Structural Geology Part 1A Structural Geology PDF

Structural Geology Part I Compiled By: Prof. Fathy Hassan The Nature of Structural Geology Structural geology addresses the architecture of the Earth - the physical components or structures that make up the Earth’s crust, and that form in response to applied forces a...

Structural Geology Part I Compiled By: Prof. Fathy Hassan The Nature of Structural Geology Structural geology addresses the architecture of the Earth - the physical components or structures that make up the Earth’s crust, and that form in response to applied forces and stresses. An understanding of rock structures helps us interpret the Earth’s history and aids in economic ventures. Oil exploration Mineral deposits Construction projects Unconformity Trap Petroleum Traps Salt Dome Trap Fault Trap Fold Trap  Faults relationship to mineral formation and precipitation  Project construction and structural geology What are Geological Structures? ▪There are several types of structures. These include: ▪1. Geological Contacts ▪Bedding planes and igneous intrusive contacts have formed at the same time as the rocks we find them in. ▪ Structures formed at this time are called: PRIMARY STRUCTURES. ▪Faults have formed after the rocks we find them in. Structures like these are called: SECONDARY (OR TECTONIC) STRUCTURES, those are produced by rock deformation. ▪So far, we can see that geological contacts can be: Primary (sedimentary or igneous) or Secondary.  Primary Shale Structures Sandstone Bedding planes & Intrusive contacts are primary structures Sandstone beds in Ain Sokhna region along Gulf of Suez Road, Egypt Folds & Faults are Secondary Structures  Folds are Secondary Structures  What are Geological Structures? ▪Another primary geological contact is an unconformity. ▪The beds below the unconformity often lie at an angle to the beds above. The beds above cross-cut the beds below. ▪Any surface which cross-cuts or terminates or interrupts another is described as: discordant. ▪Faults are often discordant. Dykes are defined as discordant tabular intrusive bodies. ▪Concordant structures are parallel to other surfaces e.g., the sill is concordant to beds.  Unconformity Younger Rocks Older Rocks Sills Dykes and Faults Dykes and Faults are Discordant Structures - Wadi Feiran Sinai  Dykes Basic Dykes Cross-cutting Younger Granite of Wadi Hawashia, Eastern Desert  What are Geological Structures? ▪2. Discontinuities within rock bodies ▪The geological contacts enclose rock bodies. Within the rock bodies there may also be surfaces which divide the rocks into blocks. ▪ These surfaces are called discontinuities, and include joints, fractures, veins, etc. ▪3. Shapes and features developed in 1. and 2. ▪ Folds are shapes defined by beds etc. ▪ Boudins are sausage- shaped structures in deformed veins. ▪ Features on surfaces are linear, e.g., ripple crests on bedding planes or striations (scratches) on fault planes. Joint sets in Younger Granite of Wadi Hawashia, Eastern Desert, Egypt Boudins Boudins- Southern Sinai  Ripple Marks Wind (current ripple) ripple Water(current ripple) ripple, Wadi Umm Latachan, Matrouh, Egypt  Symmetrical Wave Ripples Ripples - undulating bedforms produced by unidirectional or oscillating (wave) currents Fault Surface Striations  4. Patterns on the grain scale ▪ Graded bedding is a grain-size pattern showing coarsest grains at the bottom of the bed and finest ones nearest the top of the bed. Development of Graded Bedding Fining upward  4. Patterns on the grain scale ▪ Phenocrysts aligned by lava-flow (flow lineation) is an example of a grain-orientation pattern ▪ Both graded bedding and flow lineations are primary structures Development of Graded Bedding Fining upward  4. Patterns on the grain scale ▪ Mineral banding like gneissosity is an example of a grain-distribution pattern. ▪ Gneissosity is a secondary (metamorphic) structure. ▪ Cleavage is a secondary structure and is usually discordant. ▪ These grain patterns are usually small-scale structures. Metamorphic mica flakes Gneiss Banding Lineation defined by define cleavage orientation of mica flakes Measurement of Planar and Linear Structures ▪ Many structures are essentially planar ▪ Orientations of planes may be vertical, horizontal or inclined ▪ The horizontal line in a dipping plane is called the strike line (or line of strike) of the plane. The compass direction of this line is called the STRIKE of the plane. Cleavages Veins Beds Faults horizontal Joints dipping vertical c strike line Measurement of Planar and Linear Structures ▪ There are two ways to record compass directions: Azimuth and Quadrant notation ▪ To represent a compass direction as an azimuth measurement we divide the compass into 360º beginning with zero at the North and counting clockwise. The azimuth directions are always three-digit numbers e.g. 005 025 130. So, a direction 30º clockwise of North is referred to as 030. ▪ Opposite directions differ by 180º e.g., NE and SW are opposite directions. NE = 045 and SW = 225, Southeast is referred to as 135. ▪ To represent a compass direction using quadrant notation we first divide the compass into four quadrants: the NE SE SW and NW quadrants. For any direction we first determine which quadrant it lies in. In quadrant notation this direction will be written N... E, where... represents the angle from N to the direction. 000 030 N N N In this case it is 70o the NE quadrant 30o NW NE NW NE 270 W 135o E 090 W E W E SW SE SW SE So this direction is written as N70ºE S 135 180 S S  Measurement of Planar and Linear Structures ▪ After locating the strike line and measuring its compass direction, the next step is to find two other lines: 1- the dip direction of the plane (which is also at 90o to the strike line but is a horizontal line pointing in the direction of the dip); 2- the dip line of the plane (which lies in the plane and 90o from the strike line). It is the steepest line in the plane. The dip line is used to measure the angle of DIP of the plane. Together these two lines define a vertical plane. A clinometer is used to The dip direction of measure these angles the plane SW quadrant 90o 90o The dip direction is used to distinguish The dip is measured between two planes NE quadrant in this vertical plane which have the same from the dip direction strike and the same The dip line of the down to the dip line angle of dip plane  Measurement of Planar and Linear Structures ▪ So, altogether there are three measurements to be made to represent the orientation of a clinometer Strike compass planar structure. Dip Dip direction ▪ These are recorded in the following format: Strike ; Dip- Dip direction (e.g. N20ºW ; 35º SW). ▪ Linear structures are very common too parallel long axes of pebbles define a linear structure the intersection of bedding and cleavage Orientation of Lines is a linear structure The measurement of the on a plane orientation of lines is striations are linear bed easier than for planes, structures on a plane cleavage and requires only two measurements. Specifically the direction TREND a fold hinge is a of the linear structure towards which the line is plunging plunging should be linear boudins are measured. This called the structure linear TREND of the line structures  Attitude of Planar Structures ▪ The attitude of planar structures is defined by the strike, dip o Strike is the bearing of a horizontal line on the plane (a scalar), e.g., N40oE o Dip is the inclination of the plane, measured down dip is a vector; it gives the direction and amount of dip of the plane. Example for dip: 80oN 80o is the amount, N is the direction ▪ Example of planar structure: bedding, fault, fold axial plane, layering in lava, foliation.  Attitude of Dip Direction Planar Feature Strike and Dip of a Dyke- Southern Sinai  Measurement of Planar and Linear Structures ▪ Then the angle in this vertical plane from the trend line down PLUNGE to the plunging line is measured using a clinometer (Plunge). varies from 0o to 90 o ▪ Notice that a vertical line does not have a trend ▪ So, altogether there are two measurements to be made to represent the orientation of a These are recorded in the following format: linear structure. Plunge Trend 25 S40W clinometer Trend compass Plunge fault plane striations PITCH varies from 0 o to 90 o The angle from the strike of the plane down to the lineation is measured (Pitch). As with dip direction, we must also record pitch direction The pitch is measured with a clinometer simply as a quadrant e.g., NE, SW etc. Attitude of Linear Structures ▪ The attitude of linear structures is defined by the trend, plunge (together they define a vector). o Trend is the bearing of the line o Plunge is the inclination of the line ▪ Linear structure are also defined by their pitch on a given plane: o Pitch: The acute angle between the line and the strike of the plane on which the line lies ▪ How do we represent the measurements of planes and lines on a map? ▪ The first most important thing to do with structural orientation data is to put it on a map For planar measurements The longer line of the symbol represents the strike we use a symbol called the dip- and-strike symbol The short stroke represents the dip direction N For example, we will plot the dip-and-strike symbol for a bedding N40 E measurement of N40E; 60 SE at location A A First, we draw a line through A in the direction N40E and plot in strike part of the symbol 60 Then the dip stroke of the symbol is drawn. This must be in the direction of the dip Finally, the dip angle is written next to the dip stroke. In this example it is 60 degrees For linear measurements it is even easier. We use an arrow symbol N N40 E For example, we will plot the arrow symbol for a fold hinge measurement of 35 N40E at location A 35 A First, we draw a line through A in the trend direction of the hinge i.e., N40E and draw the arrow Then the plunge angle (here 35 degrees) is written at the tip of the arrow symbol The two symbols have to be modified to represent vertical and horizontal planes and lines no dip or The vertical bed is represented plunge values The horizontal hinge is represented by dip strokes in both directions need to be by a double headed arrow (because it can be considered to added here (because it can be considered to dip 90 in either direction) plunge zero in either direction) The horizontal bed has no unique the circle here The vertical hinge has no unique strike but can be considered to dip helps to trend but can be considered to plunge 90 distinguish two degrees towards all quadrants, and we use the zero towards all quadrants, and we rather similar arrow heads to show it use the dip strokes to show it looking symbols LEGEND There also needs to be different looking dip-and-strike symbols to distinguish beds, beds foliations joints veins cleavages, joints etc on a map Another set of different arrow symbols is needed for other linear These symbols must be listed and explained on fold hinges striations, boudins, mineral lineations etc. structures any map legend  You Than k