Salients, Recesses, Arcs, Oroclines, and Syntaxes - A review of Ideas Concerning the Formation of

map-view Curves in Fold-thrust Belts

Stephen MarshakDepartment of Geology, University of Illinois, Urbana, Illinois, U.S.A.

Outline

• Abstract• Terminology ( 術語；術語學 )• Historical Background• Formation of the Curves: 1.Basin-Controlled Curves 2. Result from Irregularities on Colliding Margins 3.Other Possible Causes• Conclusions

Abstract• How map-view curves in fold-thrust belts originate?• Review the advances in understanding curves• Basin controlled predeformational

sedimentary basin .• Other causes for curve formation.

Abstract• How map-view curves in fold-thrust belts originate?• Review the advances in understanding curves• Basin controlled predeformational

sedimentary basin .• Other causes for curve formation.

• Over the years, geologists have used a variety of names for the map-view curves of mountain belts

Compile a bibliography on curves• Map-view curves: A regional progressive change in an orogen’s

structure trend, regardless of the origin of the change.

Remark: orocline, arc( 弧 ), syntaxis, curve( 曲線 ), bend( 彎曲 ), virgation, salient( 凸出的 ), festoon( 花彩；花彩裝 飾物 ), arcuate range( 拱形列 ), oroflex, and recess( 凹入的 )

Terminology

Terminology• Salients & Recesses

Basic terminology for describing map-view curves.(a)The contrast between

salients and recesses. Arrow indicate the regional

transport direction

Terminology• Rotational curves & Nonrotational curves

(a) In rotational curves the trace of regional structures changes strike with progressive deformation. The short line segments represent the trends of the thrusts.

(b) (b) In nonrotational curves, structure initiate with a curved trace.

Terminology• The description of the geometry of a specific curve.

Geometric elements(endpoints, apex, amplitude, leading edge, trend lines) of a single curve. RL: reference line 參考線 A: amplitude 振幅

Historical Background

• 18 century, geologists believed that trend lines provided a clue to the origin of mountain.

• 19 century, geologists began to speculate that horizontal shortening may be involved in mountain-belt formation.

• 20 century dawned, geologists still struggled to understand the basic physical process of mountain building.

mountains form as a consequence of vertical and horizontal displacements.

Historical Background• In Alfred Wegener’s 韋格納 (1929) classic book, Origin of

Continents and Oceans, he suggested that regional curves in orogens indicate that continents move.

(b) Illustration from Wegener (1929), illustrating the curves in the Scotia Sea area.He argued that they formed as a result of the westward movement of South America and Antarctica .

Historical Background

• In mid-20 century, the curve formation issue once again gained attention paper by S. W. Carey(1955) in which he argued that curves form by the bending of orogens when crustal blocks rotate with respect to one another.

• Carey, like Wegener, used the presence of curved orogens as evidence for the mobility of the earth’s solid surface.

S. W. Carey

Historical Background

• However, only after geologists began to study subduction systems did the curve issue gain global significane once more.

• Specifically, geologists wondered why many “volcanic arcs” are , in fact, map-view arcs.

Sketch map of Asia, by Willian Hobbs, Illustrating the “arcs” of Asia.

Historical Background

• Study of map-view curves in continental orogens took a step forward when new tools became available for studying rock information.

1.Strain analysis 應變分析 2.Kinematic analysis 運動學分析 3.Paleomagnetic analysis 古地磁分析 4.Analog(sand-box and clay) modeling 類比 ( 砂盒及黏土 )

模型 5.Computer simulation 電腦模擬

1.Strain analysis 應變分析

• As Ries and Shackleton(1976) discussed, the strain distribution in a curve varies depending on whether the curve forms by variations in an amount of foreland transport along strike or by bending of a once-straight belt between 2 fixed endpoints.

Illustrate possible strain fields in curves

2.Kinematic analysis 運動學分析• Kinematic analysis defines displacement-path

trajectories ( 位移路徑的軌跡 ) in a curve.

Possible displacement-path trajectories in curves shown in map view. The arrows indicate the path of particles during the evolution of the thrust trace from its position at Time1 to its position at Time2.Particles must follow curved trajectories in oroclines, but not in nonrotational (primary) curves.

3.Paleomagnetic analysis 古地磁分析• Paleomagnetic studies, in particular, focus on

distinguishing rotational curves from nonrotational curves.

Illustration of the consequence of oroclinal bending on the pattern of magnetic declination in a thrust belt. The shaded are represents the outcrop belt of a given unit, in map view, and the arrows represent paleomagnetic declinations.

4.Analog(sand-box and clay) modeling 類比 (砂盒及黏土 )模型

• Most modeling work in the past 2 decades has used sand, because at the scale of a laboratory model, sand simulates the behavior of Coulomb materials such as the uppermost crust .

*Coulomb materials = a granular material whose bulk strength is provided by friction between grains.

Basin-Controlled Curves

• Fold-thrust-belt curves whose position and geometry reflect characteristics of predeformational sedimentary basins are called basin-controlled curves.

Basin-Controlled Curves

1.Along-strike Changes in the Depth to Detachment 2.Along-strike Changes in Slope of the Basal Detachment 3.Along-strike Changes in Strength of the Thrust Wedge 4.Along-strike Changes in Erosion and Plutonism 5.Shape of Basin-controlled Curves

1.Along-strike Changes in the Depth to Detachment• Fold-thrust-belt width depends on depth to detachment

simply to mountain volume balance for a given critical taper angle, as simple sandbox models below.

2.Along-strike Changes in Slope of the Basal detachment

• If the detachment angle is shallow, the thrust wedge thickens internally and thus propagates only a short distance into the foreland for a given displacement of the backstop.

• on the contrary…

(d,e)An increase in the detachment slope leads to an increase in the distance that the thrust wedge propagates to the foreland .

3.Along-strike Changes in Strength of the Thrust Wedge

(b,c)The width of a wedge depend on the value of the critical-taper angular, which is turn reflects the strength of the wedge or the strength of the basal detachment.

• Wedge made of strong rocks have a greater taper angle than wedges made of weak rocks.

• Thus, wedges made of weak rocks propagate relatively farther into the foreland for a given amount of regional shortening.

4.Along-strike Changes in Erosion and Plutonism

• Erosion: Erosion of strata in the hinterland of the wedge and simultaneous

accumulation of sediment at the toe of the wedge will decrease the surface slope of the wedge, and therefore will cause the wedge to become subcritical.

• Plutonism( 深成現象 ): Addition of rock to hinterland portion of a wedge by intrusion of

igneous masses will effectively increase the surface slope and cause the wedge to become supercritical, thereby leading to the foreland advance of the toe of the wedge and the formation of a salient.

5.Shape of Basin-controlled CurvesA steep-walled aulacogen( 裂陷

槽 ) yields a flat-crested salient.

An asymmetric aulacogen yields an asymmetric salient.

A spoon-shaped basin yields a parabolic( 拋物線 ) salient.

A basement obstacle yields 2 salients with an intervening recess.

Curves Resulting from Irregularities on Colliding Margins ( 碰撞邊緣的不規則 )

1.Indenter-controlled Curves2.Margin-controlled Curves3.Obstacle-controlled Curves

1.Indenter-controlled Curves

Sketches of sandbox models illustrating the consequence of impressing a hinterland indenter into a sedimentary basin.

Example of Indenter-controlled Curves

• (d,e) Sketch on a model for the development of the Himalayan curves. In this model, India is a wedge-shaped indenter, and the Himalayan/Tibet orogen is a bivergent orogen.

2.Margin-controlled Curves

Illustration of the concept of a margin-controlled curve.(a) Predeformation

condition shows an offshore arc converging with an irregular continental margin.

(b) After collision, the orogen molds to the pre-existing shape of the margin.

3.Obstacle-controlled Curves

Other Possible Causes Of Curve Formation

1.Curves Formed at the Intersection of Nonparallel Belts2.Curves Formed in Response to Strike-slip Faulting3.Curves Reflecting the Shape of the Downgoing Slab4.Curves as Artifacts of Cross Folding

1.Curves Formed at the Intersection of Nonparallel Belts

If the trend lines of different segments are not parallel, or if the different segments are separate salients, a recess forms where the segments overlap.In such recesses, younger thrusts carry older faults in their hanging wall and thereby bend the older structures around a vertical axis.

2.Curves Formed in Response to Strike-slip Faulting

• Movement on a crustal-scale strike-slip fault that cuts across a fold-thrust belt can cause a pronounced curvature to the belt.

3.Curves Reflecting the Shape of the Downgoing Slab

If the downgoing plate is curved, the thrust wedge must have a curved leading edge in order to maintain contact

with the downgoing plate.

4.Curves as Artifacts of Cross Folding

A 3-D sketch illustrating a recess forming as a consequence of the erosion of a folded thrust sheet.(a)A warped thrust surface.(b)Outcrop pattern of the thrust surface on a horizontal map plane.

Conclusions

• Fold-thrust-belt curves have fascinated geologists since the discipline of geology began, because they provide insight into the process of orogeny.

• Early studies viewed curves to be a necessary consequence of orogeny.

• In recent decades, however, it has become clear that curves form in response to local conditions- they reflect basin geometry, the impact of indenters, overprinting by strike-slip faults, buckling of the downgoing slab, overlapping of nonparallel thrust belts, and interaction with foreland obstacles.

Conclusions• Basin-controlled curves : the shape of the curve

provides constraints on the shape of the predeformational basin.

• Notably, not all curved fold-thrust belts reflect oroclinal (rotational) bending.

• For example, faults in basin-controlled and indenter-controlled curves initiate with curved traces.

Conclusions

• Future kinematic and strain studies of curves, along with paleomagnetic work, will further refine knowledge of curve formation.

• This work has applications to petroleum exploration , in that the conditions leading to oil-field( 油田 ) generation correspond to the conditions leading to basin-controlled salient formation.

• Thus, major fold-thrust-belt-related oil provinces lie at the apex of basin-controlled salients.

Thank for listening