Download Why can the breakup of the continental lithosphere take

Why can the breakup of the continental lithosphere take different paths?
Understanding the transition from magmatic to amagmatic rifting
Vadim Levin, Rutgers University
Mladen Nedimović, Dalhousie University/LDEO/UTIG
Martha Withjack, Rutgers University
Suzanne Carbotte, LDEO
Keith Louden, Dalhousie University
Chris Beaumont, Dalhousie University
Overview
The combination of land and marine instruments in the Amphibious Array makes it an
ideal tool to investigate the Nova Scotia and adjoining offshore region where key
questions of continental evolution and oceanic formation can be probed. In this region, a
clear transition from volcanic (aka magma-rich) to non-volcanic (aka magma-poor)
lithospheric breakup that took place during the opening of the Atlantic Ocean is well
recognized. The two different modes of lithospheric breakup offshore Nova Scotia may
be linked to tectonic inheritance from past rifting and orogenic episodes, or else they may
be completely independent, and instead represent a 3D evolution of the process in time
(i.e., magmatic and amagmatic stages of breakup could be diachronous). It is notable that
this along strike variation in magmatic activity observed offshore appears to agree with
onshore syn-rift magmatic activity associated with the Central Atlantic Magmatic
Province (CAMP) which extends only as far north as the Bay of Fundy and SW Nova
Scotia, but it contrasts with an apparent continuity of tectonic fabric onshore as major
terranes of the Appalachian orogeny continue along the entire length of Nova Scotia.
The deployment of the broad band ocean bottom seismometer (BB OBS) Amphibious
Array here can address a fundamental question of the plate tectonic theory: Why can the
breakup of the continental lithosphere take different paths?
Nova Scotia region is a designated “research focus area” of the GeoPRISMs initiative
(ENAM site, RIE component, Figure 1), essential for addressing the following
programmatic goals:
• The role of magmatism in rifting, breakup, and post-rift lithospheric evolution
• The along-strike transition from magma-rich to magma-poor extension at breakup
• The evolution of segmentation from initial rifting to mature seafloor spreading
Figure 1. Location of the
northern corridor within
the ENAM focus site of
the GEOPRISMS
program. Adapted from
the RIE ENAM
Implementation Plan
(http://www.geoprisms.org
/science-plan.html).
The region boasts several data sets both on land and offshore, as well as a number of
ongoing and planned experiments (see subsequent sections). The most significant data
“gap” in this area is the paucity of passive seismic data, especially offshore, and until
recently - onshore. As a consequence, the large-scale architecture of the margin is known
only in places where seismic refraction studies were undertaken (SMART, OETR,
OCTOPUS, Figure 2). As the processes shaping the margin were three dimensional,
understanding them requires observational coverage that can resolve 3D variations in
large-scale structure, spanning both onshore and offshore regions. An ongoing
cooperative passive seismic deployment (Rutgers, LDEO, University of Quebec –
Montreal, Imperial College – London) is improving the regional sampling on land,
however lateral resolution possible with the present configuration of the sites (temporary,
to operate for 2 years) is insufficient to address questions outlined above.
Figure 2. Location of the controlled
source seismic survey planned by
the Canadian scientists and existing
seismic lines offshore Nova Scotia.
Regional MCS reflection lines are
shown as solid black lines
(Lithoprobe and Frontier
Geoscience Program), dashed black
lines (NovaSpan Project of IONGXT) and yellow lines (UNCLOS
Project). OBS refraction profiles
(SMART Lines 1-3, OCTOPUS and
OETR) are marked by orange lines.
Jurassic salt over the Slope Diapiric
Province is indicated by white
patches. The red box indicates the
proposed regional 3-D controlled source OBS survey area with instrument spacing of ~15 km.
Challenging to image salt areas in the north-eastern corner are covered by the 2013 Shell wideazimuth MCS survey (dashed yellow box), the results of which Shell agreed to share with us for
research purposes. The red dashed line shows the proposed detailed 2-D OBS transect with
instrument spacing of only 1 km. The location of the proposed surveys is based on the available
data, in particular on the Smart Lines 2-3 and the ION-GXT line 5100 which gives insight to
where the main variations in the crustal structure along the margin occur.
Offshore: Available data and an ambitious plan for a new OBS facility
Canadian researchers have proposed to build a National Facility for Seismic Imaging
(NSFI) equipped with 160 compact, low-cost, next-generation ocean bottom
seismometers (OBS), each covering all the applications of a conventional short-period
OBS and a limited number of the applications of a broadband instrument. The first
experiment to be carried out with this facility is a 3D controlled source wide-angle
reflection/refraction OBS study focused on the transition from magmatic to amagmatic
rifting found offshore SW Nova Scotia (see red box in Figure 2). This experiment will
provide unprecedented insight into the structure of the crust and the uppermost few km of
the mantle if this area.
Onshore: Passive seismic observations in conjunction with the eastern deployment
of the Earthscope TA
A small number of permanent seismic observatories exist in Nova Scotia region. The
Earthscope Transportable Array (TA) on the Atlantic coast did not extend into Canadian
Maritimes, thus its ~75 km lateral sampling of the region ends at the US-Canada border.
A cooperative US-UK effort in New Brunswick and Nova Scotia did add a number of
sites in the region (Figure 3), although coverage is sparser. All temporary observatories
will collect a 2-year long data set. A number of sites in Maine will likely remain
operational through 2017 under the auspices of the Central and Eastern US Network
(CEUSN, www.usarray.org/ceusn).
ee Nipigon
Nipigon
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Figure 3. A present (September, 2014)
configuration of continuously operating
land seismic observatories in Maine,
Quebec and Canadian Maritimes.
Permanent US and Canadian stations red and white boxes. Temporary seismic
array (Summer 2013 through Summer of
2015): Earthscope TA (white circles),
Earthscope FlexArray QMIII (blue
circles), SEIS-UK array (purple
diamonds). Inset –CEUSN sites in New
England.
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Figure 4. An example plan for the layout of the passive BB OBS array in Nova Scotia region. The
yellow outline corresponds to the shape of the “Northern Corridor” in GeoPRISMS ENAM
Implementation plan. Permanent and long-term land observatories are shown by white and red
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would be sufficient for resolving lithospheric structures on ~100 km scale.