Penecontemporaneous Structures. Penecontemporaneous folds in the Maranosa Arenaci (Italian Apennines)

Transcription

1 Penecontemporaneous Structures Penecontemporaneous folds in the Maranosa Arenaci (Italian Apennines)

2 Penecontemporaneous Structures

3 Thrust fault related folds and folding Detachment fold: Folds developed above a detachment or thrust that is bedding parallel. Detachment folds require a ductile decollement layer.

4 Idealized Thrust Fault Idealized thrust fault showing kink band folding in the hangingwall and no deformation in the footwall Ramp-flat trajectories Fault-bend fold: fold generated by thrust sheet over ramp

5 Thin-skin Tectonics Thin-skin: Amount of shortening that occurred in the wedge was large and below thrust wedge, basement is not involved in deformation. Decollement (detachment): defines boundary between thrust wedge and basement, occurring along weak horizon.

6 Thick-skin tectonics: basement involved tectonics Courtesy from Lin Nina Yunong

7 Salt Structure: Halokinesis Salt is a sedimentary rock that forms by the precipitation of evaporite minerals (halite, gypsum, anhydrite, calcium sulfates) from saline water. Salt differs from other sedimentary rocks in that it is much weaker and, is able to flow like a viscous fluid under conditions in which other sedimentary rocks behave in a brittle fashion. In some case, deformation of salt is due to tectonic faulting or folding, because salt is so weak, it may deform solely in response to gravity, and thereby cause deformation of surrounding sedimentary rock.

8 Why Halokinesis Occurs? All of these factors occur in a passive-margin setting (1) The development of density inversion: Salt is a nonporous and essentially incompressible material. When it gets buried deeply in a sedimentary pile, it doesn t become denser. In fact, salt actually get less dense with depth, because at greater depths it becomes warmer and expands. Other sedimentary rocks, in contrast, form from sediments that originally had high porosity and thus become denser with depth because pressure caused by overburden make them compact. At depths greater than 6 km, salt density is about 2200 kg/m 3, whereas the density of sedimentary rock is about 2500 kg/m 3. Positive buoyancy: forces in a gravity field cause lower density material to try to rise above higher density material. Negative buoyancy: force that causes a denser material to sink through a less dense material.

9 Why Halokinesis Occurs? (2) A salt layer takes places when the downward force on the salt layer caused by the weight of overlying strata varies laterally. This may occur where there are primary variations in the thickness or composition of overlying strata, primary variations in the original surface topography of the salt layer, or changes in the thickness of the overlying due to faulting. Neutral buoyancy: depth at which it is no longer buoyant. At this level, salt has the same density as surrounding strata. The density of mildly compacted clastic strata equals that of salt at depths around m below the surface of the basin, depending on the composition. At the level of neutral buoyancy, salt may begin to flow laterally. (3) Existence of a slope at the base of a salt layer

10 Stages in the Formation of Salt Structures

11 Cross section illustrating a normal fault array over the top of a salt dome in Texas

12 Salt Dome in Zagros Mountains, Southwestern Iran

13 Zagros Mountains The Zagros Mountains in southwestern Iran present an impressive landscape of long linear ridges and valleys. Formed by collision of the Eurasian and Arabian tectonic plates, the ridges and valleys extend hundreds of kilometers. Stresses induced in the Earth s crust by the collision caused extensive folding of the preexisting layered sedimentary rocks. Subsequent erosion removed softer rocks, such as mudstone and siltstone while leaving harder rocks, such as limestone and dolomite This differential erosion formed the linear ridges of the Zagros Mountains. The depositional environment and tectonic history of the rocks were conducive to the formation and trapping of petroleum, and the Zagros region is an important part of Persian Gulf production. This astronaut photograph of the southwestern edge of the Zagros mountain belt includes another common feature of the region a salt dome (Kuh-e-Namak or mountain of salt in Farsi). Thick layers of minerals such as halite typically accumulate in closed basins during alternating wet and dry climatic conditions. Over geologic time, these layers of salt are buried under younger layers of rock. The pressure from overlying rock layers causes the lower-density salt to flow upwards, bending the overlying rock layers and creating a dome-like structure. Erosion has spectacularly revealed the uplifted tan and brown rock layers surrounding the white Kuh-e-Namak to the northwest and southeast (center of image). Radial drainage patterns indicate another salt dome is located to the southwest (image left center). If the rising plug of salt (called a salt diapir) breaches the surface, it can become a flowing salt glacier. Salt domes are an important target for oil exploration, as the impermeable salt frequently traps petroleum beneath other rock layers. Astronaut photograph ISS012-E was acquired February 28, 2006, with a Kodak 760C digital camera using a 180 mm lens, and is provided by the ISS Crew

14 Gravity-Drived Faulting and Folding

15 Types of Sheet Intrusion around a Volcano

16 Cooling Fractures: Columnar Jointing Columnar jointing in the Massif Central, France

17 Barringer Meteor Crater of Arizona Barringer Meteor Crater near Winslow, Arizona. The crater is believed to have formed from the impact of a large (30-50 m diameter) iron meteorite 50,000 years ago. The crater is about 1.2 km in diameter. Photo by D. Roddy and K. Zeller, USGS.

18 Barringer Meteor Crater Barringer Crater, also known as 'Meteor Crater', is a 1,300-meter (0.8 mile) diameter, 174-meter (570-feet) deep hole in the flat-lying desert sandstones 30 kilometers (18.6 miles) west of Winslow, Arizona. Since the 1890s geologic studies here played a leading role in developing an understanding of impact processes on the Earth, the moon and elsewhere in the solar system. This view was acquired by the Landsat 4 satellite on December 14,

19 Geological Cross Section of Impact Structures

20 Shatter cone: 岩石爆開成為碎塊, 在碎塊表面形成扇形分布的裂痕 表面具有扇形裂痕的岩石碎塊

21 Pseudotachylite or Pseudotachylyte Tachylite is a volcanic rock. Early geologists in Vredefort identified something a bit like it, and called it Pseudotachylite. It is formed largely by frictional melting along faults or after the impact event. Clasts of the country rock (here granite) are found within. In these images, the Pseudotachylitic breccia is the black stuff.

22 Homework and Assignment Table 2.3: Common Surface Markings Facing indicators Table 2.5: Terminology of Salt Structures