Chapter 7 Notes : Mountain Building and Drifting Continents
Early Theories of Orogenic Belts and Mountain Building
1. Orogenic Belts are defined as regions of intense folding and faulting accompanied by granitic intrusions and metamorphism. Orogenic belts denote areas of present or former mountain ranges and, as we shall later see, mark the collision sites of lithospheric plates.
2. Early theories of orogenic belts and mountain building (pre 1950s) did not as yet recognize the concepts of plate tectonics, seafloor spreading or lithospheric plates.
James Hall (1857)
1. Figure 7.3A: In 1857, James Hall noted that mountain belts contained much thicker sedimentary sequences than found in adjacent areas of continents and that this strata was often deformed.
2. Hall reasoned that great loads of sediments must have depressed and bent the crust until it failed. Failure or collapse of the crust, in turn, caused the enclosed strata to crumple and thus form mountains as surface expressions of the deformed strata.
J.D. Dana (1873)
1. Some years later, J.D. Dana in 1873 coined the term geosynclines in describing old belts of thick, deformed strata along the edges of continents.
Geosyncline Defined As:
(a) an elongated belt of thick strata associated with mountains.
(b) are typically folded and are displaced by thrust faults.
(c) often contain granitic batholiths and metamorphic rocks.
(d) developed within structurally unstable orogenic belts located between stable regions.
2. Figure 7.3B: Dana argued that the Earth's interior was cooling and shrinking over time. He reasoned that depressions in the earths crust formed due to buckling of the crust during cooling of the interior of the planet. These depressions then became sites of sediment accumulation. Sediment loading led to eventual crustal failure, deformation and uplift into mountain ranges.
Vertical versus Horizontal Tectonics
1. The intense folding and faulting of strata observed by many scientists led to several hypotheses as to what caused the deformation.
2. Figure 2.13: Plutonists like James Hutton (late 1700's) argued that the earth was a heat engine that caused frequent upheavals along the surface. This crustal upheaval and associated folding occurred as a result of emplacement of molten granite from the hot interior to the surface.
3. During the early 1900s, however, many geologists began to prefer Danas arguments that attributed folding and faulting in mountain belts to lateral compression resulting from a shrinking earth.
4. Figure 7.4 (top): Others, however, sided with Hall and his theory of isostatic upwarping from which emerged the gravitational sliding hypothesis. This hypothesis attributed stratal deformation to isostatic upwarping of geosynclines, followed by slippage of strata over basement rocks along a flat shear surface without actually disturbing the underlying basement rocks.
5. Figure 7.4 (bottom): An alternative interpretation, referred to as the lateral compression hypothesis, envisioned deformation of both strata and basement rocks along steep faults due to horizontal forces.
1. Figure 7.5: Serious discussion of large-scale continental displacements began in 1908 when F.B. Taylor, an American geologist, suggested that mountain belts represented the wrinkling of crust due to the drifting of continents. According to this hypothesis, the leading edges of moving continents first depressed the oceanic crust ahead to form a trough that then accumulated sediment. Further movement of the continents eventually caused upheaval of the strata to produce mountain ranges.
2. Figure 7.6: H. Baker (early 1900's) prepared the first detailed reconstruction of continents into a supercontinent. Baker argued that this supercontinent suddenly split at the end of Miocene time to form the present Arctic and Atlantic oceans. He envisioned this split as due to the close approach of the planet Venus, which tore out a large portion of the original supercontinent to form the moon. The remaining continental crust then split during slippage of the supercontinent as it filled the hole in the Pacific left by the removal of moon material. This splitting formed the present Arctic and Atlantic oceans.
Alfred Wegener (early 1900's)
(a) formulated the idea of drifting continents and continental drift.
(b) Considered paleoclimatic indicators (glacial, tropical rain forest & desert deposits) in the rock record when linking continents together.
(c) Figure 7.7: fitted together continental shelves to form supercontinent.
(d) postulated a mechanism by which subcrustal material yielded to certain forces resulting in crustal blocks flowing slowly across the upper mantle.
(e) attributed these forces to the earth's spin, tidal effects and wobble of axis.
A.L. Du Toit
(a) beginning in 1921, identified identical fossils on widely separated continents. He wondered how similar fossils could be dispersed over entire oceans and concluded that the now separated continents must have at one time been joined.
(b) published Our Wandering Continents in 1937 in support of continental drift.
1. Once the idea of continental drift took hold in the early part of the 20th century, scientists began looking for an internal mechanism by which to explain the movement of these giant land masses.
2. Figure 7.9: Arthur Holmes (1928) proposed thermal convection as the driving force for continental drift.
3. Significant opposition to the drift theory continued until as late as the 1960's when Holmess idea of convection finally took hold following the advent of plate tectonics.
4. Other mechanisms have since been proposed to explain contental drift as we shall see later.
1. Additional evidence for continental drift was supplied by studies of paleomagnetism in the rock record.
2. Physicists knew of rock magnetism as early as 1850 and recognized that certain magnetic minerals, when crystallizing, recorded the orientation of the earths magnetic field. It wasnt until the 1950s and 1960s, however, when the study of rock magnetism accelerated.
Principles of Paleomagnetism
1. Figure 7.10: Measurements of paleomagnetism in ancient rocks indicate orientations different from that expected in the magnetic field of their present location
2. Figure 7.11: Declination is measured on horizontal plane with respect to the present magnetic north pole is an indicator of paleolongitude; Inclination is the angle measured relative to the horizontal earth's surface and is an indicator of paleolatitude.
3. Figure 7.12: Paleolatitudes and Paleopoles of discordant paleomagnetic vectors can be determined by rotating each vector until it is concordant with the magnetic field. Since the Earth's magnetic pole has essentially remained in the same position, then changes in magnetic vectors for rocks of different ages must indicate continental drift.
4. Figure 7.13: Paleomangetic measurements from magnetically susceptible rocks of different ages can therefore determine the drift of a continent through time.
5. Figure 7.14: The declination of fossil magnets within ancient rocks can determine paleopole positions that aid in determining rotation and drift of continents.
1. Until the 1960s, most geologists assumed that the crust beneath the ocean basins was very old, generally flat and fixed in place. Extensive shipboard reflection surveys after WWII, however, revieled a seafloor exhibiting broad ridges, deep trenches, escarpments and countless submerged volcanoes (seamounts).
2. The most striking features of ocean floors are immense, volcanically active submarine ridges, underwater mountain ranges that snake across the ocean basins.
3. The idea of seafloor spreading can be traced back to Arthur Holmes who in 1928 presented the hypothesis of convection in order to explain continental drift. Holmes, however, did not envision mid-ocean ridges per-say but speculated that ocean basins may have formed through splitting apart of continents due to upward-flowing mantle.
4. By the 1960s, seismic imaging of the ocean floor had demonstrated the existance of vast submarine mountain ranges known as mid-ocean ridges. It was Harry Hess, a geologist from Princton University, who in 1962 envisioned mid-ocean ridges as sites of rising thermal convection cells.
5. Figure 7.19: The hypothesis of sea-floor spreading postulates that mid ocean ridges are sites where oceanic crust rifts and moves apart laterally. According to Hess, convection cells flow upward, then laterally away from oceanic ridges while carrying oceanic crust along as if on a conveyor belt. His hypothesis explains the high heat flow measurements, numerous earthquakes and submarine volcanic eruptions of basalt that have since been observed along modern active mid-ocean ridges.
Rifts and Hot Mantle Plumes
J. Tuzo Wilson (1960's-70's)
(a) seamounts are symmetrically distributed as to relative age outward from submarine ridges such that younger seamounts occurred closer to the ridge axis. The ages of seamounts are determined by sediment thickness and fossils recovered from drilling of deep-sea sediments.
(b) Figures 7.18 & 15.27: hot spots are lines of volcanic eruptions marking the trace of impingement of a hot mantle plume on moving lithosphere.
(c) Figure 7.33: Wilson Cycle
(d) Figure 7.21: Transform Faults.
(e) Figure 7.22: geometrical relationships of spreading ridge, transform faults and subduction zones.
Characteristics of Mid-Ocean Ridges and Ocean Basins
Figure 7.15: Mid-ocean ridges are submarine mountain ranges extending hundreds to thousands of kilometers in length.
(a) Sites of extensive basaltic eruptions where ocean floor is created.
(b) Characterized by high heat flow and shallow earthquakes.
(c) Ridge axes may contain numerous submarine hot springs where heated seawater is discharged.
(d) Figure 7.19: new ocean floor moves laterally away from the spreading center at rates ranging from 1-18 cm/yr.
(e) The age of oceanic crust increases away from the ridge axis.
(f) Ridges are divided into segments separated by transform faults that compensate for variable spreading rates along the ridge axis.
(g) Overlying thickness of deep-sea sediments increases away from the ridge axis.
(h) Figure 7.26: The oldest crust within an ocean basin is generally adjacent to a subduction zone or continental margin while the youngest occurs along the ridge axis.
Seafloor Magnetic Anomalies and Polarity Reversals
F.J. Vine and D.H. Matthews (1963)
(a) Figure 7.23: Studied ocean floor magnetic anomaly patterns, noting symmetry with respect to mid-ocean ridges.
(b) Established the relationships among magnetic anomalies, polarity reversals and seafloor spreading.
(c) Figure 7.24: noted symmetry of magnetic anomaly profiles with respect to the ridge axis.
(d) Figure 7.25: Vine and Matthews compared the magnetic anomaly patterns on the seafloor with the magnetic polarity time scale that had been established on land and were able to assign ages to the various magnetic stripes on the seafloor. In addition, isotopic and fossil ages, determined through deep sea drilling of sediments directly above oceanic basalt, helped bracket the ages of individual magnetic stripes.
(e) Figure 7.26: Age dating of seafloor magnetic anomaly patterns eventually led to construction of maps depicting the ages of various segments of oceanic crust.
1. The distribution of earthquake epicenters and most active volcanoes define the plate boundaries (Figures 7.27 & 7.28).
Three Types of Plate Boundaries
(a) Figure 7.19: A divergent (constructive) plate boundary is where two plates move away from one another along a mid-ocean ridge. New lithosphere is created along a divergent plate boundary.
(b) A convergent (destructive) plate boundary is where one plate is subducted beneath or collides with another plate. Lithosphere is generally destroyed along a convergent plate boundary.
(c) A transform boundary is where one plate slides against another. Lithosphere is conserved (neither destroyed nor created) along a transform boundary.
1, Figure 7.30: Four mechanisms have been suggested to drive lithospheric plates.
(a) Ridge Push: a spreading ocean ridge actually pushes the plate.
(b) Sliding: a plate slides down the slopes of an ocean ridge.
(c) Gravitational Pull: a cold, downgoing plate along a subduction zone pulls the rest of the plate behind it.
(d) Piggyback: plates are carried along the backs of convection cells.
2. Figure 7.31: There are several different types of lithosphere plate interactions:
(b) Obduction is when oceanic crust is thrust upon lighter continental crust.
(c) A zone of intensely compressed and metamorphosed ultramafic rocks and basalt define a suture zone where two plates collided.
(d) An agglomeration of microcontinents and arcs separated by suture zones is called a collage, the assembly of which is called collage tectonics.
Subsidence Of Plates And Accumulation Of Sediments
1. Figure 7.32: Six major types of sedimentary basins.
(f) Passive Margin
2. Figure 7.33: Rift Basins are associated with extensional environments such as mid-ocean ridges and certain areas of continents. Rift Basins are also found beneath Passive Margin Basins that together are related to the creation of a broad continental shelf during opening of an ocean basin.
3. Figure 7.34: Rift Basins generally contain sediments dumped into a localized, rapidly subsiding depression and may include lacustrine (lake) deposits and organic-rich shales associated with clastic sandstones and conglomerates. These rift basins are often capped by evaporites that signal the invasion of salty seawater. Passive Margins contain sedimentary rocks deposited in a broad, stable continental shelf and include sandstones, shales and possibly limestones.
4. Different basins reflect different types of subsidence:
(a) Crustal-thinning subsidence provides space for thick accumulation of sediment and typically is associated with thermal subsidence along rifts.
(b) Thermal or cooling subsidence results from cooling of the lithosphere. This type of subsidence is important in the formation of rift basins and passive margins.
(c) Figure 7.35: Sediment loading usually follows crustal thinning and thermal subsidence along passive margins and is important in all basins. The accumulation and building out of sediments actually weigh down the lithosphere, causing it to further sag.
(d) Figure 7.37: Subduction subsidence produces trench basins and is caused by the forcible depression of one lithospheric plate as it is subducted beneath another.
(e) Figure 7.36: Thrust loading involves the lateral shoving of huge slabs of rock in a series of overlapping, low-angle thrust faults. The downward flexing of lithosphere adjacent to the mountains produces a foreland sedimentary basin with a complimentary bulge, or cratonic arch, and a cratonic basin beyond the arch.
5. Table 7.2: Summarizes the major types of sedimentary basins and their characteristics.