Pullapart basin

Pull-apart basins are elongate depressions that develop along extensional steps on strike-slip faults. Pull-apart basins are features that develop in trans-tensional regions, in which the principal stresses are compressional, but some areas within the region are under extension due to the obliquity of the major stress direction with respect to the plane of failure. This results in extension of the crust along releasing bends, leading to a break in the crust and the formation of basins. some pull-apart basins show several progressive stages in their formation. others initiate along a fracture, and progress into lazy Z or S shapes, and finally progress into a basin that ranges in length-to-width ratio from 2:1 to 10:1. These types of basins are characterized by steep sides on major fault boundaries with normal faults developing on their shorter sides. Continuous movement along the major faults tends to offset deposits from their source inlet to the basin. These basins are characterized by rapid deposition and rapid facies changes along or across the width of the basin and gradual facies change along the longest axis of the basin. Pull-apart basin deposits are typically made mostly of coarse fanglomerate, conglomerate, sandstone, shales, and shallow water limestones and evaporites. Bimodal volcanics and volcanic sediments are also found interbedded within the basin deposits. These bimodal volcanics are typical of those found in rift settings, but here they are in a transtensional regime. Transcurrent faults can penetrate down deep into the crust, reaching the upper mantle and providing a conduit for magma.

See also convergent plate margin processes; divergent plate margin processes; drainage basin (drainage system); ocean basin; plate tectonics; transform plate margin processes.

FURTHER READING

Allen, Philip, A., and John R. Allen. Basin Analysis, Principles and Applications. Oxford: Blackwell Scientific Publications, 1990. Bradley, Dwight C., and Timothy M. Kusky. "Geologic Methods of Estimating Convergence Rates During Arc-Continent Collision." Journal of Geology 94 (1986): 667-681. Mann, Paul, Mark R. Hempton, Dwight C. Bradley, and Kevin Burke. "Development of Pull-Apart Basins." Journal of Geology 91 (1983): 529-554. Reading, Harold G. "Characteristics and Recognition of Strike-Slip Fault Systems." In Sedimentation in Oblique-Slip Mobile Zones, edited by Peter F. Balance and Harold G. Reading, 7-26. International Association of Sedimentology Special Publication 4, 1980.

beaches and shorelines A beach is an accumulation of sediment exposed to wave action along a coastline, whereas the shoreline environment is a more encompassing area including beaches, islands, and near-shore areas that are in some way affected by coastal processes. The beach extends from the limit of the low-tide line to the point inland where the vegetation and landforms change to that typical of the surrounding region. This may be a forest, a cliff, dune, or lagoon. Many beaches merge imperceptibly with grasslands, or forests, whereas others end abruptly at cliffs or other permanent features, including artificial seawalls that have been built in many places in the past century. A beach may occupy bays between headlands, it may form elongate strips attached (or detached, in the cases of barrier islands) to the mainland, or it may form spits that project out into the water. To understand a beach it is necessary also to consider the nearshore environment, the area extending from the low-tide line out across the surf zone. The nearshore environment may include sandbars, typically separated by troughs. The width of nearshore environments is variable, depending on the slope of the seafloor, wave dynamics, and availability of sediment. Most nearshore environments include an inner sandbar located about 100-165 feet (30-50 m) offshore, and another bar about twice as far offshore. The inner bar is often cut by rip channels that allow water that piles up between the bar and beach to escape back to sea, often generating dangerous rip currents that can drag unsuspecting swimmers rapidly out to sea.

In the eastern United States, Florida is known for wide sandy beaches, the Outer Banks of the Carolinas are famous for barrier island beaches, and Maine is well known for its beautiful rocky shorelines. The western coast has many rocky shorelines in Washington, Oregon, and California, whereas the Gulf of Mexico has low relief beaches, barrier islands, and some mangrove-dominated shorelines.

Most sandy beaches develop typical profiles that change through the seasons and include several zones. These are the ridge and runnel, foreshore, backshore, and storm ridge. The ridge and runnel is the most seaward part of the beach, characterized by a small sandbar called a ridge, and a flat-bottom trough called the runnel, and is typically fewer than 30 feet (10 m) wide. The runnel is covered by water at high tide and has many small sand ripples that get extensively burrowed into by worms, crabs, and other beach life.

The foreshore, or beach face, is a flat, seaward-sloping surface that grades seaward into the ridge and runnel, or the intertidal zone if the ridge and runnel are not present. A narrow zone of gravel or broken shells may be present at the small slope-break between the foreshore and the ridge and runnel. The foreshore contains the swash and backwash zone, where waves move sand diagonally up the beach face parallel to the wave incidence direction, and gravity pulls the water and sand directly down the beach face parallel to the slope. This diagonal, then beach-perpendicular motion produces a net transport of sand and water along the beach, known as longshore drift and longshore currents.

The backshore extends from a small ridge and change in slope at the top of the foreshore known as a berm, to the next feature (dune, seawall, forest, lagoon) toward the land. This area is generally flat

(A) Diagram of beach profile showing the major elements from the backshore to offshore; (B) different types of coastal environments, including barrier beaches, bays, and spits; (C) typical beach profiles in summer and winter showing how large winter storms erode the beach and smaller summer waves rebuild the beach

Lagoon

Swash zone

Barrier island i beach berm /

Surf zone

Wavelength

Backshore

Foreshore

Wave height o o o

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