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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 I I 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 $ 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 .,$ "I 100 Krn 0 I I . FOLD AND THRUST BELT CRATON / -_I + + + + + * + + + t , t t + + l l I t t t + I y iI - I I I , - - + , + , + , + . + - fault - A rnbent folds exposed to by C.C.Plummer mountainside in the Andes. +qy&kt! A&&&, &nitdig * 1 X> 2 > niit ii I 0 I * I I w I' mot^ ~ r " grscA'id k b~ P 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 &$&tikirk GWr4& :W mt&y .&*&: mmprrssed into the pnsent widrh of the Alps, which is 1 s rhpn ZW) kilomrers. A mmpk+x ef regionat-me-tmorphikro#d pluepaic rock is p d y hurd in . h e . ~ o u n t a i n mqps of the most intensely b & r d porciariw pf majar maunezin bdm, Most of the rnetmorpkic meks '&e= orIpinaMy d i e h r w ) . d > Older podom uf wrtte major moudlcsin Mts have undergone @ 5.9). Qw-cutring rtktioarhip show that ~ I C dWtirtg occurs d k r ,the &marion that d r a d ' in tight thrust kdand m m o r phkm and & most batholiths had famed, A6 show~lifl @5.l& ~iaa~dn~ael~dtingisaregultof~ ~ o r ~ * E i * a € & c 9 n ~ w i t h & ~~ @stra~-rharpd*fddm% , thnrst fauth& volcanic r a c h shat-had h , d c c h buried,andsubjected ro . . 4 b d * &. . intense stress and' high rhpemnrre. rWhm hmd, z n m *cZ=ning n o r 4 biting m e t i t e ~I i h t e r I a d granitic and mmunb+hiit iocb) k i b d -@,n o d - hd*. taka. p k in rhe .high, may represent hose pa+:&.& mountain: bbdtar that were d - p mof a majot mewain klt insif&~Rw with the once at even deeper W.h the crust, *here higher temperfding and ,ttwwt Gulsin of the outer part of the belt a w a caused partial melting of &. .&& (as described in (@we 5 . ~ .7,% ~ kPbes r -on wcws in .the cmtral chapter 11). Where found, & su&-p-&ce that &d *&i at-* B$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 ~