Download how continents change and grow.

how continents change and grow.
Flgun 5.4
View of glaclated peaks in one of the mountain ranges in the Andes mountain belt. A parallel but much lower range is visible at the extreme
rlght skyline of the picture.
Photo by C.C Plurnmer
A mountain, as you know, is a large terrain feature that rises
more or less abruptly from surrounding levels. VoIcanoes arc
mountains, so are erosional remnants of plateaus (mesas). In
this chapter, we will not focus on individual mountains; rather,
we are concerned here with the earth's majar m ~ w t P t nbelts,
chains thousands of kilometers long composed of numerous
mountain ranges. A mountain range is a group of closely
spaced mounrains or parallel ridges (figure 5.1). A mountain
range is likely to be composed of tectonicdly deformed sedimentary, volcanic or metamorphic rocks. It may also show a
history of intrusive igneous acriviry.
The map in figure 5.2 shows that most of the world's
mountains arc in long chains that exrend for thousands of
kilometers. The Himalaya, the Andes, the Alps, and the
Appalachians are examples of major mountain belts, each comprising numerous mountain ranges.
Geologists find working in mountain ranges ro be physically and intellectually challenging. High mountains have
sreep faces and broad cxposurcs of bedrock. This is good
because they allow a. geologist to decipher complex interrela-
tionships berween rock unia, Bur the geologist may have to
become a proficient mountain climber to access the good
exposures. (Conversely, mountain climbers who develop an
interest in the rocks t h y climb sometimes become geologists.)
On the other hand, exposures of bedrock critical, to interpreting the local geology may be buried beneath glaciers or talus
from rockfall. Furthermore, even in the highest and best
exposed mountains,we never see bedrock representative of all
of a mountain range. Significant amaunts of rack (usually
thousands of meters) once overlying the rocks we now see have
been eroded away. Moreover, thc present exposures are like the
proverbial tips of icebergs-there is much mare rock below the
exposed mountain range that we cannot observe. For instance,
the Himalaya, earth's highest mountain range, rise to 8,000
meters above sea level; yet, their mots (the earch's crust bcncarh
the mountains) atrend downward 65,000 meters. In other
words, ar bar, we have exposed ro us less than llH of the thickness of a mountain range.
Our models of how major mountain belts evolve use data
from over a century of studying the geoIogic structures and
Aleutians is continuous with the trend of the mountain ranges
e x p d in the world's many d r m p Oftea, a
that makt up much of the Maskan mainland. (See figure 5.3.
ularstudyaimtapiecet
the-chiatoryda
You will also want to refer to figure 1.5 and the gcologic map
e mountain range or-parta a ranga. h.& Gdd &,
on the inside cover as you read this chapter,) Through Alaska
ogist fcrcws on a particular typc
Ma
the ranges trend more or less east-west, but in northern
uin range. Far instan-, a g~hgist
m& &thE &aCanada they c u m inm a mare north-south trend, The n o d ns in metamorphic rocko in a mawtainaus4iCllta wi& &
south ranges include the many individual ranges of western
tent of gaining more irui t into me#morpuhic procwts,
Canada and western United Statcs. The belt is widest in the
logise working on the " ig picture," d d o p i n g hypotheUnited States where ir extends from the Coast Ranges of Caliof how major mounmin bclm CW~YC,wi&t we tfie pub
fornia and Oregon eastward to rhe Rocky Mountains of Monmults of h&
or Thourn& of I d d e s , usin8
tana, Wyoming, and Colorado. The belt narrows as the
as p k of a puzzle, (Sdencc works largely because scinorth-south trend condnues thmugh Mexico.
sts build on the work of orhers.) Modds that arently art
cly
rcgardvlg tllc evolution of mounrain Mts arc
within the b d e r f m m m r k of plate-wnic theory d
Ages of Mountain &Its and Continents
be described later in rhia draper.
Major mountain belts with higher mountain ranges tend to be
Despite the mqltwitk of idvidualthere we
geologically younger than those .where mountains are lower.
rnc chacteristia rhac are shar#l by most ar d aajw
building for rhe Himalaya, our highest mountain
~ # ~ b C 1 6 W ~ & y o u Mountain
d
belt, began only about 45 million years ago and is still taking
thethemc@a*fhiakd&ywh
m p r r v i u w ~ ( ~ & e ~ o n p hplace.
c Mountain building in the much lower Appalachians
ceased around 250 million years ago. Individual ranges within
that mi& hdp +n
mdmvd dlMCtLTiBtiC of
a mountain belt, however, may vary considerably in height
even though they arc about the same age.
F
I?
+
i Major Mountain Belts
Size and Alignment
Major mountain belts are very long compared to heir width.
For instance, the mountain belt that forms the western part of
North America (the North ArnPrican CoP$illmr) starts as the
Aleutian Island arc in southwestern Alaska. The trend o f the
Mountain regions commonly show evidence that they
were once high above sea level, were eroded to hills or low
plains, and then rose again in a Iater episode of uplift. Such
episodes af uplift and erosion may occur a number of times
during the long history of a mountain range. Ultimately,
mountain ranges stabilize and are eroded to plains.
O n thc North American continent, the Appalachian
Mountains extend from eastern Canada southward through
the eastern United States into Alabama (figure 5.3). In the
Appalachians, fossils and isotopic ages of rocks indicate that
Muuswin &In- and the Continmal Cwt
Figure 5.3
The mountain belts of North America. Some of the major ranges in the Cordillera are shown.
these mountains began to evolve earlier than the mountain belt
dong the western coast of North Amerim. The interior plains
between the AppaIachians and the Rockies are considered to
have evolved from mountain belts in the very distant geologic
past (during the Precambrian). The once deep-seated roots of
the former Precambrian mountain belts are the basment rock
for the now stable central part of the continent. Layers of Pa-
'
b i c and younger d i m m t q rock cwcr mor of that bas
ment. The great ap of the mountain-building q i d th
~
p
d rhePdmzbic sedimenradbh~hknhdby hatop
o b t i d fro)
detemtted &m dwer one bilha
tacCEd in the k s c a d locations whtrt the b e n t
hAriZOm, the C h i
q d .kc^ Eh e Grand
BM H h of h u h Jhhta, m
dome in Missouri,
0
1000 Krn
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Mountain belt
Craton
Structuyl baain
Structural doma
Precambrian shield
1.4
atic cross section through part of a mountain belt (left) and pW of a continental Intarhr (uaton). Vertkal scale is exaggerated.
in the Rocky Mountains, and the Adironclacks in New
of a continent that has been structurally staof time is called a cmmn ( f i ~
for a prolonged
5.4 and 5.5). The central part of the United States and
da is all part of a craton. Other continents are similarly
the craton in rhc central United States has a very
nly 1,000 to 2,000 m e t c r ~ stdimenf
overlying its basement. Sediment was mostly
d d o w inland seas during PaionaGoic time. Howe craton in much of eastern and northern Canada,
sedimentary MCks c o w h e uadcd
n ranges. This rcgion is a Precam*
complex of Precambrian metamorphic
nic rocks exposod over a large area Such shields and
crarom represent the roots of mounat completed the deformation process more &an
ckness and Characteristics
relatively thin cwer of sedimentary rock overlying the
t in the craton contrasts sharply with the hick sedisequence ,typical of a mountain belt. In mountain
red sedimentary rock ctlmrnonly is more than 10
ers thick. The sedimentary rock in cratons may show
rmation, or it may have been gently warped into basins
omes abwe the basement (figure 5.4). By contrast,
n belts are characterized by a variety of folds and fiults
cate moderate to very intense deformation.
ost of rhe sedimentary rock in mountains is of marine
, indicating that today's highlands were once below sea
art of a mountain belt, the rocks are similar to the
entary rocks found on the craton-limestones, shales,
sandstones. By contrast, another wide zone in a mountf n
contains great thicknesses of volcanically derived rock-
Figure 5.S
Satellite Image of pari of a uaton in Western Australia. Metamorphic
rock (dark gray) thal la 3.5 to 3 bllllon years old surrounds walshaped domes of Qraniteand gnelss (whlte) that are 2.8 to 3 3 billion
years old. Gently dipptng dlrnentary and volcrnlc rocks (tan and
reddish) unconformabty overlie the granite-metamorphic basement
oornplex. The area is 400 km across.
Landsat mosaic produced by the Remote Senslng Applications Centre
Department of Land Administration,Western Australia
Mauntai~Bela and the Continmtal Crurt
d m i c Mh, sedimcnr~from eroded volcailic rack, ab well
lava flows.
ab
Patterns of Folding
- and Paulting
Reconstructin the original position and datermitling the
thickness of aycrs of sedimentary and vntcanic rack in
1
mounrain belts iu cunlylicnrcd hecwse in host inswnces
the layered rocks have been foldcd and faulted i~solme time
r k r they were dcporited. (Refer LO figl~re5.6 ar you read
rhrough the fulluwing ynra rzphs.) Folds will bc apen in
those parts nf 3 mountain bc Iwhcrc dcformutiui~is nnr very
intenie. Tighccr folds (figure 5.7) indicate greater sacsscs.
L a r ~ ewrrturtltd and recumbent folds [figure 5.8) tilay he
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MAGMATIC ARC
Normal
mw
w4
"
I
A
W
W
m of a 'typloumournam beCt, For slmpllclty, only R few of the many layen at sdlmentuy rock are ahown. ( 8 )is the
a ma^ ui #m muntrln belt tb the right 61 ( A ) ,
Ctaa e
Fl~ure
IC.7
False color satellite Image of part of the Valley and Ridge province at the Appalachian mountain be#,near Harrlrburg, Pennsylvania. Tho
rldges are sedimentary bdr reslntant to erpsion. The pattern indiceleg tlght and open folding occurred prior to erp~l0n.
U.S. Ceologlcal Survuy.
during faulting. Beneath the lowermost fault, called a &tuchmenffisclt, the rock has remained in place.
Overall, the foIds and thrust faults in a mountain belt suggest tremendous squeezing or crustal shaaning and c w t u l
thickening, The sedimentary rocks of the Alps, for imtancc, are
.posed in more intensely deformed portion8 of mountain
!Its. Reverse faults are common, particularly in the intensely
Lded regions. Especially noteworthy are the fbld and
mt belta found in many mountainous regions. These are
~aractcrizedby large thrust faults (reverse fauits at a low
Lgle to horizontal), stacked one upon anather; the intcrvcn,g rock usually was folded while it was being transported
estimated to have covered an area of ocean floor about 500
kilometers wide when they were deposited. They were later
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.
FOLD AND THRUST BELT
CRATON
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rnbent folds exposed
to by C.C.Plummer
mountainside in the Andes.
+qy&kt! A&&&,
&nitdig
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niit ii
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mot^ ~ r " grscA'id
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rock t
hi&ki r ~f *t k llrih a sqMs'ieb., It;F p U ~apwtkdy
**"*
, c xppy :$oii&dtin:lw$hh:
>H
hg
slop&Mth &$&tikirk GWr4& :W mt&y
.&*&:
mmprrssed into the pnsent widrh of the Alps, which is 1 s
rhpn ZW) kilomrers.
A mmpk+x ef regionat-me-tmorphikro#d pluepaic rock is
p d y hurd in . h e . ~ o u n t a i n mqps of the most
intensely b & r d porciariw pf majar maunezin bdm, Most
of the rnetmorpkic meks '&e= orIpinaMy d i e h r w ) . d
>
Older podom uf wrtte major moudlcsin Mts have undergone
@
5.9). Qw-cutring rtktioarhip show
that ~ I C dWtirtg occurs d k r ,the
&marion
that d r a d ' in tight
thrust kdand m m o r phkm and & most batholiths had famed, A6 show~lifl
@5.l&
~iaa~dn~ael~dtingisaregultof~
~ o r ~ * E i * a € & c 9 n ~ w i t h &
~~
@stra~-rharpd*fddm%
,
thnrst fauth&
volcanic r a c h shat-had h , d c c h buried,andsubjected ro
. .
4
b
d
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&.
.
intense stress and' high rhpemnrre. rWhm hmd,
z
n
m
*cZ=ning
n o r 4 biting
m e t i t e ~I i h t e r I a d granitic and mmunb+hiit iocb)
k i b d -@,n o d - hd*. taka. p k in rhe .high,
may represent hose pa+:&.&
mountain: bbdtar that were
d
- p mof a majot mewain klt insif&~Rw with the
once at even deeper W.h the crust, *here higher temperfding
and ,ttwwt Gulsin of the outer part of the belt
a w a caused partial melting of &. .&& (as described in
(@we
5
. ~ .7,% ~ kPbes r -on
wcws in .the cmtral
chapter 11). Where found, & su&-p-&ce that &d
*&i
at-* B$mh e
3
h
m
e
m
S g b place in rbc
in the lower crwt must haw bwn r : m # p ~ dinto mu&
~ I S Wprdbfiaf- acrhrc mjpr mountain .belt. '
higher lev& o f , rtre, -muat during a mountain-buildi*
episodedBaehohth, mostly afgcanirc, atrcat m magma gktw
tratcd from partid mdtihg of thc l ~ w c rcrust (w upper
mantle). Magma diapirs traveled upward to collect -&d
Geophysical hwxigarians yield additional inhmtation about
solidiFy at a higher Iwel in the crust.
m o u n h and chc continend mas h diOCUSSBd in hprer 2
*
*
I
L
,
Portion of fault block eroded away
I
Sediment fromeroded fault blcoks
I
rk-Mock mauntalns wlth movamsnt along normal faults.
The Evolution
of a Mountain Belt
Vertical uplifl
Hor~zontalexteneion
kde
Althaugh tach mountain belt differs in details, geologists have
developed (and cundnue co modify) models char provide logical explanations of how mountain belts evolve. Plate-tectonic
theory provides the framework for most of our models that
explain various aspects of mountain belts.
The evolution of a mountain belt begins when sedimmc is
deposited; it ends hundreds of millions of years later when the
former mountain belt has become part of a craton. For instructional purposes we describe the history of a mountain belt
in three stages: (I) the accumulation, (2) the orogenic, and
(3) the upIih and block-faulting stages. However, therc arc no
sharp rime boundaries b m e n the stages. Processes associated
with more than one stage may bc occurring at the same time in
different parts of a mountain belt.
The Accumulation Stage
As we noted earlier, mountain belts typically contain thick
sequences of sedimentary and volcanic rocks. The accumulation of thee great thicknesses (several kilometers) of sedimentary or volcanic rocks takes place in the accumulation stage of
mountain building. Most of the sedimentary rocks and much
of the volcanic material accumulate in a marine environment.
The source for the sediment deposited in the water must be a
nearby landmass, belonging to either an adjoining continent
or part of a volcanic island arc.
The east coast of North America is a parriv~continmu/ margin
(described in the previous chapter; see figures 4.22 and 4.26).
Europe and North America have been moving away from each
other for well over 100 million years. During this time sediment
eroded from the continents has washed into the adjoining seas.
The resulting very thick sequences of sedimentary rock make up
continend shelves and slopes. The sedimenwy rocks that form
at passive margins arc predominantly shales, limestones, and
Mountain &Its and the Continental Cm,t
I17
Northwest
Sea
level
,
Present topagraphic surfam
Cross section through pert of the Alps.Thieker lines am thrust faults. Leawr fotds are not shown. Mmmsnt Is from tho rlght to Ihe left of
the diagram (southeast to northwest). Only a few arrows tare shown 10 indicate movement al the ovsrrldlng thrust black.
From S.E. Boyer and D. Elllot, 7982. M P G Bulletin. Reprinted by permission of American Association of Petroleum Geoiogists.
sandstones (mostly quam sandstone). Volcanic rocks are rare or
absent. As long as the Atlantic Ocean basin conrinues to open,
sedimentation will continue and more sedimentary layers will
accumulate on both sides of the Atlantic.
Where we find a thick sequence of shale, sandstone, and
limestone exposed in a mountain range, it is reasonable to
assume these rocks were originally deposited at a similar passive continental margin.
Acc~114tulatt"on
Along a
Conwrgmt Bounday
A large variety of rack rypes are created near a convergent plate
boundary. Volcanic rocks-mosc charaacristically andesiresaccumulate near the boundary either as pyrocIastie layers or as
flows. Sedimentary rocks, moreovtr, may accumulate in equal
or greater amounts than volcanic rocks. Limestone, however, is
usually absent or present only in minor proportions. Shales
and sandstones are the predominant sedimentary rock types.
The sandstones are mostly gruyuckes ("dirry"looking sandstones with sand-sized grains of volcanic or other rocks in a
fine-grained matrix) rather than qumr sandsrant.
The source of this sedimentary as well as volcanic material
is a mptiC dw, which, as described in chapter 4 (see figure
4.33), is a chain of volcanoes or plutons along a line (usually
curved as seen from above).
Sediment eroded h m the magmatic arc, as well as newly
erupted pyroclastic debris, is msporced to and deposited in the
basins on either side of the arc. U s d y the basin co h e seaward
side w
ill be underwater, at least initially, and in time will fill with
sediment. Once the basin has fdled, sediment may be transported
beyond it and accumulate ourward onto the deep ocean flwr,
The Orogenic Stage
Intense deformarion follows or is contemporaneous with the
accumulation stage. An o m g a y is an episode of intense defbrmation of the rocks in a region; the deformation is wually
accompanied by meramorphism and igneous activity. Layered
rocks are compressed into folds. Reverse faulting (especially
thrust faulting) is widespread during an orogeny. Normal
faulting may dso occur but is not as widespread.
The more deeply buried rocks,.subjectedto regional maamorphism, arc converted to schists and gneissw. Magma generated in the deep crust or the upper mantle may work its way
upward to erupt in volcanoes or form bazholiths.
Umgmirs and Occan-Continent
Conv~cc
The relationships among igneous activity, metamorphism, and
subduction arc described in chapters 1 I and 15. Plate convergence also accounts for the folded and reverse-faulted layered
rocks found in mountain belts. An &ccreti~tkslywedge develops
where newly-formed layers of marine sediment u e folded and
faulted as they are snawplowed off the subducring oceanic plate
(see figure 4.32 and explanation in the previous chapter).
Rock caught in and pulled down the subduction zone is
subjected to intense shearing. If rock is carried further down
the subduction zone it becomes metamorphosed (as described
in chapter 15).
Fold and thrust belrs may develop on the craton (backarc)
side of the mountain beIt (figure 5.1 I). Thrusting is away from
the magmatic arc toward the craton. The magmatic arc is at a
high elevation, because the crust is thicker and composed
largely of hot igneous and metamorphic rocks.The large thrust
sheets move toward and sometimes over the craton. (In the
Rocky Mountains, thrust faulting of the craton itself has taken
place.) The thrusting probably is largely due to the crustal
shortening caused by convergence. There is, however, some
controversy among geologisrs over additional processes that
may tqke place. Some geologists regard gravity flow (from the
high and mobile magmatic arc outward over the low and rigid
craton) to contribute significantly ro the process. Others think
that the expanding magmatic arc pushes the sedimentary (and
sometimw metamorphic and igneous) rocks outward to
become the fold and thrust belt. (The magmatic arc is likened.
to a bulldozer pushing a wedge of loose material ourward.)
Extension
Arrows ind~cate
direction of flow
Normal faults
belt
I
Metamorphic
rock moved
to a higher
level
~eep-seated
metamorphism
look place
here
f a mountain bdt in which gravitational collapse and spreading are taking place during plate convergence. Red
rook. Faulting occurs In br~ttlerock near the surface. Rock that was metamorphosed at depth flows to a hfgher
faulting may take place due ro extension in the
80s and early 1930s geologists developed a
s (1) fold and thrust belts, (2) simdtaneous
how once deep-seated metamorphic
lcvel in a mountain belt. What
at the central pan of the mountain
ecomes too high and gravitationally unstable resulting
tatianal c o h p c and spreading. The mobile central
n becomes increasingly elmeed during plate convtrpression of sedimentary and
1 as to volcanic eruptions and
After some time, the welt in the
s roo high to be supported by the underns. (Oxford University geologist
at collapse begins when the welt
ove sea level.) As shown in figure 5.12
e fbrces rack o u d as wcll as downe mountain belt, the rock is dudk
nearer the surface, rock fractures, so
ing. The rock is pushed outward
crustd shortening, the fbld and
e high, central part, the outward flowing rock results
ion (figure 5.121, therefore the brittle, near-surface
cture and normal faulting tkcs place.
The flowage pattern (as shown in figure 5.12) can also
eated metamorphic ro& (migmarites,
m upper levels of a mountain belt.
cr crusd rocks are squeezed, forcing them to flow upward
outward, bringing them closer to the surface.
:r
. ..-
It is important to note that where there are convergent
boundaries, accumulation and deformation are occurring
simultaneously. In other words, the accumulation stage and
the orogenic stage are taking place a1 the same time. This was
not evident to early ge~logisrs,who did not have the perspectivc af plate-tectonic theory for understanding mountain
building. They generally assumed that the accumulation stage
ended before the orogenic stage began.
Am-C~ndntwt~ o n v m g m c e
Sometimes an island arc collides with a continent (figure
5.13). If an intervening ocean is destroyed by subduction, (the
subduction also causes the arc) the arc will approach the conunent. When collision occurs, the arc, like a continent, is too
buoyant to be subducted. Continued convergence of the two
~latesmay cause the remaming sea floor to break away from
the arc and creare a new site of suMuction and a new trench
seaward of the arc (figure 5.13C).Note that the direction of
the new subduction is opposite to the direction of the original
subduction (these are sometimes called flipping subduction
zones), but it still may supply the arc with magma. However,
the arc has now become welded to the continent, increasing
the size of the continent.
This type of collision apparently occurred in northern
New Guinca (north of Australia). A similar collision may have
added an island arc to the Sierra Nevada complex in California
during Mesozoic time, when a subduction zone may have
existed in what now is centrd California. Many geologists
think that much of westernmost North America has formed
from a series of arcs colliding with North America (discussed
later in this chapter under "terxanes").
.I-. -1-d
Mowmin Belts and rhc Cunrimnktl Gust
North
Continent
Arrrorica
-
Afrlca
'
R.Umbrlm rupemontinent breaks up.
Sea Rmr wreadlng begins.
- , .
. .
A m a rnumbrtrn
Arc
,'
1.;.
PidMArc
..
,
ICmnbllan
,
.
Arc
Trench
.
,
Carolina Arc
North
American
craton
Piedmont
terrane
North
Pidmont Carolina
Ancestral Atlantic
inn
Ancestral Atlantic
Africa
Afrlca
-
0 Devonlan Mississippian (Acadlan Orogeny)
Piedmont
terrane
Carolina
arranr
Alleghenlan.
suture
e
Figun 5. I3
Arc-continent convergence can weld an island arc onto a
continent,The direction of subduction changes aiter impact.
--
I
Afnca
E Psnnsylvanlan- Permlan (Alleghenian Orogeny)
Morth America and Africa joined
Future site of Atlantic
Uropiese
z
d Continent-Contifient
Nmh America
I
Anrgnrnian
svtun
Afrlca
Convcrgmce
As described in rhc previous chap~r(see figure 4.341, some
mauntain belts form when an ocean basin closes and continents
collide. Mounmin belts that we fhd within canrinenrs (wirh uacons on eicher side) are believed to be products of continentcontinent convergence, The Ural Mountains resulted from the
collision of h i a and Europe. Convergence of rhe African and
European plates created the Alps. Our highest mountains are in
the Himalayan bdt. The Himalayan orogeny smrtcd around
45 million y m ago as India began colliding wirh Asia (India was
originally in the southern hemisphere). The thick sequences of
sedimentary rocks that had built up o n both continental margins
were intensely faulted and folded. Fold and thrust belts developed and w& carved by erosion into the mountain ranges that
make up the Himalaya. The mountains arc still rising and fre
quenr earthquakes attest K, continuing tectonic activity, North of
the Himalaya, Tibet rose to become what is now the highest
plateau in the world. Normal Edults in the Xbetan plateau indiGate that gravirarional collapse is d i n g place.
F Triassic - Rifting beolns. Breakup of Pangaea starts
The geolaglc twolutlon of the southern Appalachians. Modified
from R. D. Hatcher, Jr,, 1989. Tectonic synthesis of the U.S.
Appalachiane, figure 9, valume F a , Geology at North Amsrica. By
permission of Geological Society of America.
The Appalachian Mountains are an example of eonrinentbntinent convergence, but with a more complicated history.
Arc-continent convergence was also involved and the mountain belt was later split aparr by plate divergence.
A wndenscd version of orogeny in the Appalachians is as
follows (figure 5-14]: Duriig late Precambrian and exlitst kleozoic time the anccsaal Arlantic Ocean developed as sea-floor
spreading forced the passive margins of North America, Europe,
n the continents. These
now
the Calbdonide
A At end of orogsnic atage
/--
-
B Mountain belt moves upward
f /
---.,-,
,
>---,
Paqafoded away
C Erosion and renewed upllft continue until crust beneath
mountain belt Is the same thickness aa that of the craton
,
' 4 . .
Figure 4. Ib
ed and thinned samewhat by
Isostasy In a mountaln belt.The thlckness of the continental crust
is exaggerated.
rational collapse and spread&Lamimhbn, detachment
a$$ doling downward of the underlying lithaspheric m d e ,
as h r i b c d on p. 125.)
2
@ Uplift and Block-faulting Stage
& plateir relaxed,
convergence stops and the mmprclaive h a of the
here is a long period of u p k accompaniedby
During thts mge, which lasts many millions of years,
regions in the mountain belt m m v e r t i d y upward. Emkeep pace with uplifi and h e area remain low. Aiternarife may temporarily outpace erosion, resulting in
untain ranges. The present Appalachian Maune result of uplift and erosion that have taken place
ic stage ehded more than 250 m&n yago.
plains east of the present Appalachians were
f the original mountain belt, yet they were e d e d
and remained as lowhds for wer 70 million years, Even,the entire Appalachian mountain belt will be e d e d to a
and become part of the North American craton.
&Appalachians and most ocher oidw mountain belts, we
amibute the gealogically most recene uplift to * t i c
a&shncnt of the crwt that was thickened during rht orogcnic
stage (as explained in chapter 2 and as shown in figure 5.15).
According to the concept of isostasy, lighter, less dense continental crusr "floats" hi8hcr on the mantle than the denser
oceanic crust. The craton has achieved an equilibrium and is
floating at the proper level for its thickness. Mountains, being
thicker continental crust, "floatn higher than the stable coneinent. As marerial is removed fram mountains by erosion, the
range floats upward to regain its isostatic balance. Isostatic
adjustment does not take place instantaneously. Usually there
will be a considerable time lag between erosion and isostatic
adjustment.
At most places on continents, the altitude above sea level is
related to local crustal thickness. Beneath the 5-kilometer-high
'liberan Plateau, the crust is 75 kilometers thick. Under
Kansas, the crusr is 44 kilometers thick and beneath Denver,
the "mile high city," the crust is 50 kilometers thick. (If the
United States ever joins the rest of the world and goes metric,
Denver will be known as h e " 1 .G kilometer high ciry.") Just
wcst of Dcnvcr, the altitude ofthe Rocky Mountains jumps to
2 kilometers higher &an that at Denver. Scicnrisrs expected ro
find a corresponding thickening of the crust beneath these
Mountain &1b a d the Continental Cmir
121
Bl
"and:~p;O$/#'r&tiv#ffw
moun.b;cwctn tfiecie.vdn%.the rnt re r w n a hlock will
han-lsa>mase
a d w1llx&ar: irgr;aduliy $ward. AAin rbe
case of the plarmu; fii a v h g e surkce w d d rise to a level
h e r fkm bdm e~&loh. Ho-r,
i t s average surface is
summkere fiktwwnV'&cpqg d boetohs of d k y s .
Mthatgkthi a~refagk;bight'&the block r i w to a t i 1
eprus h a n y deep. wit*
I t&
i
1
1
II
ro r n d erosion.
Erasion and dimate can inhence tectonid ~ ' & I L;R&
exampie, the extent and type of erosion can help ~ C P E T mine whether a. highhnd gms higher or lower with time.
If a high plateau, dissected by only a f&v valleys, undergoes erosion, & pkateau is eroded downward uaifomly
(box figure 11.. Fullwing erosion, isostasy mutts in the
plateau floating i l ~ a x P t but
j
rro~uptn its nriginal Itvel, Irs
average surface, which essentially is iks actual surface, is at
a lower rLYlti~nthpl befo~eemion lwlt place. If ew jo+
"
2
,
:!<:]$:$$x
*f,,.
:j;:;j.::2
a*. ,aa
mountains. They were surprised by 1995 seismic studies that
indicated that the crust is no thicker beneath the Rockies than
at Denver. (Similar discrepancies between crustal thickness
a d mountain elevations have been reported for the southern
Sierra Nevada.) Zb orplain the higher elevations, geolagisrs
the mantle as hatttr a d therefore less dense beneath
&&wd&c
kb,
Thc crwt plus less dense mantle rn
$
2ijj!t;q{;k
:/\;im;f;
;ti"!:!
>r($~;$;T:>;:jb
:*:; :!4d.G:">:;i;
:.I?$
$><<
, ~ 1 +
,
,
:$%:
,7'
floating on deeper denser mantle. Seismic wave studies verify
rhar rhe mantle here is hot and appears ro be asthenosphere
that is at a shallower lwel in rhe earth than usual.
It is impnrtant to mention that isastatic adjustment takes
place during h e orogenic sage as well, but the isostatic forces
during an omgeny are overshadowed by forces due to plate
convergence.
I
rs are charzctcristic of this stage. The crust breaks
-bounded blocks. If an upthmn block is large
becomes a fiult-block mountain range. The norting implies hurizunwl &mion, the rcgiond pulling
of the crust. Isostatic vertical adjustment af a fault black
bly occurs at the same time.
Although most fault-block mountain ranges are bounded
by normal faults on either side of the range, some are tilted
fault blocks in which thc uplift has been p a t along one side of
thc range while the other side af the range has pivoted as if
hinged (figure 5.16). The Sierra Nwada (California) and Teton
(Wyoming) Range arc tilted fault-blot& mountain6 (figure
5.17).
Mountain Belts and
~ o n t i n r nCrarst
~l
.
-
flqCw,8.1 7
f+mTern Range, Wyomiw, a fpuJt-bkh raqp. The rudm apowd are n r h l m l m o r p h i o end I g m * m&a that were laMed
brtrnJlnpartuhi.tkn tr Wty ~ b I 0 k r ~ ~mitm.
i . d
Photo by C.C. Plumrner
t
in and Range and adjoining geological provinces.
Scientific American by Thomas H. Jordan and J. Bernard Minster
B and computer generated image prapared for
Extension
I
I
Hat, buoyant mantle cause6 extension, thinning, and block-fauttlng
Dehmipocction
the 1990s were paying considerable attention to
of lithospheric delamination and the role it may
ution of mountain belts. For instance, delaminaexplain the block-faulting,rhin crust, and gmlogically young volcanic activity of the Basin and Range.
Mountain Brb and tht Cantinmkd h t
125
Lithasphcric delamination (or simply thhnhtiox~)is
detachment of part of the m d e portion of the fichosp
beneath a mountain belt (figure 5.20). As you know, the li
sphere consiecs of the crusr and the underlying, rigid m
Beneath the lithosphere is rhe hotter, plastic mantle of
asthenosphere. During an ora ny the crust as we1 ab
underlying lirhosphere mmrle t 'ckeni.The lith~spherr
tle is moler and denser than the asthenosphere mantle. As i
cated in figure 5.20, the thickned portion of the lithoap
mande is gravinrionally unstable so it bre& off and si
through the asthenosphere to a lower Ievd in the mantle.
asthenosphcrc mantle flows in w replace the foundered, ca
mantle. Heating of the crust foll~ws,allowing thc lowcr crust
flow. The once thick crust becomes rhinncr than that of adjo
ing regions of the mountain belr. Extension results in bf
faulting in the upper part of thc cruet (as in figure 5.19).
Delamination beneath the Basin and Range helps
the extet.lpive rhyoliric and basalric eruptions &at occurre
of million years aftcr thc end of the orogcnic stage. Heating
the lower part of thc crust to 700°C would have %encrated
lick magma thar erupted as rhyolite. Basaluc magma wo
have hrmed from partid melting of the asthenosphere w
moved upward (replacing the foundered lithosphere m
and pressure was reduced (as explained in chapter 11).
the crust was o n u thicker in the Basin and Range is suppo
by recent studies of fossil plants indicating thar the Badin
Range was 3 kilomerefs higher than at present.
Delamination is dm being invoked to help explain wlqd
when Pangaea brokc up, North Amtrim split from Europe a d
Africa more or less along the old suture zone. Gwvixanbml caG
h p x muld have contributed to the weakening and thinning d
this once thick part of the mountain belt during the orogenic
stage.The breakup of h e supemncinenr began around 30 million years after the urogenic stage ended, Delamination of the
underlying lithos here mantle would have resulred in heating
and thinning of c werlying, xmf ning lithosphere. Rifting of
the supercontinent began with normal fiulting (see previous
chapter, figure 4.22) and was accompanied by basaltic eruptio~
and inmions. The Appalachians split from the European Caledonides. Europe, Africa, and North America went their separate
ways d~ the A h t i c opened and widened.
Delamination (like gavity collapse) is an example of an
hypothesis that builds on plate-tectonic theory. It w a ~thought
up and proposed bccaust it explains data better than othcr i
concepw do. It still needs hrther testing- to become widely i
acccpkd as a theory
!
Central part of mountain bdt
t
- r
A Thlck continental cruw of a mountain belt produced
durlng orogeny.
- .
,
B Delamination of gravitatic
/ unstable lithosphere
mantle. Hot asthenosphere flows irrlo place and heats
overlying lithosphere.
Normal fauhlna
i
C Extension with hot lower crust flowing outward.
Delamination and thinning of continental crust following orogeny.
Not drawn to scale.
Based on J. F. Dewey, 1988. Extensional collapse of orogens. hctonics, v.
7, pp, 1123-1 139, and K, D, Nelson, 1992, Are crustal thickness variatlons In old mountaln belts llke the Appalach~ansa consequence of
ithospheric delamination? Geoiogy v. 20. pp. 498602.
-
The Growth of Continents
Continents grow bigger as mountain bdts evolve along their .
margins. Accumuiation and igneous activity add new contin e n d crust beyond former coasdincs. In h e Paleozoic Era the
Appalachians were added to eastern North America, and during the Mesozoic and Cenozdc eras the continent grew westward because of accumulation and orogenic processes in many
at is now the CordiIlera. Therefore, if we isotopicks that had been through an orogeny, starting in
Shield and working toward rhe east and west
,we should find the rocks to be progressively younger. In
general way, this seems to be the case. However, there are
rather glaring exuptions.
pect and Exotic Terranes
any parrs of mouncain belts are regions where rhe age
eristics of the bedrock appear unrelated to that of
ions. To help understand the geology of moun-
tain belts, geologists have in recent years begun dividing
major mountain belts into tectonostratigraphic terranes
(or,more simply, terranes), regions within which there is
geoIogic continuity. The geology in one terrane is markedly
different from a neighboring terrane. Terrane boundaries are
usually faults. Typically, a terrane covers thousands of square
kilometers, but some terranes are considerably smaller.
Alaska and western Canada have been subdivided by some
geologists into over fifty terranes (figure 5.21). Terranes are
named after major geographic features; for instance,
Wrangeliia, parts of which are now in Alaska and Canada
(and with fragments in Washington and Idaho, according to
Mountain &hand tbt Continental C w t
and have been called
mrmna, that 'is,tcrrancs &.t
may hot have hrmcd at dwit pmmt site. If evidence i n d i d
b t a t m m did mt krm at its present site on a continent,
-.
4
is q p c k d as an d
Accreted terraria that can
& showit to have t
d @at h c c s arc knmn as m
rdc
imwm.
A Buspeer rerrane will htvt mck types and ages & f f e r c ~
from adjoining t a w , h a ro p v c that it came from elsewhtre in the world ( a d Therefore is an accreted terrane),
geologists may compare hull assemblages or determine tu
paltomagnetic poles (see &per 4) of the rcrrane's rocks. If
the terrane is exotic, its fossil assemblage should indicate
very different climrric or environmental setting compared td#
that of the adjoining rerrane. For an exotic terrane, the pa:
leomagnetic poles for the to& in the terrae will plot a i
some part of the world very distant from pales of adjoiningi
terranes that formed in place. This indicates that a particula
urrane formed in a different part of the earth and drift4
into th continent of which it is now a part. Some accreted
terranes were island arcs and some might have been micro+
continma (such as present-day New Zealand) that moved
considerable distances before crashing inro other landmasses. Others may have been fragments of distant continents that split off and moved a long distance because of
transform faulting. Imagine what might happen if the San
Andreas fault remains active for another 100 million years or
so. Not only would Los Angeles continue northward toward
San Francisw and bypass it in about 25 million yean (see
box 6.3), but the block ofcoastal California west of the fault
would continue moving out to sea, becoming a large island
with continental crust that drifrs northward across the
Pacific. Ultimately it would crash into and suture onto
Alaska.
Figure 5.22 shows a tentative reconsauction of how parts
ofAlaska might haw migrated in time. This is based on paleamagnetic h a that indicate that parts of Alaska originally
formed south of the equator and moved many thousands of
kilomcttts to become part of rhe Cordillera. Note from the
diagram that the path of migration was not simple. Plates split,
plates joined, and the direction of movement changed from
time to time.
The A~pdachiansas well as mountain belts in other continents h a v i i s o been divided into terranes. Even rhe Canadian
Shield has been subdivided into terranes. Some geologists
think rhey can determine, despite the great age and complexity
of the shield's rocks, the extent t~ which some rerranes rraveled
before crashing together.
We should caution the reader thar geologists do not always
agree on the nature and boundaries of terranes. While most
would probably agree that some tcrrancs are exotic, many geol'ogists think the subdividing of daska and western Canada
into fifty terranes is overdoing it and not supported by suKcient evidence. Only time and more painstaking gathering of
evidence will allow geologists to determine the history of each
alleged terrane.
3
I
Sonomia
Francimn
and
Great Valley
Figure 5.21
Some terranes In wastern North America. Note Wrangellia is in
Alaska, Britlsh Columbia, and Idaho.
After U.S. Geological Survey Open File Map 83-716.
some geologisrs) was named after
the Wrangell Mountains of
Alaska.
Many terranes appear to have formed essentially in place
as a result of accumulation and orogrny along the continent's
margin. Other terranes have rock types and ages that do not
seem related to the rest of the geology of the mountain belt
Flgum 6.22
How fragments of the Southern Pacific Crust
may have b o r n e part of Alaska.
ModHied from D B.Stone, 5. C Panuska, and
D R. Packer, 1982, "Paleoiatltudesversus tlme for
southern Alaska," Journal of Geophyscat
Research, vol. 87 (pp. 3697-3707),copyrlghted
by American Geophysical Union.
.. .
:dths Continsnd Cwt
Concluding
Only a couple decades ago, rnany geologists thought that
through the applimtion of plate tectonic theory, we could mily determine the processes at work in each mountain belt and
work back in time to understand the history of each of the
earth's continents. Some s u g p t e d that there would hardly be
major problems for earth scientists to solve in the future. Plate
tectonia was a breakthrough, and a great maiy problems were
solved; but with this p t leap forward in the science we have
identified new problems. New generations of geologists will
have no shorrage of challenges and no less excitement from
solving newly d i m d problem than did their predecessors
who saw the dawn of the plate tectoniw breakthrough. Science
present builds on science past.
C
Mujur mountain belt$ are made up o f a
number of mounrain ranger. Mountain belts
are generally several thousand kilometers
long but only a few hundred kilometers
wide.
Mountain belu
evolve as hllows. First, a thick squtnce of sedimtntaq
and volcanic rock accumulates (the accumprLtion stugc). Second, the accumulation stage
is either accompanied or followed by an omgenic s q e , which involves intense deforma-
tion of the layered rocks into folds and
reverse (including thrust) faults, along with
metamorphism and igneous activity. Third,
the area is then subjected to a long period of
uplift, often with block-faulting, and erosion. Eventualy the mountain belt is eroded
down to a plain and incorporated into the
w t o n , or stable interior of the continent.
According to the theory ofplate tectonics, mountains on the edge of continents
are formed by continent-oceanic conver-
gence, and mountains in the interior ofconrinents are formed by continent-continent
collisions.
The uplift of a region following termination of an orogeny is generally attributed
to isostatic adjustment of continental crust.
Continents grow larger when new
mountain belts evolve along continental
margins. They may also grow by the addition
of ttrrancs that m a y have traveled great distances before colliding with a continent.
gravitational collapse and spreading 1 19
lithospheric delamination
(or delamination) 126
major mountain belt 1 10
mounrain range 110
orogeny 118
Premtnbrian shield I 13
suspect terrane 128
terrane (tectonostratigraphic tcrrane) 127
Terms to Ren~eltnber
accreted terrane 128
accumulation stagc 1 17
craton 113
fault-block mounrain range 123
fold and rhrwr belts 1I5
L
'l'esting Your Know lctlge
iiTg;<ig3:$+;;@&ii
::;;,:.,,:ic<,
Use the questions bclaw to preparc far exams based on this Ehaptc
I , What does a fo1d.d
. .thrust belt tell us about what accurred
during an orogeny?
2. What is the difference between the f o r m that could explain
fault-black mountains and the Farces that could account for an
omgenic srage? .
'
3. Explain how erosion and isaswy evenrually produce arablc,
relatively thin, continental crust.
4. Mow do the squenres of sedirnenrq m& in crato~lsdiffer
from rhose in mountain
belts?
.
. .
5. What sequence of events accounts far a mountain bdt that is
bou~idcdo n eithe~side by cratons? . .
6. The mountain belt that forms the mstern part of North
America is dl4 the (3Appdachians (b) North American
Cordillera (c) Minialaya {d) tan& .(el RockiPs
,
.
.
130
I
I
.
.
,
Tfy
~
L
7
i
7. The craton (a) covtrs the ~enrralpart af the United States and
Canada (b) ha only 1,000-2,000m of sedimenary rock
overlying basement rock Sc) is relatively undehrmcd Id) all of
the abouc
8. The Precambrian shield (a) conwins geologically young rocks
(b) occurs only In mauntainaw regions (c) is a complex of
Precambrian metamorphic and plutonic tpcb
exposed over a
la& ma (dl di of the above
9. Folds and mrse U t s in a mountain bdt sumest (a1 ctwtal
shorrening (b) tensional stress (c) deep water deposition of the
sediment (d) all of the above
10. Which is nor a stagc in the history o f a mountain belt?
(a) subsidence b)aecumularion (c) orogenic (d) uplift and
block-Fadring
11.
-
-
To explain fold and thrust belts, simultaneous normal faulting;
&.f#-*? f ideep-seared
~
metamorphic rocks rise to an uppe-
l!,l::~*:t~
>tbl t
,
,
.
level in a mounrain belt, geologists use a madel called
(a) rrctonism (b) gravirarional collapse and spreading
(c) orogeny (dl faulting
12. The Wilson Cycle describes (a) thc cycle of uplifr and eiosion of
mountains (b) the movement of nrhenuaphere (c) the blockfaulting that occurs at mountains (d) the cycle of splitting of a
supercontinent, opening of an ocean basin, hilowed by closing
of the basin and collision of continents
13. T h e detachment of part of the mantle portion of &e
lithosphere beneath a mountain belt is called la) grmitatiod
collapse (b) rifting (c) lith~s~heric
dihmination (d) none of the
above
15* Which is a source fol
(b) fiagrnenw of distant c
(d) all of rhe above
16. Block-faulting may be due tc
adjustmenr (b) aubdustion (c) gr
(d) lithospheric delaminatipn
17. A mounrain belt formed through ocean-conrinent
convergence may mnrain (choose all that apply) (a) fold and
thrust belts 6)
thick cumulations of marine sediment
(c] normal FauItc (d) meramorphism
18. Place these stages ~Fdevelopmentof a mountain beIc in order
(a)
uplift and black faulting (b) accumulation (r) orogeny
14. Which is not a type of terranc? (a) accumulated (b) exotic
(c) swpect (d) rectonosuatignphic (e) accreted
I
1.
HOWare unconformiries wcd to
determine when orogenies occurred?
2. How has seismic tomography
contribuud ro our understanding of
1
mountain belts?
m
Condic, K. C. 1999. lnlate tectonics
and m u 1 evolutiuio~r.4th ed.
Stoneham, MA: RutterwrthlHeinen~ann.
M e r , H. D., Jr. 1394. The role of extension
in mountain-belt life q c l e 9 . JoumdI of
Geohgicaf i%ucatim, v. 42, pp. 2 12-1 9.
McPhee, J. A. 198 1. B k n and range. New
York Farrar, Srraus & Giroux.
. 1983. la rw-cct~ m z i nNew
.
York:
Farm, Straw & Giroux.
. 1986. Rksingfim tbtpkim, New
I
5 . How could fossils in a terrane's rocks be
used to indicate that it is an exotic
terranc?
3. How do basalr and ulrramafic rocks
from the oceanic lithosphere become
part of mountain belts?
4. Why is a waron locally warped into
basins and domes?
Geological Socicty of America 1983-94.
ThfgmhD of North MCU.
For
information bn a particular part of North
America refer to the appropriate volume in
19 vol. Boulder, Colorado: Geological
geologic history for this part ofthe
Appalachians.
httpiiv~u.~nau.edulpoo~jhwI
TMbat,M
Ek*:
A M-l FieU T ~ D
You
. can take a
rrip through the ~ibccan'i'latcau into thc
Society ofAmerica.
Thc Appdchian stay Arlanuc
Geoscience Sociev, Atlantic Independent
Media: Halifax, Nova Scotia.
Himalaya. Good summaries o f the geology
*fthe l-;betan piacFauand ~
i
Mountains are accessible through this site.
.h&/,+
m,edur*I
Continmtalcrwt: A n h t and m&m. The
Media Guild: San Dicgo, California.
tth&umns~cravtl.htmf
. York: Farrar, Saaus & Giroux.
-'.
1993.Assmbling C~Iffirnia.
New
i York Farm, Srraus & Giroux,
E. M., ed. 1990, Shping the
Tecmnics of Continene a d OWUHS.
New York: W. H.Freeman.
' Moores,
firtk-
'
Gmhgy of Grand Zmn Nmional Park,
,
Hawickc O k~ptruzl~~~u
rptp.A field
trip through put of the Appalachians in
central New York statc. The site includes a
Wrning A p h m , map, mdtm
description of the spectacular Grand Teton
Range and its geologic history
~