Structural Geology and Isostasy Notes

Rock Deformation

Permanent Deformation
  • Occurs when applied stress exceeds the rock's strength.
  • Examples: Folding and faulting.
Nonpermanent (Elastic) Deformation
  • Occurs when weak stresses are applied.
  • Stress and strain are proportional; the rock returns to its original form when stress is removed.

Isostasy and Glacial Rebound

Glacial Impact on Lithosphere
  • During the last glacial cycle (25,000-15,000 years ago), large ice sheets covered northern North America.
  • The weight of a 12,000-foot thick ice sheet depressed the lithosphere by one-third of its thickness.
  • The asthenosphere was displaced by the ice load.
    • The lithosphere depression related to density differences
Isostatic Equilibrium
  • The lithosphere is in isostatic equilibrium with the asthenosphere, "floating" on it.
  • A load, like an ice sheet, causes the lithosphere to depress.
  • When the ice melts, the lithosphere rebounds.
  • Examples of areas experiencing isostatic rebound:
    • Hudson Bay
    • Puget Lowland
    • Great Lakes
Vertical Crustal Motions
  • Areas where the ice sheet was thickest, like Hudson Bay, are still rebounding (up to +18 mm/yr).
  • Subsidence (down to -6 mm/yr) occurs as mantle material moves away from these rebounding areas.
  • Strandlines form as wave-cut shorelines emerge from below sea level.
Great Lakes Example
  • The northern Great Lakes area is rebounding at 5 mm per year.
  • Currently, the Great Lakes drain east via the St. Lawrence River into the Atlantic Ocean.
  • Eventually, the Great Lakes may drain south to the Gulf of New Mexico via the Mississippi River system as the land recovers.
Reservoirs and Isostasy
  • The lithosphere can rise and depress with the filling and emptying of a reservoir (1-2 cm/yr).
  • Puget Sound's tectonic setting and isostatic considerations influence its persistence.

Rock Deformation Types

Stress Types
  • Permanent deformation can be compressional, tensile, or translational (shearing).
  • The tectonic setting influences the stress type.
    • Compressional stresses versus tensile or shearing stresses.
Deformation Mechanisms
  • Permanent deformation occurs when shear stress exceeds shear strength.
  • Brittle fracture (faulting) or ductile deformation (folding) can result.
Temperature and Ductility
  • Temperature is related to rock ductility (glass blowing analogy).
Ductile Deformation Conditions
  • Glacial ice can deform ductily under high confining pressure and low strain rate.
  • Marble, typically brittle, can deform ductily when the strain rate is low.
  • Interlayered sandstone and shale can deform both ductily and by brittle fracture.

Faults

Vertical Fault Classification
  • Classified by offset of the hanging wall relative to the footwall.
    • Normal fault: Hanging wall is down-dropped.
    • Reversed or thrust fault: Hanging wall is upthrown (thrust faults are low-angled).
Basin and Range Province
  • Characterized by normal faulting.
  • Extensional stress field.
Overturned Folds and Thrust Faults
  • Overturned folds often evolve into thrust faults under continued compressive stress.
  • Collisional tectonic mountain ranges (e.g., Canadian Rocky Mountains) often exhibit fold and thrust fault structures, placing older Precambrian rock over younger Paleozoic rock.
Strike-Slip Faults
  • Classified by relative motion of fault blocks.
San Andreas Fault
  • A right-lateral strike-slip fault.
  • Horizontal motion between tectonic plates defines a transform fault.
Bends in Strike-Slip Faults
  • Can result in compression mountain ranges and pull-apart basins.
  • The Transverse Ranges in southern California formed where a restraining bend occurs within the San Andreas Fault.

Folds

Fold Formation
  • Ductile deformation results in fold structures.
  • Compressive stress causes folding.
  • Folds have two limbs and an axis.
  • The axial plane is an imaginary plane that extends through the axis.
Fold Classification
  • Based on the symmetry of limbs relative to the axial plane.
Monoclines
  • Form when the stress field is unidirectional.
  • Often associated with the reactivation of older faults in brittle basement rock.
  • The fold structure is confined to the ductile sedimentary rock.
  • Greatest erosion occurs proximal to the axis of the fold.
Anticlines and Synclines
  • Symmetric folds that form from bi-directional compressive stress.
  • Plunging anticlines:
    • The anticline is plunging into the slide (north).
Erosion Patterns of Folds
  • Anticlines expose the oldest rock in the center (near the axis) as they erode.
  • Note the difference in outcrop pattern of plunging versus non-plunging folds.
Plunging Folds
  • The fold pattern of a plunging anticline closes in the direction of plunge.
  • The fold pattern of a plunging syncline opens in the direction of plunge.
Zigzag Folds
  • Form as a series of plunging anticlines and synclines are eroded following tectonic uplift.
  • Common in uplifted fold and thrust belt mountain ranges, like the Appalachian Mountains.
  • Observed in the valley and ridge province of the Appalachian Mountains due to erosion of plunging folds.
Domes and Basins
  • Special cases of anticlinal and synclinal fold structures.
  • Oldest rock is exposed in the center of an eroding dome.
  • Youngest rock is exposed in the center of an eroding basin.
Richat Structure
  • An uplifted volcanic dome located in Mauritania, Africa.