1、www.helsinki./yliopisto Intro to Quantitative GeologyClass overview today-December 2,2019Part I-Basic concepts of thermochronology Basic concepts of thermochronologyEstimating closure temperaturesPart II-Low-temperature thermochronology(online only)Denition of low-temperature thermochronologyThree c
2、ommon low-temperature thermochronometersPart III-Quantifying erosion with thermochronology(online only)Basic concepts of heat transfer as a result of erosionEstimation of exhumation rates from thermochronometers2 BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
3、BBBBBBBBBBBBBBBBBBBBBBBB WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWI
4、ntro to Quantitative Geology www.helsinki./yliopistoIntroduction to Quantitative Geology Lesson 6.2 Low-temperature thermochronologyLecturer:David Whippdavid.whipphelsinki.2.12.193 BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB WWWWWWW
5、WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWwww.helsinki./yliopisto Intro to
6、Quantitative GeologyGoals of this lectureDene low-temperature thermochronology Introduce three common types of low-temperature thermochronometersHelium dating(The(U-Th)/He method)Fission-track dating(The FT method)Argon dating(The 40Ar/39Ar method)4 BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
7、BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
8、WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWwww.helsinki./yliopisto Intro to Quantitative GeologyWhat is low-temperature thermochronology?Low-T thermochronology uses thermochronometers with effective closure temperatures below 300C5 BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
9、BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
10、WWWWWWWWWWWwww.helsinki./yliopisto Intro to Quantitative Geology0 100 200 300 400 500 600Effective closure temperature CWhat is low-temperature thermochronology?Low-T thermochronology uses thermochronometers with effective closure temperatures below 300C6Ar-based systems(U-Th)/He systemsFission-trac
11、k systemsHornblende(50050C)Muscovite(35050C)Biotite(30050C)K-Feldspar(150-350C)Zircon(200-230C)Titanite(150-200C)Apatite(755C)Titanite(265-310C)Zircon(24020C)Apatite(11010C)BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB WWWWWWWWWWWWWWW
12、WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWwww.helsinki./yliopisto Intro to Quantita
13、tive GeologyWhy is thermochronology useful?Thermochronometer ages provide a constraint on the time-temperature history of a rock sampleIn many cases,the age is the time since the sample cooled below the system-specic effective closure temperature7the timing,rate,and extent of motion on the fault7,11
14、.The lower closure temperature of the apa-tite He system makes it possible to detect andquantify degrees of tectonically induced coolingthat are too small to be recorded by higher tem-perature systems.Additionally,for very young orrapid cooling events the He method oers betterprecision.Other tectoni
15、c problems can be ad-dressed by pairing apatite He ages with thosefrom higher temperature systems to better docu-ment the last few hundred degrees of rock cooling12,13.Thus apatite He ages can supplement andin some cases replace other dating techniques in arange of tectonic studies.Apatite He ages a
16、re strongly inuenced by per-turbations in the thermal eld of the shallowcrust.Most notably,crustal isotherms mimic sur-face topography,with the relief on isothermsbecoming increasingly signicant closer to thesurface(e.g.Fig.1).The lower the closuretemperature of the system,the greater the inu-ence o
17、f topography on cooling ages.Although thepotential for topography to confound cooling agepatterns has been recognized 14,15,the sensitiv-ity of apatite He ages is such that they can beused to infer the existence and even the evolutionof topography in the past 6.Of course this to-pography may be prod
18、uced by faulting(Fig.1),so tectonic interpretations of apatite He ages can-not be made in isolation from the eects of sur-face topography.This interplay underscores theneed for quantitative models that link the thermaleects of tectonics with surface processes suchas relief development,river incision
19、 and glacialerosion which ultimately control the timetem-perature history of rocks in the uppermost crustas well as the long-term evolution of the land-scape.In this regard apatite He ages may providean important tool for linking geomorphology,which relies largely on analyses of modern topog-raphy a
20、nd recent surface processes,with longertime scale eects documented by structural geol-ogy.In this paper we discuss the principles,tech-niques and limitations of the apatite He datingmethod and illustrate the use of He ages to inves-tigate tectonic and geomorphologic processes.Weemphasize general con
21、cepts over specic detailsand applications,which have been reviewed inother papers(e.g.5).Fig.1.Thermal processes in a normal-fault bounded range that inuence the interpretation of apatite(UTh)/He data.Isotherms(dashed lines)are curved from advection of mass and heat(red arrows)in the footwall and ha
22、nging wall,and by topographic re-lief.Rocks in the subsurface(open circles)are exhumed and sampled at the surface(lled circles).The stippled red zone betweenV45 andV75C represents the helium partial retention zone(HePRZ)where helium diusion is neither fast enough to maintaina zero concentration,nor
23、slow enough for complete retention of helium.Modied from 11.EPSL 6478 13-1-03 Cyaan Magenta Geel ZwartT.A.Ehlers,K.A.Farley/Earth and Planetary Science Letters 206(2003)114 2Ehlers and Farley,2003BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
24、BBBBBBB WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWwww.helsinki./ylio
25、pisto Intro to Quantitative GeologyWhy is thermochronology useful?Because the temperatures to which thermochronometers are sensitive generally occur at depths of 1 to 15 km and ages are typically 1 to 100s of Ma,they record long-term cooling through the upper part of the crust and can be used to cal
26、culate long-term average rates of tectonics and erosion8the timing,rate,and extent of motion on the fault7,11.The lower closure temperature of the apa-tite He system makes it possible to detect andquantify degrees of tectonically induced coolingthat are too small to be recorded by higher tem-peratur
27、e systems.Additionally,for very young orrapid cooling events the He method oers betterprecision.Other tectonic problems can be ad-dressed by pairing apatite He ages with thosefrom higher temperature systems to better docu-ment the last few hundred degrees of rock cooling12,13.Thus apatite He ages ca
28、n supplement andin some cases replace other dating techniques in arange of tectonic studies.Apatite He ages are strongly inuenced by per-turbations in the thermal eld of the shallowcrust.Most notably,crustal isotherms mimic sur-face topography,with the relief on isothermsbecoming increasingly signic
29、ant closer to thesurface(e.g.Fig.1).The lower the closuretemperature of the system,the greater the inu-ence of topography on cooling ages.Although thepotential for topography to confound cooling agepatterns has been recognized 14,15,the sensitiv-ity of apatite He ages is such that they can beused to
30、 infer the existence and even the evolutionof topography in the past 6.Of course this to-pography may be produced by faulting(Fig.1),so tectonic interpretations of apatite He ages can-not be made in isolation from the eects of sur-face topography.This interplay underscores theneed for quantitative m
31、odels that link the thermaleects of tectonics with surface processes suchas relief development,river incision and glacialerosion which ultimately control the timetem-perature history of rocks in the uppermost crustas well as the long-term evolution of the land-scape.In this regard apatite He ages ma
32、y providean important tool for linking geomorphology,which relies largely on analyses of modern topog-raphy and recent surface processes,with longertime scale eects documented by structural geol-ogy.In this paper we discuss the principles,tech-niques and limitations of the apatite He datingmethod an
33、d illustrate the use of He ages to inves-tigate tectonic and geomorphologic processes.Weemphasize general concepts over specic detailsand applications,which have been reviewed inother papers(e.g.5).Fig.1.Thermal processes in a normal-fault bounded range that inuence the interpretation of apatite(UTh
34、)/He data.Isotherms(dashed lines)are curved from advection of mass and heat(red arrows)in the footwall and hanging wall,and by topographic re-lief.Rocks in the subsurface(open circles)are exhumed and sampled at the surface(lled circles).The stippled red zone betweenV45 andV75C represents the helium
35、partial retention zone(HePRZ)where helium diusion is neither fast enough to maintaina zero concentration,nor slow enough for complete retention of helium.Modied from 11.EPSL 6478 13-1-03 Cyaan Magenta Geel ZwartT.A.Ehlers,K.A.Farley/Earth and Planetary Science Letters 206(2003)114 2Ehlers and Farley
36、,2003BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
37、WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWwww.helsinki./yliopisto Intro to Quantitative GeologyWhy is low-T thermochronology useful?Low-temperature thermochronometers are unique because of their increased sensitivity to topography,erosional and tecton
38、ic processes9the timing,rate,and extent of motion on the fault7,11.The lower closure temperature of the apa-tite He system makes it possible to detect andquantify degrees of tectonically induced coolingthat are too small to be recorded by higher tem-perature systems.Additionally,for very young orrap
39、id cooling events the He method oers betterprecision.Other tectonic problems can be ad-dressed by pairing apatite He ages with thosefrom higher temperature systems to better docu-ment the last few hundred degrees of rock cooling12,13.Thus apatite He ages can supplement andin some cases replace other
40、 dating techniques in arange of tectonic studies.Apatite He ages are strongly inuenced by per-turbations in the thermal eld of the shallowcrust.Most notably,crustal isotherms mimic sur-face topography,with the relief on isothermsbecoming increasingly signicant closer to thesurface(e.g.Fig.1).The low
41、er the closuretemperature of the system,the greater the inu-ence of topography on cooling ages.Although thepotential for topography to confound cooling agepatterns has been recognized 14,15,the sensitiv-ity of apatite He ages is such that they can beused to infer the existence and even the evolution
42、of topography in the past 6.Of course this to-pography may be produced by faulting(Fig.1),so tectonic interpretations of apatite He ages can-not be made in isolation from the eects of sur-face topography.This interplay underscores theneed for quantitative models that link the thermaleects of tectoni
43、cs with surface processes suchas relief development,river incision and glacialerosion which ultimately control the timetem-perature history of rocks in the uppermost crustas well as the long-term evolution of the land-scape.In this regard apatite He ages may providean important tool for linking geom
44、orphology,which relies largely on analyses of modern topog-raphy and recent surface processes,with longertime scale eects documented by structural geol-ogy.In this paper we discuss the principles,tech-niques and limitations of the apatite He datingmethod and illustrate the use of He ages to inves-ti
45、gate tectonic and geomorphologic processes.Weemphasize general concepts over specic detailsand applications,which have been reviewed inother papers(e.g.5).Fig.1.Thermal processes in a normal-fault bounded range that inuence the interpretation of apatite(UTh)/He data.Isotherms(dashed lines)are curved
46、 from advection of mass and heat(red arrows)in the footwall and hanging wall,and by topographic re-lief.Rocks in the subsurface(open circles)are exhumed and sampled at the surface(lled circles).The stippled red zone betweenV45 andV75C represents the helium partial retention zone(HePRZ)where helium d
47、iusion is neither fast enough to maintaina zero concentration,nor slow enough for complete retention of helium.Modied from 11.EPSL 6478 13-1-03 Cyaan Magenta Geel ZwartT.A.Ehlers,K.A.Farley/Earth and Planetary Science Letters 206(2003)114 2Ehlers and Farley,2003BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
48、BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
49、WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWwww.helsinki./yliopisto Intro to Quantitative GeologyHigh temperature=no topography sensitivityFor thermochronometers with a high effective closure temperature,the closure temperature isotherm will not be inuenced by surface topographyNote that age wil
50、l increase with elevation as a result of the topography10the vicinity of the surface,the isotherm followsexactly the surface topography and K=1.Consequently,in an actively uplifting and erod-ing area characterized by a nite topography,there should be a well-dened relationship be-tween height and app
