Table Of ContentSedimentology and Thennal
Mechanical History of Basins in the
Central Appalachian Orogen
Pittsburgh, Pennsylvania to Wallops Island, Virginia
July 1-8, 1989
Field Trip Guidebook T152
Leaders:
Rudy Slingerland Kevin Furlong
Associate Leaders:
warren Manspeizer Christopher Beaumont John Diemer
wayne Newell Jacqueline Huntoon
American Geophysical Union, Washington, D.C.
COVER Synthetic-aperture radar image of the Harrisburg, Pennsylvania region (north
to the top).
Leaders:
RUdy Slingerland and Kevin Furlong
Department of Geosciences
Pennsylvania State University
University Park, PA 16802
Copyright 1989 American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-615-X
Printed in the United States of America
IGC FIELD TRIP TIS2:
SEDIMENTOLOGY AND THERMAL-MECHANICAL lDSTORY
OF BASINS IN THE CENTRAL APPALACIDAN OROGEN
Rudy Slingerlandl, Kevin P. Furlongl, Warren Manspeizer2,
Jacqueline Huntoon1, Mark Lucas2, Christopher Beaumont3
4
John Diemer
INTRODUCTION Brief Overview of the Appalachian Orogen
This sedimentary basin workshop and field trip will 'fhe Appalachian orogen sensu stricto, was created
examine the interplay between basin tectonics and as a result of Late Proterozoic (610-630 Ma) rifting of
sedimentary deposits in foreland, rift, and, to a lesser Gondwana (Africa and South America) and Laurentia
extent, passive margin basins of the well-studied (proto-North America) (Cook et aI., 1983; Cook and
central Appalachian orogen (Fig. 1). We will describe Oliver, 1981), along a trend approximately coincident
the tectonic characteristics of each basin type using with the axis of the present Appalachians (see
thermal-mechanical models of the crust and then Bouguer gravity gradient in Fig. 18 of Slingerland and
discuss the nature of their basin fills in that light. Beaumont, this volume). Metamorphic and plutonic
In this way we hope to better understand the rocks of the Grenville Province, with radiometric ages
relationships between tectonic characteristics---basin of 1.3 to 1 Ga, were stretched to produce a series of
geometry, subsidence history, and relative topographic grabens filled with thick sequences of Eocambrian
relief, for example---and basin-fill characteristics such sedimentary rocks such as the Ghilhowee Gp. (Fig. 2)
as depositional environments and resulting lithofacies, and volcanic rocks such as the Catoctin Fm. in
isopach patterns, and on-or off-lap patterns. southeastern Pennsylvania. As the large, near-surface
The material presented here, limited by publication thermal gradients associated with rifting decayed, a
factors, is arguably the minimum necessary to passive nlargin developed upon which a thin Lower
accomplish these goals. Following these comments, a Cambrian transgressive clastic sequence (eg. Antietam
few paragraphs outline the geologic history of the Fm.) was succeeded by a 4 km thick sequence of
Appalachian orogen. This is followed by a Cambro-Ordovician platform carbonates.
presentation of a foreland flexural model and its The passive margin in the central Appalachians was
application to the Late Paleozoic foreland basin of the disrupted in Caradocian time when eastern Laurentia
central Appalachians. A similar article presents a rift (North America plus Greenland, Scotland, and northern
model and its application to the Mesozoic basins of Ireland) collided with an island arc and a set of
eastern North America, and in particular, the Newark microcontinents along an outboard-dipping subduction
Basin. No passive margin model is presented as such; zone (see Fig. 4, panels I-V in Slingerland and
the important considerations are presented under the Beaumont, this volume) (for a detailed account in New
rift model. These are followed by the description of England see Stanley and Ratcliffe, 1985). The
specific field localities in the central Appalachian resulting overthrusting event, called the Taconian
region, chosen to illustrate the database upon which orogeny, depressed the foreland and allowed
many of the arguments rest. accumulation of over 1.8 km of sediment in
Pennsylvania between Middle Ordovician and Early
Acknowledgments Silurian time. This is the first of three eastward
derived clastic wedges, the Taconian wedge,
We thank John S. Bridge, William Duke, Terry represented in central Pennsylvania by the Antes
Engelder, Edward S. Belt, and Mark Sholes who each Shale through Tuscarora Formation (Fig. 2)- (Lash,
reviewed a part of the manuscript. This research was 1987; Lash and Drake, 1984; Rodgers, 1970).
partially supported by The Pennsylvania State Modelling by Beaumont et aI. (1988) of the type
University Earth System Science Center, Elf Aquitaine described in Slingerland and Beaumont (this volume),
SA, and the Donors of the Petroleum Research Fund, indicates that between 8 and 12 km of overthrust load
administered by the American Chemical Society (Grant is necessary to accommodate the maximum 3 km of
No. 16560-AC2). Taconian detritus preserved in the basin. As
explained later, the outboard region of a rifted
cratonic margin can accumulate up to about 20 km of
1Department of Geosciences, The Pennsylvania State overthrust material before a mountain range of any
University, University Park, Pennsylvania. consequence is created. This arises because seaward
2Department of Geology, Rutgers University, of the Bouguer gravity gradient marking the
Newark, New Jersey. continent-ocean crustal transition, thrust sheets
30ceanography Department, Dalhousie University, replace \\later and load an attenuated continental crust
Halifax, Nova Scotia, Canada. and oceanic lithosphere. Thus the Taconian
4Department of Geology, Franklin and Marshall overthrusts loading the Cambro-Ordovician slope and
College, Lancaster, Pennsylvania rise probably were of modest subaerial topographic
T152: 1
FIGURE 1 General physiography of the central Appalachian orogen and course of the field trip. Numbers
represent days from the start of the trip. Latitude and longitude lines are at 20 intervals; 20 longitude equals
approximately 200 km. From southeast to northwest the orogen consists of an unconsolidated coastal plain of
Cenozoic passive margin sediments; a low relief piedmont underlain by·igneous and metamorphic rocks·of late
Precambrian to Pennsylvanian Age, occasionally interrupted by lowlands of the Mesozoic basins; a crystalline
mountain range (labeled Blue Ridge)· of Precambrian and earliest Paleozoic age; ridges and valleys of the
Alleghanian fold and thrust belt; and a heavily dissected plateau formed on the more nearly flat-lying rocks of
the fold and thrust belt (Modified from Raisz, 1954).
T152: 2
--'--·-------'-t-M>>-,FAaINrSlml--aplon[drnIe\gali--IlalaiIi\nnmla.loIni nioiao\nnliiaVannVI0I"I'\AINIofc>azc=1::f::: >xx iV:::I'\IAooUNZw ----....0052107.2.228.3799728 fF-oSr-BecFFkhmr-mnaeFolrmlr-biesr-· ~H~lroFacvmikenVTaSrMRlliemboyFhrcmm.k*ers - !~~ ~ c 0cZZ0~::x: -P-lHeFa-imlsl*a-nt-?I--El-bFrom-ok-- ,--1--:< e:::g-E u~acccZ::otx:::
>- U Harrell·1 Waynesboro*Fm
""i:~ ~ 1r~:: Ifa~f:-i: .g'..E..g.~~,Sy~hMe~orinmlSlehnbeRerlildRogi·"dMMbg're" --:J~.~...- 385 ~~~ .~~ TomFstmown* ~ ~
~~'~~Vl:-\s-Iov\r-A~n7e~~.-c.rD-t_s-s-c:u,.t-"-j:rf.YY:--oork:ar-kt"H~:J-aHa;vcaue:-knvse~-o;-tnny·-tw'ytpp:a-e~el-.dbl;.a0.a-slas-lo-_tM--MU:rVJYlCvY:.~!'~Q~~~~;N.Y_rI3mV:I[.\.~_.r!.!JA[~N"1u0=r~cWt:>~I:l:M[[OIVUIVrIVI111-29405000 ~~igE~:i~~--~CNShRSa-eGMunIsneal;Mny'aMdis;vebnribrR~drSe'ngcr;>~uO~DrkVla:MelemybSIR:larLtiiShaduJg~,els'-MM:~,bb~rr' i1jIl~Ilj~! -....~<w]Q?1ij-':i:E5 0U 339950 :W;;/~qn~r;f1fJ!t!fj,~-~5j1C1a1::tA:1o:H:n:cBattiFrie~nMplIa;emoIr-MFnsImrtbaFrI-lmto-' -£z<~olc: I ua«aa~Z~::l:: 570
~ ~ !! N N r~~~ - Q..- 90U
BrunFsmwick ~o Z :=J uUaCc::/x5:): 0~(WJ) ~215 Corriganvillels lomwberr I~e;, 51ecn .3 0«.W....J _400 .if~ :'~C::i 160G
~LOCkaIOngFmL I- ~220 NewCreekls Q.. 405 FIGURE 2 Stratigraphic
SlocklonFm ,~... c KeyserFm -0 correlation chart for central
I II I " I TonolowayFm Pennsylvania (from Berg, et aI.,
'ililllIlll[l[[[ ~ BloomsburgFm u_;;~ 1983). Absolute scale in
~1;u1r7~~:._ f---;-:;I-- acZ::x:: 415 millions of years before present.
J;; :3
:i RochesterMbr i,- U5 42
RoseHillFm z ~
__________ .--.?--.--+---+--4-----I 290
Tuscarora·Fm ~
~-::-----t
,+---~--::,.--f-~-+-+-+----l 43
..N\/VV'\I\-?,/\/\IV\/\,?./\/VVVVV 0T~ ~le'FJmun~iata*~Fm j i:i~
Conemaugh!?lGp .~_e- 310 Reedsville·Sh Martinsburg ~~-
AlleghenyGp Fm 45
SalonaFm
PottsvilleGp ~::'::::.'C.hFa~mbersbu~rg·,
8 ~nyller tm
Fm
I I 330 I.H,alter0·11F'm11 ~CD!1~ - :U;c:>2:x: 46
Loys.r~ 5t.Paul 0
MauchFmChunk 340 b~~'ofIMilroUyp"peMrbri PineGsbpurg f--;~:; c00c 475
IIlB--Ou-rY-5g-ols--oln---h--l'"-n'P-no-FcI1moFnom_~.li.E.eo.B.w-~.e..M~c--km:b-vr-lK~il-l-e• 355 ~i~E~3: rNAiJ-itxtae-nFmyma~*-mn-nbF*~rm ~RRoSuctnaFk~tmdiFoamlne ]~><~ceC;[;;,f4; 485
• Mbr
Rockwell '1''yyy?f larke*JS~_lo_ne_hen_ge_'_Fm-l
Fm J)J..?.}v F~- /Stoufferstown*
-------mfASpechty. :--?....L...,--+--+--i-----i :lGS _ I--~---Mbr-- 500
1--------r-----;¥J1v(!JJ i Mines*MbrShadygrove'Fm
Upper
! sandy
, mbr
Duncannon E'-
Mbr l: !Ore!Hill_'Mbr
~ lower
Hampshire ~ ---'s!an!d!y!!.-
~
Fm I I Slac'Mbr
~I'
Sherman
Creek- .::::
Mbr ~
---~
T152: 3
relief. Our interpretation is that the orogen varied whole eastern half of the orogen was subjected to
along strike among states 111-V in Figure 4 of folding and thrusting, and, to a lesser extent,
Slingerland and Beaumont (this volume) by the end metamorphism and plutonism from relative
of the Taconian orogeny. transpression. (see Slingerland .and Beaumont, this
Following the Taconian orogeny, sedimentation rates volume for details).
declined in the basin. Approximately 900 m of The Permian and Early Triassic history of the
carbonates, salt, fine-grained clastics, and thin, Appalachian orogen is uncertain, because there are no
mature shelf sandstones were deposited during Middle preserved deposits of that age. It is clear however
Silurian to Early Devonian time" (Fig. 2), reflecting (Fig. 2), that by the Carnian or late Landinian (230
relative tectonic quiescence along the orogen. 225 Ma) sediments had begun accumulating in basins
Although plate convergence continued along the along reactivated strike-slip and thrust faults
eastern Laurentian margin during this interval (Van (Manspeizer and Cousminer, 1988; Traverse, 1987),
der Voo, 1988), crustal loading by overthrusting recording the initial breakup of Pangea (days 6 and
apparently was minor. 7). Rupture occurred roughly along the present
Commencing in the Early Devonian in New England continental shelf edge (see Manspeizer and Huntoon,
and ending in the Early Mississippian in Pennsylvania,' this volume, for details) and sea-floor spreading
convergence between Laurentia and an unspecified began'between late Early to Middle Jurassic (190-175
plate (Ferrill and Thomas, 1988) produced a Ma)(Klitgord and Schouten, 1986, p.364).
metamorphic, plutonic, and loading event called the A second passive margin developed, of broad
Acadian orogeny. The resulting foreland basin fill in platforms having fairly thin sediment cover and basins
the central Appalachians is called the Catskill-Pocono whose margins probably mark the sites of transform
clastic wedge (Marcellus through Pocono Formations, faults' active during the initial breakup (Folger et aI.,
Fig. 2), and is the subject of our field trip on days 3 1979). Jurassic sediments of the passive margin tend
and 5. to be terrigenous lagoonal, fluvial, or deltaic
Closing of the proto-Atlantic continued during the nearshore lithosomes ponded behind widespread
Mississippian to Permian, culminating in the collision carbonate build-ups at the shelf edge. During the
of Gondwana with eastern North America and the Cretaceous and into the Cenozoic, a thick sequence of
third Paleozoic deformation event, the Alleghanian fluvial, deltaic, and shelf sediments prograded seaward
orogeny. Outboard loading rejuvenated the Acadian to form a w~ll defined sl9pe and rise. The result is
foreland basin, and it received a minimum of 7.5 km an eastward-thickening wedge of prinlarily
of sediments from the orogenic highlands to the east unconsolidated sedilnellts, about 2.4 km thick in the
(Mauch Chunk through Conemaugh Fms. of Fig. 2 seen Delmarva area, thickening to 9 km· in the Baltimore
on field trip days 4, 5, and 6). Subsequently the Canyon Trough (Folger et aI., 1979) (day 8.)
TECTONICS AND SEDIMENTATION
OF THE UPPER PALEOZOIC FORELAND BASIN
IN THE CENTRAL APPALACHIANS
Rudy Slingerland and Christopher Beaumont
INTRODUCTION Our intention here is to illustrate just such an
interplay between tectonics and sedimentation in a
Foreland basins are sedimentary basins lying particularly revealing example, the Appalachian
cratonward of major compressional zones. They are foreland basin of the Appalachian Orogenic Belt. Our
formed during continent-continent collisions as a method is to first describe some concepts of basin
result of outboard crustal loading, or by a creation using models of flexural response of the
combination of loading and subduction of oceanic lithosphere and then to describe and interpret the
lithosphere. Those due primarily to outboard loading character of two orogenies---the Acadian and
are especially interesting because the creation of the Alleghanian---and the foreland clastic wedges that
basin and the source terrain both arise from the same resulted from them. The treatment is general; details
cause --- thickening of the crust by overthrusting. of the geodynamic modelling can be found in Quinlan
In these basins we expect to see a pattern of and Beaumont (1984), Stockmal et aI. (1986), Beaumont
evolution that reflects adjustments .to the size and et aI. (1987), Beaumont et al. (1988), and Jamieson and
rate of application of the overthrust load, variations Beaumont (1988). More in-depth discussions of the
in time and space of the lithospheric rheology, and field relationships and tectonic evolution can be found
feedback between sedimentation in the basin and rates in Fisher et al. (1970), Williams and Hatcher (1982),
of erosion of the thrust stack.
T152: 4
relief. Our interpretation is that the orogen varied whole eastern half of the orogen was subjected to
along strike among states 111-V in Figure 4 of folding and thrusting, and, to a lesser extent,
Slingerland and Beaumont (this volume) by the end metamorphism and plutonism from relative
of the Taconian orogeny. transpression. (see Slingerland .and Beaumont, this
Following the Taconian orogeny, sedimentation rates volume for details).
declined in the basin. Approximately 900 m of The Permian and Early Triassic history of the
carbonates, salt, fine-grained clastics, and thin, Appalachian orogen is uncertain, because there are no
mature shelf sandstones were deposited during Middle preserved deposits of that age. It is clear however
Silurian to Early Devonian time" (Fig. 2), reflecting (Fig. 2), that by the Carnian or late Landinian (230
relative tectonic quiescence along the orogen. 225 Ma) sediments had begun accumulating in basins
Although plate convergence continued along the along reactivated strike-slip and thrust faults
eastern Laurentian margin during this interval (Van (Manspeizer and Cousminer, 1988; Traverse, 1987),
der Voo, 1988), crustal loading by overthrusting recording the initial breakup of Pangea (days 6 and
apparently was minor. 7). Rupture occurred roughly along the present
Commencing in the Early Devonian in New England continental shelf edge (see Manspeizer and Huntoon,
and ending in the Early Mississippian in Pennsylvania,' this volume, for details) and sea-floor spreading
convergence between Laurentia and an unspecified began'between late Early to Middle Jurassic (190-175
plate (Ferrill and Thomas, 1988) produced a Ma)(Klitgord and Schouten, 1986, p.364).
metamorphic, plutonic, and loading event called the A second passive margin developed, of broad
Acadian orogeny. The resulting foreland basin fill in platforms having fairly thin sediment cover and basins
the central Appalachians is called the Catskill-Pocono whose margins probably mark the sites of transform
clastic wedge (Marcellus through Pocono Formations, faults' active during the initial breakup (Folger et aI.,
Fig. 2), and is the subject of our field trip on days 3 1979). Jurassic sediments of the passive margin tend
and 5. to be terrigenous lagoonal, fluvial, or deltaic
Closing of the proto-Atlantic continued during the nearshore lithosomes ponded behind widespread
Mississippian to Permian, culminating in the collision carbonate build-ups at the shelf edge. During the
of Gondwana with eastern North America and the Cretaceous and into the Cenozoic, a thick sequence of
third Paleozoic deformation event, the Alleghanian fluvial, deltaic, and shelf sediments prograded seaward
orogeny. Outboard loading rejuvenated the Acadian to form a w~ll defined sl9pe and rise. The result is
foreland basin, and it received a minimum of 7.5 km an eastward-thickening wedge of prinlarily
of sediments from the orogenic highlands to the east unconsolidated sedilnellts, about 2.4 km thick in the
(Mauch Chunk through Conemaugh Fms. of Fig. 2 seen Delmarva area, thickening to 9 km· in the Baltimore
on field trip days 4, 5, and 6). Subsequently the Canyon Trough (Folger et aI., 1979) (day 8.)
TECTONICS AND SEDIMENTATION
OF THE UPPER PALEOZOIC FORELAND BASIN
IN THE CENTRAL APPALACHIANS
Rudy Slingerland and Christopher Beaumont
INTRODUCTION Our intention here is to illustrate just such an
interplay between tectonics and sedimentation in a
Foreland basins are sedimentary basins lying particularly revealing example, the Appalachian
cratonward of major compressional zones. They are foreland basin of the Appalachian Orogenic Belt. Our
formed during continent-continent collisions as a method is to first describe some concepts of basin
result of outboard crustal loading, or by a creation using models of flexural response of the
combination of loading and subduction of oceanic lithosphere and then to describe and interpret the
lithosphere. Those due primarily to outboard loading character of two orogenies---the Acadian and
are especially interesting because the creation of the Alleghanian---and the foreland clastic wedges that
basin and the source terrain both arise from the same resulted from them. The treatment is general; details
cause --- thickening of the crust by overthrusting. of the geodynamic modelling can be found in Quinlan
In these basins we expect to see a pattern of and Beaumont (1984), Stockmal et aI. (1986), Beaumont
evolution that reflects adjustments .to the size and et aI. (1987), Beaumont et al. (1988), and Jamieson and
rate of application of the overthrust load, variations Beaumont (1988). More in-depth discussions of the
in time and space of the lithospheric rheology, and field relationships and tectonic evolution can be found
feedback between sedimentation in the basin and rates in Fisher et al. (1970), Williams and Hatcher (1982),
of erosion of the thrust stack.
T152: 4
Donaldson and Shumaker (1981), Tankard (1986), rocks decreases with increasing temperature and that
Rodgers (1987), and Van der Voo (1988). the viscosity of the mantle apparently determines the
approximately 104-105 year relaxation timescale of
glacial rebound, relaxation times spanning the range
FLEXURAL MODELS: 105-108 years are expected for. the lithosphere. Note
CONCEPTS AND BASIC RESULTS that in Figure 1 the peripheral bulge adjacent to the
flexurally downwarped region migrates toward the
The best starting point for a discussion of the surface load as stress is relaxed and the basin
models is a review of the flexural response of the deepens and narrows. This migration may uplift and
lithosphere to supracrustal loading. The lithosphere's allow erosion of sediments deposited earlier within
flexural properties determine the form of the foreland the foreland basin. In principle therefore, erosional
basin produced by a given overthrust load as shown patterns at the distal edge of the basin can be used
diagrammatically in the cross section cartoon of to determine whether the lithosphere is able to relax
Figure 1. A load emplaced on the surface of an stress and the timescale over which this relaxation
originally flat lithosphere deforms the plate into the occurs. However, there are other mechanisms, such
profile indicated by curve 1. If the lithosphere's as sea level change, that may also create
response is effectively elastic, then it will maintain unconformities, and it is therefore difficult to
attribute any particular unconformity unequivocally to
lithospheric stress relaxation.
Flexura! Response of a LIthosphere that Relaxes Stress
Panel (b) of Figure 1 assumes that part of the
orogenic load is removed from a surface made
a) Loading horizontal by erosion of uplifted areas and
sedimentary infilling of depressed areas (curve 3).
Note that the foreland response to unloading is a
mirror image of the response to loading. Uplift first
occurs over a broad region (curve 4) and becomes
successively concentrated near the unloaded region
(curves 5 and 6) if there is stress relaxation. Net
reduction of orogenic loading should therefore be
recorded in the foreland stratigraphy as an erosional
unconforlnity present over wide areas and having the
greatest missing section near the unloaded orogenic
b) Unloading region.
Two additional points can be made from these
simple concepts. First, each load change applied to
the lithosphere evolves through the same sequence of
flexural deformation. If the lithospheric response to
loading is linear, their superimposed effect in time
and space is the sum of the individual effects.
Second, an overthrust load that migrates laterally
toward the foreland faster than relaxation allows the
peripheral bulge to migrate in the opposite direction
will create an unconformity as the peripheral bulge is
driven across the foreland ahead of the overthrust
load (Jacobi, 1981; Quinlan and Beaumont, 1984).
FIGURE 1 Qualitative representation of the loading
These concepts can be combined to give a first
and unloading response of a model lithosphere that
order explanation of the sequence of events in the
releases stress by some form of thermally controlled
development of a multistage foreland basin, like the
creep mechanism. See text for discussion.
Appalachian basin (Fig. 2). The first stage shows the
development of a basin-wide unconformity as the
this flexural shape while the surface load changes.
peripheral bulge migrates ahead of the thrust loads.
If, however, the lithosphere can relax the bending
This phase is followed by subsidence and the
stresses set up by the surface load by creep, then its
formation of a foreland basin. During the quiescent
flexural profile will evolve through time to assume the
(relaxation) phase, the peripheral bulge is uplifted and
shapes indicated by curves 2 and 3, even though the
migrates toward the thrust load, only to be halted by
magnitude of the load remains constant. The
the next orogeny and loading phase which,
timescale over which stress relaxation occurs depends
superimposes the next major sedimentary package;of
on the mechanism by which stress is relaxed. If
the foreland basin. Thus, as earlier worke(s'
viscoelasticity provides a valid model of the relaxation
recognized in principle, the stratigraphy and
mechanism(~QuinlanandBeaumont, 1984;Beaumont
sedimentology of the basin fill and the positions of
et aI., 1988), then it is the viscosity distribution
the unconformities in space and time contain
within the lithospheric plate that determines the
important evidence on activity··in the adjacent
relaxation timescale. Given that the viscosity of
orogen, a point we will return to later.
T152: 5
The question of antecedent conditions and many compressional orogens (Fig. 5) may be
inheritance is important for the style of foreland interpreted as the superposition of the anomaly from
basins. Although the role of these conditions and the inherited rifted margin (the steep gradient above
details of their effect have yet to be \vorked out in the transitional zone of crustal thinning, Figure 5)
detail, some aspects have been modelled (Karner and
Watts, 1983; Royden and Karner, 1984; Stockmal et aI., PASSIVE MARGIN
1986; Stockmal and Beaumont, 1987). Figure' 3
illustrates how Stockmal et aI., (1986) incorporated
thermal effects and lateral changes in the flexural
properties of the lithosphere into models of rifting,.
passive margin development, plate collision, and
overthrusting. Simple elastic plates, the bases of
which are defined by a given isotherm were used in
the flexural models (Beaumont et aI., 1982; Keen and
Beaumont, in press).
The significance of these model results (Fig. 4) is
that some sense of the geometrical relationship
between the, overthrusts, their topography, and the
flexed crust of the inherited margin is obtained. For
example: about 20 km thick loads can overthrust the
outboard part of the margin before they need be
subaerially exposed (Panel IV); mountain roots beneath
ACTIVE MARGIN
orogens of Himalayan proportions may be in excess of
Nexltr'~~~~'hon '''~
60 km thick (Panel VII); the ultimate preservation of
a foreland basin, once the orogen has been eroded to __
base level, can be attributed to that part of the
overthrust load that still remains on or outboard of ~=oo~~~L_-- -
the antecedent rifted margin (Panel VIII), and; the --------..--
/"
characteristic Bouguer gravity anomaly common to
(a) Beekmantown-Knox Unconformity
I-4- ~~----
(b) Ordovician Foreland Basin
(c) Relaxation Phose FIGURE 3 Schematic diagram of the quantitative
approach to modelling the transition from passive
(rifted) to active (convergent) margin used by
Stockmal et at., (1986). Stages are constructed in
steps following geologically instantaneous rifting;
stretched continental crust beneath the margin is
(d) Next Overthrust Phase located between vertical dashed lines of the upper
panel. Steps involving the addition of sediments to a
specified bathymetric profile (upper panel) are
alternated with thermal time steps during which
thermal relocation occurs (shown schematically as a
single isotherm T at two times, tj and tj+l). The
flexural response of the lithosphere changes through
time because the thermally controlled effective
FIGURE 2 Cartoon illustrating the development of a thickness of the lithosphere also changes. The
multi-stage foreland basin on a lithosphere that tectonic switch from passive to active margin is
relaxes load-induced stress. The uplift of the modelled by overthrusting loads sequentially onto the
peripheral bulge is shown exaggerated by a factor of passive margin (lower panel). These loads are
lOin (c). Circles represent conglomerates, dots shoveled into position up to a specified topography
represent sandstone, dashed pattern represents shale, (dashed line of lower panel) instead of being pushed
and the brick pattern represents carbonates. Bold in a geologically correct "manner. This approximation
arrows show overthrust ~and peripheral bulge is reasonable when considering subsidence and
migration. Fine arrows illustrate'active overthrusting. sedimentation in the undisturbed part of the foreland.
T152: 6
and the longer wavelength flexural component above ~ 134 m.y. ,
the foreland. The position of the steep Bouguer.
,
gradient may therefore give the approximate location • " I
, ,
of the inherited rifted margin beneath an orogen , \ ~'I'
(Stockmal and Beaumont, 1987). The change in i' ,~ I
geometry with increasing amounts of convergence Foreland Basin" ~ I'
between the overthrusts and the inherited margin can , I
also explain the major change in the associated I -
mnmrr
sedimentary facies from flysch to molasse. Figure 4 I
Moho
(Panels II and IV) also shows that in the early stages I
of convergence, before the overthrusts have 1 Mantle
completely mounted the margin, the foreland basin "'1
may take the form of a deep asymmetric trough that .-1
1
does not have a characteristic flexural shape. I
-,-, '-. ',-, __
:~
"SZl136 m.y.
1"·,,-- ~ ~ "-'Y"-r
~ "')l~
''':2l
-7-
'_, I
I', '-..m D
I ---1---
" I -
I/
II I
125 m,y.
I
/,'-: " _,I
I
'- I'~'/I- I
.-1
I
J
I
I
'SZIl138 m.y I
I ' ..
ill
128 m.y. "
, I
, /, -, I/ - " - .. /
1 ~ "I - , ,/ _/", -
,
II -1\ -" / _"
/
NI3Im.y.
~,,- -'\ ' , 'I I' ~ I ,
\ -/ I\ / - 1 -, I- \ -\ I
'SZIIl188 m.y.
,
- I ,
Mot,o " .... I , .... " n:::::--
I ,
Mantle
, ,- I ~~
.•IIITTnlIIIIIIII
FIGURE 4 Selected steps in the evolution of the
model shown in Figure 3 (from Stockmal et aI., 1986)
Moho
in which an orogen is built on a 120 MyoId rifted
Mantle
margin and then eroded. The vertical exaggeration is
15:1. Random-line pattern is continental and
stretched continental crust. Vertical ruled pattern is ocean crust. Bold lines with bold arrows represent the
decollement. Bold wavy line represents an unconformity. Stipple pattern marks sedimentary basins. Solid
triangles nlark the position of the peripheral bulge. Bold dashed line (panel VII) marks the depth within the
orogen that is exhumed to the surface during erosion and isostatic rebound between stages VII and VIII.
T152: 7
