new Flashcards
What are the three lines of evidence that folds are influenced by wind?
- Cross sections showed the magnitude of folding in the structural relief of the folding by finding the difference between the structural relief and the topographic relief which is the depth of material which must have been blown away to reveal the structural highs.
- Radiometric dating of 100-120 ka lake deposits in the region show the relative rates of wind erosion by finding the distance between the highest and lowest stratigraphically indicative layers.
- Seismic reflections show that growth strata began as aridification occurred (~3 ma) related to the “upwelling” of antiformal structures indicated by onlap. This accelerated during the 3-2.5 mya dry period.
Classes of fault-related folds
There are fault-bend folds where layers bend over the thrust. The upper layers will be thinned. It is similar to a rollover monocline.
fault propagation folds These are rock layers that fold ahead of the propagation tip. These create the turned over antiformal folds in the hanging wall. The syncline axis in the footwall is near the propagation tip.
During Continental continental collisions there are also “snake head folds” which occur in relatively weak, less brittle crust where a large lobe basically plops onto the footwall.
Cleavage
This is a structure that forms in the lower part of the brittle regime (~10 km) common with passive folding and transposition. It is indicated by a staple symbol when mapping.
Geometry: They are closely spaced, planar, woody surfaces associated with folds, and may be oblique to bedding. Generally, it has a “domainal” nature where parts are deformed more than others, creating a very finite “banding” of platy minerals like mica.
Stress/Strain: max shortening perpendicular to sigma 1 and through pressure solution and recrystallization there is a decrease in wavelength and limb thickness
Most common is axial planar cleavage which is perpendicular to sigma 1 and parallel to axial planes.
Coloumb failure criterion
This is a line given by sigma = Co + tan(30) sigma n
If any stress state is beyond the failure criterion then it fails.
Core complex cross-sectional geometry
Metamorphic core complexes tend to be flanked on one side my a low angle detachement fault overlayed by high angle normal faults and interfaced with a mylonitic gneiss (think about this side of the Catalinas). At their center is a granite core related to decrompression melting. The other side is characterized by a listric breakaway fault and a basin with domino style faulting.
Across the whole complex there is the same sense of shear creating the mylonitic fabric
Decollment zone
This is when there is detachment from lower layers and the formation of a cupsate. It is common in concentric folds.
This underlies the formation of large duplexes and thrust faults too.
Detachment Faults and Surfaces
Detachment faults are low angle (<30o) which form underneath graben systems and at the interface of the brittle crust and the metamorphic core complexes. In the latter case they are marked by chloritic alteration (green color) which forms due to fluid alteration during the process of exhumation.
They can be confused with thrust faults.
Drag folds
These are folds that occur along the interface of a fault and appear to resemble frictional resistance to sliding along the fault. They are actually caused by the propagation of the fault surface from depth. For example, in a normal fault (which represents extension) before the shear plane physically divides strata in a normal fault, the hanging wall will be extended and the materials near the fault “collapse” deforming those on the surface upwards. You are basically removing the support below the layers causing slumpling.
Duplex
This is the uppermost thrust. It forms the earliest and is rotated to steeper and steeper dips as it overrides the foreland material. These occur when the inactive fault is thrusted above the active ramp to be on the surface of the previously adjecent layers.
Duplex architecture
There are generally several thrust sheets that layer onto one another. In the foreland are the youngest thrusts and the hinterland has the oldest thrusts.
The lower layer where “decoupling” occurs is the floor fault and the roof fault is the boundary between the thrusted layers and the overlying rock.
A horse is where the thrusted layer interfaces with the inclined layer below.
Fault rocks as a function of depth
0-4 km of depth: Breccia/gouge which has angular clasts, can be incohesive, and comminuted material. Gouge is very fine breccia.
4-10 km: cohesive cataclasite (cement) which is angular clasts surrounding by hard muddy material. It is cohesive and usually fluid related. (can become breccia during exhumation)
~8-10 km: migmatites in high shear zones
10+: mylonites This is when quartz begins to ductily deform (200-300 C) and you have extremely strained fabrics. This is defined by the recrystalization of the rock
Fault-related folds
Fault-bend folds are layers that fold because of a change in fault geometry/trajectory. Note that in the fault bend fold there is horizontal displacement of the uppermost layers to the right.
Fault-propagation folds are folds that fold ahead of the propagating tip of a blind thrust. There is not a horizontal displacement of layers above the thrust.
Foreland Basins
These are depressions that form on the frontside of fold-thrust belts (in the foreland) where the topographic load of the fold-thrust belt causes lithospheric flex. Foredeeps seperate the fold-thrust belt from the forebulge.
General architecture of fold-thrust belts
From low to high elevation. The foreland is the inner-continental basinward part of the belt and made of younger, lower grade rocks. It will be in front of an emergent imbricate fan which has newer thrusts at lower elevations. This merges with the plateau aka the hinterland where the duplex roof exists (the first thrust fault) These are underlain by younger thrusts too. This is where the highly sheared, old, and high-grade rocks exist.
All of this sits above the decollment fault which is the “main fault” that the other faults feed into. It is relatively flat but dips towards the hinterland. It represents a decoupling within the crust
There are usually turned over antiformal folds in the hanging wall and synclines in the footwall.
General shear
Any mix of pure and simple shearing. It is also in plane. This will result in a mixture of smooshing and translation.
Gneissic Structure
Penetrative plane layering with compositional banding, mineral laminae, and quartz eigens
Horst and Graben structures
These are characterized by high areas flanked by normal faulting on both sides. They are consistent with andersons theory of faulting (highs have normal lows have thrust) often underlined by a detachment fault defined as a low-angle normal fault most commonly found at depth. This fault determines if individual structures are synthetic (dip in the same direction) or antithetic (dip in the opposite direction)
How are large thrust sheets emplaced?
The integral of the frictional forces needed to forces 100’s of km of thrust sheet onto another rock is beyond the magnitude of conventional tectonics. The rock would shatter before thrusting in this matter. The caveat is water pressure.
Water pressure acts as a negative weight and enables the thrust sheet to more effectively slide onto the material below it. It decreases mean stress without changing differential stress (drives the process)
Additionally, thrust sheets move in piecewise dislocations. movement and strain is not homogeneous. This is known as creep
How does rock strength change in each rheologic regime
In the brittle regime strength is depth dependent. In the ductile regime strength decreases exponentially with temperature and then in the viscous regime it is strain rate dependent.
How does strength change with increasing strain rate, temperature, and depth?
With strain rate strength increases
With temperature strength decreases
With depth strength increases
How does strength vary with depth and lithology?
Rocks that are weak on the surface (mafic rocks) are strong at depth and rocks that are strong in the brittle regime (quartzite) are weak at depth.
The rheology of quartzite controls rock deformation.
How does strength vary with temperature?
As temperature increases strain rate increases exponentially and strength decreases exponentially.
Metamorphic Core complex tectonostratigraphy with depth
Starting at the detachment fault (low angle fault separating brittle crust from core complex)
chloritic breccia (green breccia between cm and 10s of m thick)
cataclasite resistant layer (~1 m)\
mylonitic shear zone (gneisses with hundreds to km of m thick)
injection complex (dikes in the gneisses)
undeformed core
Relay Ramps
Within a series of partially connected normal faults the relay ramps refer to the inclines between tip points.
Strength vs. depth curve/chart
This shows three main parts:
An elastic part given by mohrs failure criterion where stress is linearly related to strain by youngs modulus and brittle deformation dominates. At these depths joints then faults occur. At the lower depths, quartz becomes plastic (T300-350 C)
The yield strength (~10-15 km) indicates the elastic-plastic transition and is where earthquakes nucleate (greatest stress). It is also where von mises failure begins.
After this an exponential decay of strength occurs marked by the brittle-ductile transition. It begins with flexural flow fold then plastic flow (qtz) and viscous flow (strain dependent so stress is constant)
Thrust belt propagation
The first ramp thrusts over the flat which then eventually begins to thrust too. This increases the dip of the dip of the earlier thrusts and also makes the youngest faults those that are closest to the foreland.
Like normal faults, thrusts cut up-section (originate at depth). They are connected like stair-steps with ramps and flats that progressively reach the surface. This is because they take advantage of planes of weakness (parallel to bedding)
What is the hypothesis for the development of Tibetan Rift systems?
They start as half-grabens with a structural low adjacent to the normal fault high. This low causes isostatic rebound at depth that starts to “bulge” the half graben basin into a basin high. At depth this causes the normal fault to flatten. The flattening of the normal fault initiates the onset of breakaway faults in the growth strata. The continual decline of the hanging wall repeats this process, and the basin high continues to enlarge until you reach the point where there is a detachment fault at the surface.
Cross section of tucson from NW to SE
Starting in the SE:
- folded cretaceous Bisbee group rocks tilting away from Tucson mountains with ~perpendicular angular unconformity with the cat mountain volcanics.
- Tucson mountains gives way to tuscon basin. Buried normal faulting on detachment fault.
- Rounded dome gives way to breakaway faults (Guliaros) with tops leaning away from tucson
What is the architecture of an ocean-continent collision?
From oceanward to inland.
the trench goes to the forearc and then arc.
This gives way to the highly elevated, hot and thick plateau and retroarc thrust belt (hintenland towards plateau and foreland towards newer thrusts)
The hinterland basin at the front of the thrust sheet forms because of the excess mass of the thrust belt
thin skinned thrust belt architecture
Starting in the lo elevation foreland there is a blind thrust and a small fault propagation fold. Then there is the emergent imbricate fan which are high angle thrusts that intersect the surface.
Topography levels out and less extensive higher angle thrusts from the duplex. The uppermost thrust is the duplex roof and the lowermost is the duplex floor.
These all align with the decollement zone which is a place of sliding that dips slightly towards the hinterland (high elevation)
What does a restraining band look like in map view?
When there is a bend in a transform fault (sigma 1 is 45 degrees off). Riedel shears form 15 degrees from the transform and feed slip into the system.
The depression is bounded by normal faults (add U and D) and the elevated by thrust faults and anticlines.
