik (tectonic unit), sedangkan tektonik dan geotektonik dianggap sebagai suatu studi dengan skala yang lebih besar, yang mempelajari obyek-obyek geologi seperti cekungan sedimentasi, rangkaian pegunungan, lantai samudera, dan sebagainya.(http://geodynamics.gc.itb.ac.id/index.php?option=com_content&task=blogcategory&id=21&Itemid=70)

I. Strike and dipare attitudes or characteristics in the rocks produced by geologic forces and present after the rocks are folded (bent) or faulted (cracked and moved along the crack appreciable distance)

A. Strikeis an imaginary line with compass direction constructed on the surface of a sedimentary bed or fault in which all points on that line are of equal elevation--the compass direction is usually expressed as a bearing

B. Dipis an imaginary line constructed down-slope on a sedimentary bed or fault--the dip direction is perpendicular to the strike direction and usually expressed in bearing and an angle of tilt (dip) measured from the horizontal plane to the top of a bed or fault--a dip angle may not exceed 90 degreesclick here to see strike and dip

II. Folded structuresare warps in rock layers and occur bended upwards, downwards, or sideways--compressional forces are the prime causes of folds—folds are important causes of mountain formation

A. Kinds of folds1. Anticlineis a series of up-arched strata with side portions (limbs) dipping in opposite directions away from the central portion of fold split by a plane called the axial plane and observed in the top (plan) view as the fold axis--an eroded surface indicates the rocks become progressively younger away from the fold axis2. Synclineis a series of down-arched strata with limbs dipping inwards in opposite directions towards the fold axis--an eroded surface indicates the rocks become progressively older away from the fold axis

B. Specific types of anticlines and synclines1. Symmetrical fold--is a fold with the sides showing a mirror image with respect to the axial plane2. Asymmetrical fold--a fold without a mirror image in respect to the axial plane3. Overturned fold--a fold in which the axial plane is tilted and beds may dip in same direction on both sides of the axial plane4. Recumbent fold--a fold with the axial plane horizontal5. non-plunging and plunging foldsplunging is the tilting of the fold towards the front or back end--all anticlines and synclines have some degree of plunge. in non-plunging folds contact lines separating formations shown in the surface view are parallel and straight---the contact lines in one of the two profile views are horizontal and parallel---the contacts in the other profile view are arched up or down--in plunging folds contact lines in the surface view are curved---contact lines in one of the profile views dip in the direction of plunge---contact lines in the other profile view are arched up or down--the surface contact lines between formations are convexed (closed) in the direction of plunge for the anticline and concave (open) in the direction of plunge for the syncline6. formation and location of petroleum and natural gas is associated with anticlines and synclines

3. Dome

is an up-arched series of strata with beds on all sides dipping away from the center throughout 360 degrees--an eroded surface indicates the rocks become progressively younger away from the center of the structure

4. Basinis a down-arched series of strata with beds on all sides dipping in towards the center throughout 360 degrees--an eroded surface indicates the rocks become progressively older away from the center of the structure5. Monoclineis a bend in the strata resulting in a local steepening in dip of the strata which is almost flat lying on both sides away from the bend-- there is only one direction of dip in the monocline

III. Fault structures

A. Definitionis a structure with major displacement of rock material along a crack in a rockB. Types of faultsthe types of faults are based on relative movement along the cracked rock and include horizontal, vertical, or a combination of movements—faults are other structures which can form mountains and mountain chains1. Vertical or dip slip faultis that type with movement along the dip of the fault--the concepts of hanging wall and footwall are important in defining the different types of vertical faultsif the angle of dip is not 90 degrees the side of the fault with the portion of land which appears to be hanging over a lower portion before movement is called the hanging wall and that on the bottom, the footwallNormal fault--hanging wall moves down in respect to footwallReverse fault--hanging wall moves up in respect to footwall--a low angled reverse fault is called a thrust faultHorst and graben--a wedge of land which moves up (horst) or down (graben) between 2 normal faults--best example is the Rhine River Graben in Europe2. Horizontal or strike slip faultis that type with horizontal movement along the cracked rock--movement can be left lateral or right lateral--best example of right lateral strike slip fault is the San Andreas fault in California--looking down the fault line in the front plan view, the right side of the fault appears to have moved towards you if a right lateral fault or left side moves towards you if a left lateral fault3. Oblique faultis a fault which has a major dip slip and strike slip component—see page 212 in text to see an example of an oblique slip fault

IV. Joint structuresA. Definition of jointsfractures in the rocks in which there is no appreciable displacement along the crack--often joints occur in 2 sets of cracks intersecting between 45-90 degrees dividing the rocks into rectangular blocksB. Causes of joints1. Unloading or sheeting effects (see Weathering)2. Stresses in a cooling magmacan cause columnar jointing in which there are 6-sided fractures comprising the jointing pattern--best example is Devil's Tower, Wyoming

V. Unconformity structuresA. Definitionis a surface of non-deposition or erosion which represents a break in the rock record--is a structure comprised of a sequence of geologic events in which there is often a significant portion of the geologic history lost through massive erosion--unconformities are used to help separate geologic time divisions

B. Kinds of unconformities 1. Disconformitya series of sedimentary rocks form, next a non-deposition or massive erosion event occurs, then a series of more sedimentary rocks are deposited2. Angular unconformitya series of sedimentary rocks form, next rocks are folded or tilted, next a non-deposition or massive erosion occurs, then a series of more sedimentary rocks form3. Nonconformityigneous or metamorphic rock form, next a non-deposition or massive erosion event occurs, then a series of sedimentary rocks form

VI. Importance of geologic structuresOil and natural gas are formed and found trapped in subsurface foldsFaults, joints, and fractures can act as a passageway for groundwater and a host for valuable mineral deposits as ores of gold, silver, copper etc.Unconformities can be used to mark geologic time boundaries for eras, periods, and epochs

(http://courses.missouristate.edu/emantei/creative/glg110/geostruct.html)

This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.

Summary Of Structural GeologyStructural geology brings vividly before us the innumerable changes through which the earth's surface has passed, and which are recorded in the rocks. The sedimentary rocks, originally laid down under water in approximately horizontal positions, have been upheaved into land surfaces, either without losing that horizontality, or being tilted, folded, compressed, or even violently overturned. Or, they may be fractured and dislocated in great faults and thrusts. These movements we have found to be due to enormous lateral compression set up within the crust of the earth, a compression generated in some manner not yet clearly understood. Whether folding or faulting shall result from a given compression depends upon the rigidity of the strata, upon the load which overlies them, and the sudden or gradual way in which compression is applied. The results of compression on a large scale are accompanied by certain minor changes not less characteristic. Compressed rocks are cleaved, fissile, or schistose, according to the intensity of the action, and whether the rocks affected are in the shell of flowage or of fracture. These changes may go so far as completely to reconstruct the minerals of the rocks, destroying the old, generating new, and obliterating the original character of the strata.

Thus, displacements, dislocations, cleavage, fissility, and dynamic metamorphism are but the varying results of lateral compression, acting under different conditions and at varying depths.

Another class of rocks - the igneous, massive, or unstratified - we found to have penetrated and overflowed the strata, and to have consolidated in the fissures and cavities which they have made for themselves, or to have been poured out freely on the surface. According to the circumstances under which these masses have cooled, the resulting rock is of glassy, porphyritic, finely or coarsely crystalline texture. When solidified as sheets or dykes, the igneous rocks may be folded, faulted, cleaved, or metamorphosed like the strata, and when a region has been long and repeatedly subjected to compression, its structure may become excessively complex, and the metamorphosis of its rocks so complete that not even the most careful examination will suffice to distinguish those rocks which were originally sedimentary from those which were igneous.

Highly heated waters circulating through fissures and along the joint-planes of the rocks deposit the substances which form the mineral and metalliferous veins, though concerning the source of these substances and of the solvent waters there is much difference of opinion.

Our study has taught us that many of these processes go on deep within the earth's crust, and hence cannot be directly observed, but must be inferred from their results. Encouraging progress has already been made in this work, but very much more remains to be done before our knowledge of structure and its full meaning shall be even approximately complete.

(http://chestofbooks.com/science/geology/Intro/Summary-Of-Structural-Geology.html)