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Keurusselän Pirstekartioiden Tarkastelua KeurusselänKeurusselän pirstekartioidenpirstekartioiden tarkasteluatarkastelua KARI A. KINNUNEN JA SATU HIETALA uonna 2003 geologian opiskelija Satu Hieta- la löysi Keurusselän ran- takallioilta rakenteita, jot- ka silmämääräisesti muis- tuttivatV impaktirakenteista kuvattuja pirstekartioita. Niiden perusteella Keu- russelkä tulkittiin maamme 11. impak- tirakenteeksi (Hietala ja Moilanen 2004 a ja b, Pesonen et al. 2004, Pesonen et al 2005, Pesonen et al. 2006, Ruotsa- lainen et al. 2006 a ja b, Hietala ja Moilanen 2007, Schmieder et al. 2009). Keurusselän pirstekartioita ei kuitenkaan ole näissä julkaisuissa tar- kemmin tutkittu, lukuun ottamatta esiintymisaluetta ja havaintojen mää- rää sekä valokuvia. Sama puute koskee myös muita Suomen impaktikraatterei- 68 GEOLOGI 61 (2009) Keurusselän länsirannan Jylhänniemen tiheään rakoillut metavulkaniittinen silokallio, jossa hiertymien kautta avautuneilla raoilla on pirstekartiomainen pintarakenne. Muut raot ovat tasapintaisia. Kastumi- nen korostaa kalliopinnan rakenteita. Kivinäytteen läpimitta 8 cm. Kuvat: Kari A. Kinnunen. ta. Pirstekartiorakenteita on kuvattu ja pidet- ja Dietz 1988). Näistä tunnetuimpia raken- ty todisteena meteoriitin törmäyksestä mm. teita ovat Etelä-Afrikan Vredefort, Saksan Ries Ahvenanmaan Lumparnilla, Petäjäveden Ka- ja Steinheim sekä Kanadan Sudbury. Pirste- rikkoselällä, Taivalkosken Saarijärvellä sekä kartiot kuvattiin impaktikraatteriin liittyväk- Suvasveden eteläisen Haapaselän rannoilta. si ensimmäistä kertaa vuonna 1947 Kentlan- Impaktirakenteita ja -kraattereita tunne- dista Indianan osavaltiosta. Dietz (1947) esit- taan maapallolta tällä hetkellä n. 186 ja noin ti alueen kalkkikivissä kartiomaisten ja kohol- puolesta on kuvattu pirstekartioita (McHone laan olevien viuhkamaisten rakenteiden liitty- GEOLOGI 61 (2009) 69 vän meteoriitin törmäykseen. Sittemmin sa- man kovia yksittäisiä todisteita. Lopullista var- manlaisia rakenteita löytyi mm. Sudburysta, muutta voi olla vaikeaa saavuttaa – kuten tie- joka on yksi maailman kookkaimmista impak- teessä yleensäkin. tikraattereista (halkaisija ~250 km). Myöhem- Pirstekartioiden yleisyydestä huolimatta min pirstekartioista sekä mm. Sudburyn im- niiden mineralogiaa ja mikrorakenteita on tut- paktialkuperästä on esitetty kritiikkiä (mm. kittu hyvin vähän, Suomessa ei ollenkaan. Fleet 1979). Nykyään pirstekartiot ja meteo- Harvat tutkimukset suurimmista vanhoista riittikraatterit ovat geologiassa ja varsinkin impaktirakenteista, kuten Sudburystä ja Vre- geofysiikassa lähes paradigman asemassa ja defortista, ovat lisäksi synnyttäneet uusia ky- usein ainut selitysmalli uusien kohteiden tut- symyksiä tuomalla esiin piirteitä, joita on ol- kimuksessa. lut vaikea selittää varsinaisten shokkivaikutus- ten avulla (ks. Fleet 1979, Nicolaysen ja Rei- Pirstekartioita tutkittu mold 1999). hyvin vähän Tästä syystä katsoimme aiheelliseksi tar- kastella Keurusselän rakenteita lähemmin. Viitteitä jättimeteoriitin, asteroidin tai komee- Huolellinen mikroskopointi osoitti pirstekar- tan, törmäyksestä maankuoreen ovat kraatte- tioiden olevan erikoinen yhdistelmä semiduk- rimainen morfologia ja siihen paikallisesti liit- tiilin vaiheen hiertymiä ja hauraan vaiheen tyvät geofysikaaliset anomaliat. Kiistattomia murtumia. Nämä rakenteet, jotka ulottuvat tuntomerkkejä ovat merkit shokkimetamor- myöhäismetamorfisista hiertopinnoista aina foosista, meteoriittisen aineksen palaset tai hyvin pinnalliseen rakoiluun, ovat vaatineet geokemialliset jäämät (Koeberl 2002, Masai- syntyäkseen shokista poiketen geologisesti pit- tes 2005). Kiistattomia todisteita löydetään kän ajanjakson. Osoituksena tästä ovat klo- yleensä vain geologisesti melko nuorista im- riitilla silautuneet hiertopinnat ja niitä myö- paktikraattereista. Syvälle kuluneista prekam- ten mutkitteleva kartiomaisia ja viirukkeisia brisista impaktirakenteista ne sitä vastoin pintarakenteita muodostava kaoliinipitoinen useimmiten puuttuvat. Tästä syystä varsinkin myöhäinen rakoilu. Myös muut shokkivaiku- Kanadan ja Fennoskandian kilpialueilla on tuksesta (minimi 8 GPa) kertovat piirteet pirstekartoita käytetty vuosikymmeniä impak- puuttuvat Keurusselän näytteistä, mikä toisaal- tirakenteiden tuntomerkkeinä. ta oli odotettavaakin ottaen huomioon pirste- Varmoja kriteerejä aidon pirstekartiora- kartioiden synnyn vaatiman alhaisen shokki- kenteen tunnistamiselle ei kuitenkaan ole esi- paineen, noin 2 GPa (ks. French 1998). tetty, ja jopa itse rakenteen tarkka syntytapa on yhä kiistanalaista (Koeberl 1997). Monien Pirstekartioiden maankuoren luontaistenkin murtumaraken- määritelmiä teiden tuntemus on yhä vajavaista, ja Suomes- sakin ollaan hauraan deformaation tutkimus- French (1998) on koonnut kirjaansa yhden perinteitä vasta luomassa, mitä ovat edesaut- harvoista kattavista yhteenvedoista pirstekar- taneet rakennettavuustutkimukset ja ydinjät- tioiden merkittävistä piirteistä. French mää- teiden sijoitustutkimukset. Kilometrien syvyy- rittelee pirstekartiot kartiomaisiksi rakoilupin- delle kuluneiden prekambristen impaktiraken- noiksi, joita voi syntyä vain asteroidin tai ko- teiden tunnistaminen on useimmiten hajanais- meetan törmätessä kallioperään. Pirstekartiot ten viitteiden kokoamista ja punnitsemista il- syntyvät, kun shokkiaalto eli voimakas paine- 70 GEOLOGI 61 (2009) ja lämpöaalto kohtaa kallioperän ja kulkee sen kuoren tavanomaisen deformaation eräistä lävitse. Niitä esiintyy yleensä törmäyskraatte- harvinaisemmista murtuma- ja hiertoraken- reiden pohjilla ja keskuskohoumissa sekä kraat- teista. Gibsonin kolmas kohta puolestaan lie- terin reunan ulkopuolella. Pirstekartioille neekin otettu mukaan määritelmään sediment- ominainen piirre on, että ne ovat kiven läpi- tikivien kartiorakenteiden (engl. cone-in-cone) kotainen rakenne, eivät ainoastaan pintakuvi- erottamiseksi (ks. Lugli et al. 2005). ointia. Näin ollen pirstekartiopintoja voidaan Pirstekartioiden määritelmät ovat siis ul- lohkoa esiin kivien sisältä. Ne näyttävät uur- konäköön perustuvia. Oikeastaan ainoa todiste teilta, jotka kulkevat kaareutuen kiven eri pin- siitä, että ne liittyvät törmäystapahtumaan on noilla ja muodostavat viuhkamaisia hevosen- se, että niitä esiintyy usein meteoriittikraatte- häntää muistuttavia kuvioita. rien yhteydessä. Pirstekartioiden oletetaan syn- Nykykäsityksen mukaan pirstekartioita ei tyneen törmäyksen puristumisvaiheessa varsi- voi sekoittaa mihinkään muuhun geologisissa naisen shokkiaallon tai sen jälkeisen ohentu- prosesseissa syntyvään rakenteeseen. Virhetun- misaallon vaikutuksesta ns. dynaamisina ra- nistuksen riski liittyy Frenchin (1998) mukaan koina (Sagy et al. 2001, 2002, Baratoux ja tavallisiin rakoilu-, liuskeisuus- ja haarniska- Melosh 2003). Tällöin ne olisivat muodostu- pintoihin (engl. slickensides). Pirstekartiot on neet samaan aikaan kuin kvartsin shokkilamel- erotettu näistä siten, että niiden pinnan tulisi lit (vrt. Reimold 1998) Toisaalta ne voivat olla ainakin jonkin verran kaareva ja uurtei- olla syntyneet myös myöhäisemmässä vaihees- den kulku on konvergoivaa. Kartioiden huip- sa hiertoina, kun törmäyksessä kokoon puris- pukulmat (engl. apical angle) vaihtelevat välil- tunut maankuoren yläosa palautui litostaatti- lä 66°–122°, mutta ovat useimmiten 90°:n seen tasapainoon. Vredefortin muuten vaikeas- tuntumassa. Pirstekartioita syntyy kivilajista ti ymmärrettävät viirukkeiset rakoryhmät riippumatta, mutta karkearakeisissa kivissä (engl. multiple striated joint sets, MSJS) ja nii- rakenne on usein vaikeasti havaittavissa. Kar- hin liittyvät pirstekartiot on haluttu selittää tiot ovat näyttävimmillään kivissä, joiden lu- näin (Nicolaysen ja Reimold 1999). Kummas- juusominaisuudet ovat heikot, esimerkiksi se- sakin tapauksessa pirstekartioiden katsotaan dimenttikivissä sekä hienorakeisissa vulkaani- syntyneen mallinnusten, kokeellisten töiden ja sissa kivissä. Kooltaan pirstekartiot voivat olla kraatterihavaintojen perusteella 2–6 GPa:n muutamista millimetreistä pariinkymmeneen paineessa. Näin alhainen shokkipaine olisi metriin saakka. mahdollistanut ainoastaan kvartsin asemata- Pirstekartioiden tunnusmerkit ovat vuo- son suuntaisten shokkilamellien syntymisen, sikymmenien ajan olleet samoja. Gibson ja mutta ei vielä muita mineralogisia merkkejä Spray (1997) ovat esittäneet pirstekartiolle tapahtumasta. tuntomerkkejä, jotka ovat käytännössä samo- Jos pirstekartiot käsitetään dynaamisiksi ja kuin jo aikoinaan Dietzillä (1960): 1) ra- raoiksi (ks. Sagy et al. 2001, 2002), ne olisivat kenne on kartiomainen tai osittain kartiomai- avautuneet hyvin lähellä Rayleigh-aallon no- nen rikkoutumispinta, 2) harjannekouruvii- peutta (nopeus yli 0,9-kertainen Rayleigh-aal- rutus (engl. ridge-and-groove) kapenee karti- toon verrattuna) (Sagy etal. 2004), siis jopa on kärkeen tai keskiviirun (engl. central striae) kilometrejä sekunnissa. Näin suuri nopeus ja 3) rakenteen pitää olla läpikotainen, ei pin- erottaisi ne endogeenisistä rakojärjestelmistä nallinen. Tämän kuvauksen perusteella mah- ja osoittaisi ne varmuudella impaktiperäisiksi. dollisia pirstekartioita on vaikea erottaa maan- Tunnistamiseen voi tällöin periaatteessa käyt- GEOLOGI 61 (2009) 71 tää poikkileikkauksen haa- roittumisrakennetta ja pinnan mikrorakenteita, kuten Sagy työtovereineen on menetellyt. Impaktissa kokoonpuristu- neen kallioperän palautumi- seen mahdollisesti liittyvien hiertojen erottaminen maan- kuoren oman tektoniikan ai- heuttamista hierroista vaikut- taa sitä vastoin vielä mahdot- tomalta tehtävältä, eritoten jos esiintymispaikalta ei löy- detä varsinaisia todisteita sho- kista.
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