Geological Structure

Geological Structure: Refers to the deformation and displacement of rock layers or masses that make up the Earth's crust under the influence of internal and external geological forces, such as folds, joints, faults, cleavages, as well as various linear and planar structural features.

The scale of geological structures can range from thousands of kilometers—large enough to require integrated analysis of geological and geophysical data, combined with interpretation of remote sensing information—for identification, such as lithospheric plate tectonics. On the other hand, smaller features, measured in millimeters or even micrometers, can only be observed using optical or electron microscopes—for example, deformations of mineral grains or lattice dislocations.

Fold structure: The bending of rock layers under lateral compressive stress is called a fold. A fold refers specifically to a single curvature in the rock layers, whereas a continuous series of such curvatures is known as a fold structure, or simply a fold. There are two basic types of folds: anticlines and synclines. An anticline is a fold where the rocks at the core are relatively older, while those on either side are younger. Conversely, a syncline is a fold characterized by younger rocks at its core, surrounded by older rocks on the flanks.

Fault structures: When rocks deform under stress, and if the stress exceeds a certain strength, the rocks will fracture—or even undergo displacement along the fracture plane—resulting in a breakdown of the rock layers' continuity and integrity. This phenomenon is known as fracture tectonics. Fracture tectonics encompasses two main types: based on whether there is significant movement of the rock on either side of the fracture, it can be classified into joints and faults.

(1) Classified according to the chronological order of their formation

According to the chronological order of their formation Geological structures can be classified into primary and secondary structures, with secondary structures being the primary focus of structural geology research.

Native Structure : Structures formed during diagenesis. The primary structures of igneous rocks are mainly created by the forces generated from magma flow and the contraction caused by cooling and solidification. These primary structures of igneous rocks can be broadly categorized into two types: primary flow structures and primary fracture structures. Primary flow structures refer to oriented features that develop as solid materials align themselves within the flowing liquid magma—for instance, flow planes formed by the directional arrangement of flaky, platy minerals, flattened segregations, or enclaves that crystallized early in the magma; or flow lines resulting from the aligned orientation of columnar and acicular minerals that first emerged during magma solidification. Primary fracture structures include columnar joints, as well as longitudinal, transverse, oblique, and bedding-parallel joints, which are further classified based on their spatial relationships with flow planes or flow lines.

The primary structures of sedimentary rocks are mainly formed by the action of surface processes. These include bedding, planar structures, intra-bed structures, and cross-bedded structures.

① Bedding structures refer to the layered formations found in sedimentary rocks, where layering becomes evident through variations in the composition, grain size, and color of the sediments. Structural bedding serves as a fundamental reference plane for studying secondary structures and is itself an important primary structural feature.

② Stratigraphic structures primarily refer to features such as ripple marks, mud cracks, raindrop impressions, imprints, and trace fossils. These structures typically develop on the top or bottom surfaces of sedimentary rock layers and can be used in the field to identify the upper and lower boundaries of strata, helping determine whether the stratigraphic sequence is normal or inverted.

③ The intra-layer structures primarily refer to primary structures confined within a single rock layer, such as graded bedding, cross-bedding, intra-layer folds, and intra-layer faults. Among these, with the exception of intra-layer faults, all can also be used to identify the top or bottom surfaces of the strata based on their distinctive internal structural features.

④ The through-layer structure primarily refers to anticlines or faults developed within multiple layers of rock strata, known as synsedimentary anticlines or synsedimentary faults. Synsedimentary anticlines gradually form in locally uplifted areas of basins undergoing regional crustal subsidence and continuous sedimentation. As a result, they retain their original, upwardly arched curvature, with the stratigraphic thickness at the axial zone being thinner than that of the flanking limbs. Additionally, the grain size of clastic sediments in the axial zone is coarser compared to the corresponding layer sediments in the limb zones.

A co-sedimentary fault, also known as a growth fault, refers to a fault that moves concurrently with sedimentary processes. These faults typically develop along the margins of sedimentary basins and exhibit normal fault characteristics. In such basins, the area on the downthrown side of the fault features significantly thicker sedimentary layers compared to the upthrown side, and the displacement increases with depth—meaning the older the strata, the greater the fault offset. Additionally, rhythmic layers are another type of primary cross-bedding structure, formed by the sequential stacking of multiple graded bedding units.

Secondary structure : Tectonic deformation resulting from the action of tectonic movements after rock formation is the primary subject of study in structural geology. Secondary structures include folds, joints, faults, cleavages, and lineations, among others.

(2) Classified according to deformation characteristics

Continuous Deformation Structure : A structure where the continuity of rock layers remains intact, exhibiting plastic deformation characteristics, such as folding.

Discontinuous Deformation Structure : A structure in which rock layers are cut, losing their original continuity, and classified as brittle deformation features, such as joints and faults.

(3) Classified by Geometric Elements

Planar structure: Structures represented by geometrically defined surfaces, such as fold axial planes, fracture planes, fault planes, and cleavage planes. Systematic planar structures are typically referred to as foliation.

Linear structures: Structures represented by geometrically defined lines—such as fold hinges, fault striations, the oriented arrangement of non-equant minerals, and the intersection lines of two structural planes—can be categorized into two types: abstract (e.g., fold axial planes, hinges, or intersections of two structural planes), which exist purely in a geometric sense without physical presence, and delineative, which refer to physically real, distinct planar or linear features.

Based on the distribution characteristics of planar and linear structures within geological bodies, they can be classified as:

① Transmissive structures refer to planar and linear features that are uniformly distributed, continuous, and arranged in a specific pattern throughout a geological body at a certain scale—such as cleavage, gneissic banding, schistosity, and various types of minor lineations.

② Non-penetrative structures refer to planar and linear features that are non-uniform, discontinuous, and occur within geological bodies in a segregative manner—such as joint planes, fault planes, as well as large-scale rod-like or louvered structures. The distinction between penetrative and non-penetrative structures is relative to the scale at which the feature is observed; for instance, penetrative structures on a larger scale may appear non-penetrative when viewed at a smaller scale. Typically, the term "penetrative structure" is applied specifically to two scales: small-scale structures and microstructures.

Folds

Fold structure: Under the powerful compressive forces of crustal movements, rock layers undergo plastic deformation, resulting in a series of wave-like bends known as folds. In principle, fold formation is a fairly straightforward process—but because the causes behind their creation vary, folds can be classified into many different types.

Hub: On each transverse section of the fold, the line connecting the maximum curvature points of the same folded surface is called the hinge.

Axial plane: Also known as the hinge plane, it refers to the surface formed by connecting the hinges on adjacent folded surfaces.

Axial trace: The intersection line of the axis plane with the ground or any other plane.

Slot line: On the transverse sections of the same folded surface in an anticline or syncline, the lowest points are called "valleys," and their connecting line is known as the valley line.

Ridge line: On the transverse sections of the same folded surface in an anticline or syncline, the highest points are called "ridges," and their connecting line is the ridge line.

Syncline: An anticline is a downwardly curved fold, with the youngest layers found closer to its center. Typically, an anticline arches upward, pointing toward the sky. Over time, its crest may even get eroded away, leaving us to see only the edges where it was originally formed.

Anticline: Anticlines are similar to synclines—only they’re the reverse. An anticline is a convex-folded structure, with the oldest layers located closer to its center; the flanks typically slope downward into synclinal formations. However, in reality, fractures and erosion often occur, causing these two features to become separated. Anticlines frequently give rise to numerous excellent hydrocarbon-trapping areas, making them ideal for oil exploration.

Sharp-crested folds: This type of fold is often referred to by oil companies as a V-shaped fold, which, as the name suggests, typically appears in a V-like configuration. It usually forms due to local compressive stresses, but its development requires several very specific conditions. In total, there are four distinct stages: fold nucleation, parallel folding, wing spreading/central sharpening, and finally, the tightening phase leading to sharp-crested folds.

Horizontal fold: A recumbent fold is an overturned fold—a fold that has been flipped or completely reversed, featuring an axial plane inclined at a certain angle, with one side of the strata having been turned upside down. The axis of a recumbent fold remains essentially horizontal.

Isoclinal folds: The angles of the two limbs of the isoclinal fold range from 0° to 10°, and the two limbs are nearly parallel.

Dome: In geological structural terms, a dome is formed when symmetrical anticlines intersect and penetrate each other at their crests. Its formation is driven by recumbent horizontal stresses, combined with atmospheric impact/piercing forces (resulting in vertical displacement as deeper, lighter portions move upward toward the surface).

Basin: A basin is, in a sense, the opposite of a dome-shaped fold, as the deformation of previously flat-lying rock layers leads to a large-scale reorganization of the stratigraphic structure. A structural basin is essentially a geological depression.

Co-sedimentary folding – Folds formed gradually as rock layers were deformed during their formation.

Partition-style folds: It consists of a series of alternating parallel anticlines and synclines, with the anticlines being tightly folded and the synclines relatively more open.

Slot-type fold: It consists of a series of folds arranged alternately as parallel anticlines and synclines, with the synclines being more tightly closed and the anticlines more gently open.

Longitudinal Flexural Folding: Rock layers fold under the influence of layer-parallel compressive forces, a process known as longitudinal flexural folding.

Transverse flexural folding: When rock layers are subjected to external forces perpendicular to their bedding planes, causing them to fold, this phenomenon is known as flexural-slip folding.

Bending-slip action: Refers to the process by which a series of rock layers are bent into folds through interlayer sliding.

Bend flow effect: When longitudinal flexural folding causes rock layers to bend and deform, not only interlayer sliding occurs, but also material flow phenomena appear within certain rock layers.

Shear-fold interaction: Also known as flexural-slip folding, this process causes differential sliding along a series of closely spaced cleavage planes that are not parallel to the original bedding planes, ultimately forming "folds." In this type of folding, the original bedding planes no longer exert control; instead, they merely serve as indicators reflecting the outcome of the sliding motion. Hence, it is also referred to as passive folding.

Ryūryū Fold Structure: Refers to highly ductile rocks (such as rock salt, gypsum, and coal seams) or rocks that, when subjected to high temperature and high-pressure conditions, transform into ductile bodies capable of flowing and deforming like viscous fluids under external forces, ultimately giving rise to intricate folds.

Knee-bending effect: It is a unique type of folding that combines the characteristics of both flexural-slip folding and shear folding.

Joint

Joints: A fracture structure where the rock blocks on either side of the fracture have not experienced significant displacement along the fracture plane is called a joint.

Shear joint: Joints formed by shear stress.

Zhang Jie Li: Joints formed by the action of tensile stress.

Vertical joints: A joint whose strike is parallel to the fold hinge.

Cross-joints: A joint whose strike is perpendicular to the fold hinge.

Oblique Joint: Joints whose strike is oblique to the fold axis.

Heading toward Joint 1 : A joint whose strike is roughly parallel to the strike of the rock layer in which it occurs.

Trend Fracture 2: A joint whose strike is roughly perpendicular to the strike of the rock layer in which it occurs.

Oblique Joint 3: A joint whose strike is roughly oblique to the strike of the rock layer in which it occurs.

Stratigraphic Joint 4: A joint whose plane closely matches the bedding attitude of the surrounding rock layer.

Tensile Joint: Primarily developed at the hinge zones of anticlines, they appear in a fan-like arrangement when viewed in cross-sections of the fold, with individual fractures shaped like downward-pointing wedges.

Horizontal Joint: Cross-faults that form before rock layers undergo bending typically follow early, planar X-faults, extending in a jagged, saw-tooth pattern. After the rock layers are deformed by bending, two types of cross-faults emerge: one develops in the core of synclines, often tracing late-stage planar X-faults and also appearing in a serrated, saw-tooth configuration; the other forms in areas where folds exhibit pronounced plunges, with a strike oriented perpendicular to the hinge line. These faults are oriented vertically relative to the local bedding planes and dip in the opposite direction from the fold’s plunge, while their dip angle is complementary to the fold’s plunge angle.

Fault

Fault: A fault is a fracture structure characterized by significant displacement of rock blocks on either side of the fracture surface.

Slip distance: Refers to the actual displacement distance between the two sides of a fault, measured as the real distance between two corresponding points after the movement, based on a specific point before the displacement occurred.

Break distance: Refers to the relative distance between corresponding layers of a wrongly identified rock stratum on both sides.

Normal fault: The upper block slides downward relative to the fault plane, while the lower block moves upward in comparison.

Reverse Fault: The upper block slides upward relative to the fault plane, while the lower block slides downward in comparison.

Translational Fault: The two plates move past each other along the fault plane.

Graben: It consists of two (or more) faults that trend roughly parallel to each other, dip in opposite directions, and share a common downthrown block between them.

Horst: It consists of two (or more) faults that trend roughly parallel to each other, dip in opposite directions, and share a common uplifted block between them.

Stepped fault: It is composed of several normal faults with roughly similar orientations.

Stacked fault structure: It is composed of several reverse faults with roughly similar attitudes.

Rift Valley: It refers to a large, complex graben system characterized by fault-bounded valleys against the backdrop of regional uplift, exhibiting distinct features in geological and geophysical aspects.

Deep Major Fault: It is vast in scale, extending hundreds or even thousands of kilometers and cutting deeply into the Earth. Its downward erosion can reach as far as the Mohorovičić discontinuity, potentially penetrating the crust or even the lithosphere. Often, it serves as a boundary between distinct regional tectonic units with differing geological structures and evolutionary histories.

Syndepositional fault: Also known as growth faults, these primarily develop along the margins of sedimentary basins. As the basin continuously subsides and sediments accumulate during its formation and evolution, the outer edges of the basin gradually uplift—processes all driven by the ongoing activity of faults that control the basin's margins.

Scratch marks: Traces left on the fault plane due to friction and the scraping action of debris during the relative displacement of rock blocks on either side of the fault appear as a set of parallel, uniform fine lines. These features can be used to determine the presence of the fault and the direction of its relative movement.

Steps: Traces left on the fault plane due to friction and the scratching action of debris as rock blocks on either side of the fault move past each other—these appear as a series of tiny, steep steps roughly perpendicular to the striations. They are formed either by localized differences in resistance or through intermittent, jerky movements along the fault.

Resilient Fault: Also known as a ductile shear zone, it is an intensely deformed zone formed by shear action under the plastic state of rocks. It exhibits fault-like displacement but lacks a distinct fracture surface.

Schistosity

Schistosity: It is a secondary planar structure that divides rocks into parallel, closely spaced sheets or plates in a specific direction.

Flow foliation: It is formed by the parallel arrangement of flaky, plate-like, or flattened mineral aggregates.

Fracture cleavage refers to a series of closely spaced (with intervals less than 10 mm) and parallel fracture surfaces within rock, exhibiting mechanical properties identical to those of shear fractures.

Schistosity cleavage—also known as strain-induced schistosity cleavage or fold cleavage—is a set of fracture surfaces that cut through earlier-formed cleavages (flow cleavages), with subsequent slip along these fractures, thereby inducing minute folds in the original foliation.

Axial-plane foliation: Foliation that is parallel to, or nearly parallel to, the fold axial plane. It is primarily developed in rock layers undergoing intense folding.

Lineation

Lineation: Lineation is a descriptive term that broadly refers to various parallel linear structures found within or on the surface of rocks.

Stretch lineation: Composed of elongated gravel, oolites, rock fragments, mineral grains, or mineral aggregates arranged in a parallel orientation.

Stone sausage structure—also known as pudding structure—is a type of geological formation that develops when layers of rocks with contrasting properties are alternately arranged and subjected to vertical compression under conditions characterized by significant differences in ductility. Its cross-sectional shape resembles a sausage, hence the name "stone sausage."

Window frame structure: It is a large-scale lineation found within intensely folded rock layers, characterized by a series of cylindrical or gently undulating, rounded prismatic structures.

Pencil-shaped structure: It is a common structural feature found in folded mudstone or siltstone slates. One explanation for its formation is that it results from the intersection and segmentation of two or more sets of parallel planar structures. Some instances involve the intersection of bedding planes with cleavage, while others arise from the intersection of cleavage surfaces themselves.

Rod-shaped structure: Composed of quartz, calcite, or other single-mineral, tough rocks, these features appear in bands and clusters within specific metamorphic rock layers. Interestingly, the composition of their elongated "rods" differs from that of the surrounding rock matrix. Typically, they form as a result of rolling and shearing forces acting perpendicular to the hinge line of the fold, with their extension oriented parallel to the hinge and perpendicular to the direction of movement—characterizing a b-axis lineation.

The material is sourced from the internet.