Download cordilleran tectonics: the roles of lithoplate and mesoplate boundaries

2004 Denver Annual Meeting (November 7–
10, 2004)
Paper No. 217-10
Presentation Time: 8:00 AM-12:00 PM
(Wednesday)
CORDILLERAN TECTONICS: THE ROLES OF
LITHOPLATE AND MESOPLATE BOUNDARIES
PILGER, Rex H.
Landmark Graphics Corporation
1805 Shea Center Drive, Suite 400
Highlands Ranch, Colorado 80004 USA
[email protected]
CORDILLERAN TECTONICS: THE ROLES OF
LITHOPLATE AND MESOPLATE BOUNDARIES
Mesoplates are a new heuristic for quantifying the motion of lithoplates relative to the mesosphere.
The three major mesoplates are Hawaiian (primarily beneath the oceanic plates of the Pacific),
Tristan (beneath most of the plates of the Atlantic and Indian Oceans), and Icelandic (beneath
Eurasia, the northernmost Atlantic, Arctic Ocean). While lithospheric plate interaction is the
principal control on mountain belt evolution, mesoplate interactions also contribute to the overall
“style” of deformation.
The principal evidence for mesoplates is distinctiveness of hotspot reference frames for each of the
three regions (Hawaiian, Tristan, and Icelandic). While hotspots within each frame show very
little relative motion, the three frames demonstrate significant relative motion among themselves
over the past 130 to 80 my. Shallowness of the reference frames is indicated by correspondence
of stress fields with motions in the hotspot frames, as well as lithospheric thickness controls on
the origin of minor hotspot traces in the Pacific and South Atlantic. Because the three reference
frames are shallow, interaction with subduction zones is implied. Continuity of deep subduction
zones implies that the zones are the probable boundaries between adjacent mesoplates. Where
medium-to-deep subduction zones are absent, the boundaries between mesoplates are likely
determined by kinematics.
For the North American Laramide (~80 to 48Ma), lithoplate and mesoplate kinematics imply North
America moved to the southwest relative to Tristan, while motion between the Hawaiian and
Tristan mesoplates was divergent, and relative motion of North America and the Hawaiian
mesoplate was minimal. The growing “gap” between the two mesoplates was “filled” by the lowangle subducting Farallon (or Kula) plate.
Pacific plate motion at ~48 Ma (Hawaiian-Emperor bend) changed relative to the Hawaiian
mesoplate and North America. Concurrently, the boundary between the Hawaiian and Tristan
mesoplate was gradually unconstrained as younger plate began to be subducted, and then
subduction progressively ceased. Thus, the mesoplate boundary began to move beneath western
North America, contributing to the complex extensional tectonics and uplift of the western United
States in the Late Cenozoic.
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What are Mesoplates?
Mesoplates and overlying lithoplates are separated by the asthenosphere. The base of mesoplates
may correspond with either the 410 or 600 km discontinuity.
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What are Mesoplates?
Mesoplates: Kinematically rigid
layers in the upper
mesosphere. Three are
defined: Hawaiian, Tristan,
Icelandic.
Boundaries are based on hotspot sets,
intracontinental stress field coherence, and
subduction zones. Original mesoplate
boundaries in blue (Pilger, 2003). Recently
revised in green.
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Evidence for Mesoplates (1):
North
American
lithoplate and
Hawaiian
mesoplate
moved in
concert 75-25
Ma.
Western
boundary of
North
American
lithoplate
(bold) and
Hawaiian
mesoplate
(light)
reconstructed
to Tristan
mesoplate
(Atlantic-Indian
Ocean hotspot
set).
Motion of North American lithoplate and
Hawaiian mesoplate may be correlated.
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Evidence for Mesoplates (1):
Myth: Hawaiian-Emperor bend has no
kinematic correspondence in relative
plate motions.
(Bend age is 47-48 Ma.)
Note bends (~47 Ma) in loci of North
and South American lithoplates and
Hawaiian hotspot (mesoplate) relative to
Pacific lithoplate.
Pacific Loci @ Hawaii
50
Latitude (deg)
40
Data
PC-EU
PC-NA
PC-SA
PC-AN
PC-AU
PC-NZ
PC-FA
PC-HW
30
20
10
-240
-210
-180
-150
Longitude (deg)
Hawaiian-Emperor bend is kinematically significant.
6
Note parallelism of North
American lithoplate (circles) and
Hawaiian mesoplate (squares)
loci relative to Tristan mesoplate.
Note lesser motion of North
America relative to Hawaiian
mesoplate.
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Imagining the North American Cordillera and adjacent Pacific in the Early Cenozoic.
8
Evidence for Mesoplates (3):
Isotopic ages of basalts,
western United States.
Contemporary and paleostresses (s1): colored bars.
Loci of North American
lithoplate motion relative to
Hawaiian (diamonds) and
Tristan (circles) mesoplates,
anchored at “hotspots”.
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Isotopic ages of rhyolites,
western United States.
Contemporary and paleostresses (s1): colored bars.
Loci of North American
lithoplate motion relative to
Hawaiian (diamonds) and
Tristan (circles) mesoplates,
anchored at “hotspots”.
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Basalts appear to reflect both hotspots and Basin Range extension.
Rhyolites seem to have less relation to hotspots.
North American-Hawaiian locus fits Yellowstone better. North AmericanTristan locus better fits Raton.
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Yellowstone
25
15
NOAM-HAWA
NOAM-TRIS
Basalt
Rhyolite
10
Loci of North American lithoplate
relative to Hawaiian (blue) and Tristan
(lavender) mesoplates.
5
0
0
5
10
Raton
Distance (deg)
25
Volcanic and stress patterns imply that
Yellowstone hotspot has been
embedded within Hawaiian mesoplate
since 25 Ma; Raton hotspot is within
Tristan mesoplate. Late Cenozoic
Mesoplate boundary corresponds with
tectonic boundaries of the Cordillera.
20
Age (Ma)
Age (Ma)
20
Distance from Yellowstone (left) and
Raton (right) versus isotopic ages from
basalts and rhyolites and loci.
Inception of activity was followed by
persistence almost to the present.
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NOAM-HAWA
NOAM-TRIS
Basalt
Rhyolite
10
5
0
0
5
Distance (deg)
10
12
Evidence for Mesoplates (4):
0-10 Ma
10-20 Ma
Bars: s1.
Small circles: Instantaneous flow-lines.
Color is age-coded.
13
20-30 Ma
30-40 Ma
Bars: s1.
Small circles: Instantaneous flow-lines.
Color is age-coded.
14
40-50 Ma
50-60 Ma
Bars: s1.
Small circles: Instantaneous flow-lines.
Color is age-coded.
15
60-70 Ma
70-80 Ma
Bars: s1.
Small circles: Instantaneous flow-lines.
Color is age-coded.
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80-90 Ma
Bars: s1.
Small circles: Instantaneous flow-lines.
Color is age-coded.
Contemporary and paleo- stresses (s1)
within stable (cratonic) North America are
parallel with motion of North American
lithoplate relative to Tristan mesoplate.
Intraplate stress fields reflect interaction with thin asthenosphere and underlying mesosphere.
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Correspondence with Tristan hotspot frame implies
hotspots are embedded within kinematically rigid
upper mesosphere (mesoplate).
Contemporary Hawaiian-Tristan boundary beneath
western United States corresponds with boundary
between stable continental interior and actively
extending Cordillera.
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Key References:
Mϋller et al. (1993) Geology, 21, 275-278.
Pilger (2003) Geokinematics: Prelude to Geodynamics, Springer-Verlag.
Raymond et al. (2000) AGU Monograph 121, 359-375.
Zartman et al. (1995) USGS DDS 14.
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